Isis Volume 52 issue 3 1961 [doi 10.2307%2F228079] Richard C. Dales -- Robert Grosseteste's Scientific Works.pdf

Robert Grosseteste's Scientific WorksAuthor(s): Richard C. DalesSource: Isis, Vol. 52, No. 3 (Sep., 1961), pp. 381-402Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/228079 .Accessed: 09/05/2014 10:18

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

The University of Chicago Press and The History of Science Society are collaborating with JSTOR to digitize,preserve and extend access to Isis.

http://www.jstor.org

This content downloaded from 195.78.109.69 on Fri, 9 May 2014 10:18:41 AMAll use subject to JSTOR Terms and Conditions

Robert Grosseteste's Scientific Works

By Richard C. Dales*

A great deal of attention and interest has recently been directed at Robert Grosseteste, particularly because of his scientific methodology. The most

recent and perhaps the most brilliant work dealing with this aspect of Grosse- teste's many-sided activity is A. C. Crombie's Robert Grosseteste and the Origins of Experimental Science.' In this book Dr. Crombie is primarily interested in Grosseteste's methodology, and in his scientific works only as they illustrate the methodology. He proposes and argues well that Grosseteste first worked out a theory of scientific method, notably in his commentary on the Posterior Analytics, and then applied the method in his own scientific investigations. In developing his thesis, Crombie usually begins his discussion of each part of Grosseteste's method with an appropriate quotation from the commentary on the Posterior Analytics and then, where possible, cites sections of the scientific works to illustrate how the idea was put into practice; or he cites methodological remarks made in the scientific works to illustrate methodological principles presumably already arrived at before the work had been undertaken.

This procedure has several difficulties. In the first place, it assumes that the commentary on the Posterior Analytics precedes all the scientific works, an assumption which, it will be shown below, there is some reason to question. And in the second place, it does not pay adequate attention to the chronological relationship between Grosseteste's scientific works and the development of his ideas in them.

I have selected for treatment in this article several of Grosseteste's trac- tates which may be classified as natural science.2 It shall be my purpose primarily to present an exposition of the contents of these works, secondarily to call attention to Grosseteste's methodology in the course of the exposition, and finally to attempt where possible to establish their chronological order.

* Lewis and Clark College. A preliminary portance and their difficulty demand that they version of this paper was read at the 1958 be given special treatment; I have excluded meeting of the Pacific Coast Branch of the De operacionibus solis because it adds nothing American Historical Association. Further re- significant to the other scientific works and search was aided by a grant from the Danforth because it is not an investigation or even an Foundation. argument, but a number of assertions in the

1A. C. Crombie, Robert Grosseteste and form of a commentary on Ecclesiasticus the Origins of Experimental Science, 1100- XLIII, 1-5. Cf. S. H. Thomson, "Grosse- 1700 (Oxford, 1953). teste's Questio de Calore, De Cometis, and De

2 I have excluded De sphera and the other Operacionibus Solis," Medievalia et Humanis- astronomical works because their great im- tica, 1957, 11: 43.

381


With regard to my primary purpose, it has in most cases seemed advisable to give quite a full account of each work and to use much of Grosseteste's language. I have tried to omit nothing essential. I have, however, occasion- ally re-arranged sentences in the interest of clarity and order.

Regarding my secondary purpose, I have found the role of experiment in Grosseteste's scientific works to be somewhat more extensive than was described by Dr. Crombie,3 who says that after the resolutio, or analysis of a complex phenomenon into its principles, and compositio, or deduction of the observed events from the discovered principles, Grosseteste then deduces consequences of the hypotheses introduced in the compositio and verifies or falsifies them by experience or experiment. In addition to this, as will be seen, Grosseteste uses experiment to aid in the resolutio and for suggestions in framing the hypotheses of the compositio.

The last part of my purpose is by far the most complex and its results the least conclusive. We may, however, adopt certain principles which enable us in some cases to establish chronological relationships. Grosseteste had several favorite notions which appear in various forms in his scientific works, and he had some pet experiments which he sometimes used for quite different pur- poses. It may be assumed that when a notion appears in a rough, general form in one work, and in a clear, fully-developed and often modified form in another, the former work is of earlier composition. Also, when new concepts are employed, such as the subordination of sciences, or the incorporation of rays in the elements, which should have been used in another work but were not, we may assume that this work was written prior to the one which employs the new concepts. The use of these principles is somewhat complicated by Grosseteste's habit of rapidly summarizing the results of a previously detailed investigation when it serves his purpose in a new context. One must consequently be careful to distinguish between a crude statement on the one hand and a brief summary of a carefully worked out theory on the other.

Grosseteste's career as a scientist may be considered to have extended from about 1220 to 1235 when he became Bishop of Lincoln. Fortunately, many great scholars, notably Ludwig Baur, A. C. Crombie, D. A. Callus, Ezio Fran- ceschini, J. C. Russell, and S. Harrison Thomson, have done a great deal toward establishing the probable composition dates of many of Grosseteste's works. Of the works we will be considering, those about which there is relative cer- tainty are De generacione stellarum (about 1220),4 De lineis, angulis et figuris and De natura locorum (about 1230-31).5 By relating the other works to these, we may gain an approximate knowledge of their composition dates and of the development of Grosseteste's knowledge of the natural world.

3 A. C. Crombie, op. cit. and "Grosseteste's 20...." The crudity of the method and con- Position in the History of Science," in D. A. cepts employed in the work would indicate Callus, ed., Robert Grosseteste: Scholar and that it is not much later than 1220. Bishop (Oxford, 1955). 5 De lineis contains two almost certain ref-

4 A. C. Crombie, Robert Grosseteste and erences to Averroes (cf. Crombie, op. cit., the Origins of Experimental Science, p. 48: 49-50); De natura locorum presupposes De "A reference... to 'Aristoteles in XVIII de lineis and uses Michael Scot's translation of animalibus' would place his work after 1217- Avicenna's Abbreviatio de animalibus.

382 RICHARD C. DALES


ROBERT GROSSETESTE'S SCIENTIFIC WORKS

II A. Early Works

I. De generatione stellarum6 De generatione stellarum is not a particularly noteworthy essay. Method-

ologically it has little to recommend it. Grosseteste proves of stars that they are not of the same nature as their spheres; that they are not simple bodies but are composed of the four elements; that they are not perfectly transparent, but vary in their transparency; that they are not parts of their own spheres, since the sphere and the star are both circular and do not have the same center; a star and its sphere are not uniform, but differ essentially in their light, and so are of a diverse creation. The treatise closes with a criticism of the chemists for supposing that the fifth essence is present through humiliation in mixed bodies. The essay is as loose in structure and as indefi- nite in purpose as this summary makes it appear. It proceeds by stating a syllogism, then proving the minor premise by other syllogisms, then the major, citing in the process any authorities which seem pertinent. Then follows another syllogism not necessarily related to the preceding and the process is repeated.

Two aspects of this work should be especially noted for comparison with Grosseteste's later works: the crudity of its discussion of color, completely dependent on Aristotle,7 and the belaboring of the assertion, derived from Aristotle, that there are degrees of transparency.8 This second point Grosse- teste takes for granted in his later works and he even makes it an essential part of his theories of color (cf. De colore and De iride) and heat (cf. De calore solis).

2. De generacione sonorum9 A more interesting, but still probably a quite early work, is De generacione

sonorum. The first part of this tractate is remarkably similar, in some cases 6 Ludwig Baur, ed., Die Philosophischen

Werke des Robert Grosseteste, Bischofs von Lincoln (Miinster, 1912), (Bd. IX of Beitrdge zur Geschichte der Philosophie des Mittel- alters), pp. 32-36. For bibliographical information, cf. S. H. Thomson, The Writings of Robert Grosseteste, Bishop of Lincoln 1235- 1253 (Cambridge, 1940), p. 100. In this work, Grosseteste cites Albumazar and Aristotle (De generatione et corruptione, ii; De animalibus; De anima, ii; De caelo et mundo, i and iii; and Predicamenta).

7 "Omne coloratum est mixtum. Stellae sunt corpora colorata. Ergo stellae sunt corpora mixta. -Prima patet duplici ratione, quarum prima haec est: Colores sunt qualitates secundae ex primis generatae, ut dicitur in Praedicamentis. Dicuntur enim passibiles qualitates, non quia generent passionem, sed quia ex passionibus generantur. Ergo erit coloratum elementum ex necessitate.

"Item alia ratione sic: 'Color est lux in

extremitate perspicui in corpore terminato.' Sed corpus terminatum est corpus mixtum, et corpus coloratum est corpus terminatum. Ergo corpus coloratum est corpus mixtum. -Quod maior huius syllogismi sit vera, patet. Si enim diceret aliquis, quod corpus diaphanum esset terminatum, ut quinta essentia et ignis, aer autem et aqua non sunt terminata, quia fluxi- bilia sunt, tunc esset coloratum caelum, quia esset ibi lux in extremitate perspicui in corpore terminato. Et etiam dicit Aristoteles, quod cum lux non sit, nisi in corpore terminato, non est in corpore simplici. Ergo corpus terminatum non est simplex, sed mixtum ex simplicibus. -Minor autem huius syllogismi patet per illud, quod dicit Aristoteles, quod 'proprium sensibile visus est color.' Ergo stellae non sunt visibiles nisi per colorem. Sunt igitur coloratae" (Baur, op. cit., pp. 33-34).

8 Ibid., pp. 34-35. 9 Ibid., pp. 7-10; cf. Thomson, op. cit., p. 99; cites Priscian and Isidore of Seville.

383


word for word, to parts of Grosseteste's treatise on the Liberal Arts and his commentary on Aristotle's Posterior Analytics. Dr. Baur has printed the appropriate sections of these three works in parallel columns in order to facilitate comparison.?1 In none of these works is the investigation of the generation of sound the main object. De generacione sonorum is closely connected in spirit with De artibus liberalibus; its primary interest is in phonetics, the relation of sounds to the shapes of letters, etc., and the discussion of sound in general is simply an introduction to the main part of the essay.

A comparison of the three discussions of sound reveals that they are of almost the same length, although De generacione sonorum is longest by a few lines. It is extremely difficult to decide on the basis of the texts which is the earliest. Baur has suggested1 that De generacione sonorum is the first draft of the section on sound in the commentary on the Posterior Analytics. Al- though it was probably written before the commentary, it cannot for this reason be considered a "rough draft." The treatment of sound in De generacione sonorum is the most comprehensive of the three and excels the other two in five respects. First, the presentation is systematic and tight rather than discursive; second, appeal to experience, limited though it is, is an integral part of the method of investigation, as it is not in the other two; third, the treatment of the transmission of the pulses of the vibrating body is much expanded; fourth, the reception of these pulses by the ear is mentioned only in De generacione sonorum; and, fifth, the important distinction between a sensation and a perception is made only in this work.

De generacione sonorum omits the resolutio-whether because Grosseteste felt it unnecessary in view of his purpose or because of the early composition date of the work is not clear-and presents the reader at once with a hypothesis: When an object capable of making a sound is struck, parts of the object go forth from their natural place. The nature of the object then exerts a force reinclining these parts to their natural place; but in their return the parts overshoot the mark and the process is repeated in the opposite direction. This creates a subtle vibration in the outer parts of the sounding object, which is evident both to sight and touch. When these parts go forth from their natural place, there result an extension of the parts along the longitudinal diameter and a constriction along the transverse diameter; and the opposite occurs when they return. When these parts vibrate, they move the air contiguous to them in a manner similar to their own motion, and the air thus moved reaches the ears.12 This creates a sensation of the body which the soul then takes notice of, and there arises a heard sense.

The remainder of the essay concerns phonetics and related topics and lies outside the scope of this investigation. It will be noted that the use of experience is limited to verification of the vibration of a sounding object by sight and touch, and that the introduction of geometrical considerations concerning the extension and constriction of the longitudinal and transverse diameters is really superfluous, since no deduction follows from it.

10 Baur, op. cit., pp. 58*-59*. "Idcirco difinitur sonus: Aeris percussio indis- "Ibid., p. 59*. soluta usque ad auditum" (Migne, Pat. Lat., 2Cf. Boethii, De musica, lib. I, cap. 3: LXIII, col. 1173).




B. The Middle Period After these two early works, dating probably from the first years of the

1220's, there follow three works which we may assign to the "middle period" of Grosseteste's scientific career, extending from about 1223 or 1224 to about 1230. These works show Grosseteste's increasing methodological precision and an ever-growing range of reading on scientific subjects. In all three works, he assumes that heavenly bodies cause change on Earth by their light. In the earliest of these three works, De impressionibus elementorum, there is as yet no notion of the incorporation of these rays by the elements. Rather, change is caused by heat or visible light resulting from the reflection and condensation of the rays themselves. (Grosseteste's first mention of the incorporation of the rays in a dense medium and the production of heat by scattering is in De accessione et recessione maris and is used by him regularly thereafter.) Elements are transmuted through sublimation and assimilation. There is as yet apparently no knowledge of refraction in any of these three works (except possibly in the last of them, De cometis), no use of the principle of subordination of sciences; and although mathematics is employed, especially concerning the power of incident and reflected rays, the precision of De lineis and De natura locorum is not yet reached. During the middle period, Grosse- teste's physical thought was essentially qualitative in nature.

1. De impressionibus elementorum13 In De impressionibus elementorum, the method of resolutio and compositio

together with the use of experimental verification and falsification is fully developed, although only slight use is made of mathematics. In the resolutio, five principles are put forth from which the production of the elements, i.e., dew, rain, snow, and hail, may be deduced. First, it is asserted (assuming the "light metaphysics" of De luce) that the rays of celestial bodies descending upon corporal things are the foremost cause of change. Second, it is the reflected and condensed rays, not the hot body, of the sun, which cause heat among us. This is established by an appeal to experience: There is greater heat in valleys than on mountains, as is evident from the fact that snow remains longer on mountain tops than in valleys and that birds of prey fly high in the summertime to cool themselves; if the sun heated us as a hot body does, mountain tops, being closer to it, would be hotter than valleys; but the opposite of this has just been shown. Third, rays descend into the depths of water, since water is a transparent body, and in the depths of water there is a reflection. Therefore there is greater heat in the depth than on the surface. This may be verified experimentally by noting that fish in winter are in the depth of the water, but in summer are on the surface and that water is congealed on the surface but not in the depth. Fourth, water, in addition to being cold and humid, is by nature congealed rather than fluid. Its fluidity results from enclosed heat. And fifth, rays reflected from a concave mirror generate fire, and flax placed opposite the mirror is ignited.

1s Baur, op. cit., pp. 87-89; cf. Thomson, op. cit., p. 104; cites Aristotle (Philosophus).

385


From these five principles, Grosseteste, in the compositio, deduces the observed phenomena of dew, rain, snow and hail. When rays are condensed in the depth of the water, he says, the water will become hot. But insofar as it becomes hot it does not remain under the nature of water, but passes over to the nature of air. Then, since it is not the nature of air to be under water, it rises in a bubble above the water. If anyone wishes to see this, let him put clear water in a clear vessel and place it over a flame; he will see the bubbles generated and ascending because of the heat of the fire.

When many bubbles rise above the water at the same time, they maintain themselves because of their humid nature, and from these comes vapor or steam from which clouds arise. But the qualities and properties of the bubbles vary according to the proportion of each of the four elements in them, for there are all four elements in such a bubble: earth, because of the place of generation; water, for obvious reasons; fire, in the generation of heat; and the generated air, resulting from the heat. When water predominates in the generated bubbles, they are called "humid vapor"; when earth predominates they are called "dry smoke"; and when air is abundant, they will be a dense vapor. Therefore, the bubbles are more subtle or more gross according to the subtlety or grossness of the generating heat.

In the morning and evening, when the heat is weak, subtle bubbles flutter upwards from the surface of the water. And when these little bubbles are destroyed by the heat they fall to the surface of the earth and become dew.

But if the heat is greater, it makes the bubbles-or cloud-rise to the first (of the three) interstices of the air. When heat destroys the bubbles here, drops of rain fall.

But when a cloud ascends to the second interstice, there is made a greater abstraction of heat and the bubbles are destroyed there by the heat succes- sively only, not suddenly, wherefore that which is soft is relinquished, just like wool, and becomes snow.

If, however, a cloud be suddenly driven upwards to the second interstice, it is suddenly destroyed by the heat, and each round bubble becomes a round stone, or hail. This occurs especially when the heat is great.

This essay is to be compared to four other works, De calore solis (which it anticipates in several respects), De accessione et recessione maris, De lineis, angulis et figuris, and De natura locorum. One is struck by Grosseteste's remarks on the reflection of rays in the depths of water in De impressionibus elementorum. It is obvious from this that this work preceded De lineis (in which the laws of reflection and refraction are carefully studied), De iride, and De calore solis, since in it Grosseteste is apparently unaware that rays would be reflected from the surface of the water and would be refracted at the surface. And since he does not use the concepts of "incorporation" and "scattering," but attributes change simply to the heating of water by condensed rays, it would also be earlier than De cometis, De accessione et recessione maris, and De colore. Another point on which this essay is to be compared to De calore solis and De natura locorum is the discussion of the way in which the sun heats. That the sun does not generate heat as a hot body is a point which is incorporated in De calore solis. In the latter work,




however, the proof of this assertion is more elaborate, involving the immuta- bility of the fifth essence and the non-intersection of rays on high and is much more mathematical than in De impressionibus elementorum, where the propo- sition that the sun generates heat as a hot body is falsified experimentally.

One part of the falsification of this point creates a problem in determining the relationship of De calore solis to De natura locorum and De accessione et recessione maris. In De impressionibus elementorum, Grosseteste mentions that there is greater heat in valleys than on mountain tops. This same observation is used for slightly different reasons in De accessione et recessione maris and De calore solis. In De natura locorum, however, where Grosseteste is concerned to prove the rules that the shorter and straighter the line and the shorter the pyramids the paths of rays take, the stronger is their action, he says that essentialiter mountains are hotter than valleys, but accidentaliter coldness sometimes dominates mountain tops because of the higher-blowing winds or because some mountains reach up into the middle interstice of the air, which is very cold. By every other test, De calore solis seems to be later than De natura locorum, while De impressionibus elementorum and De accessione et recessione maris are certainly earlier than either. Why then did Grosseteste first alter an opinion for geometric reasons, and then return to his original position without explaining away what he had said in De natura locorum?

2. De accessione et recessione maris14 In his study of the tides, entitled De accessione et recessione maris, or De

fluxu et refluxu maris, Grosseteste does not use the structural scheme of resolutio and compositio. He begins by showing the causa materialis communis and the causa materialis propria of motion among us. First communis: The spheres of the four elements are so arranged that earth is in the center and is not further condensible; fire is on the outside and is not further capable of rarefaction; but water and air, between these two, are particularly suited to be moved because they can be condensed and rarefied. Then propria: In air and water, since they themselves are moved greatly by condensation and rarefaction, is every generable and corruptible motion.

He then states that he will omit speaking of the motion of air and speak rather of the motion of water. This discussion is divided into three major

14MSS. Assisi, Comun. 138, fols. 261D- 262B; Florence, Marucell. c. 163, fols. 18A-19C; Prague, Nat. Mus. XII E 5, fols. 410-42A; Vatican, Barb. lat. 165, fols. 402D-403B; cf. Thomson, op. cit., p. 89; and F. Pelster, "Zwei unbekannte philosophische Tractate des Robert Grosseteste," Scholastik, 1926, 1: 572-573; cites Alpetragius. This work has been edited by Ezio Franceschini-"Un inedito di Roberto Grossatesta: la 'Quaestio de accessu et recessu maris,' Rivista di Filosofia Neo-Scholastica, 1952, 44: 11-21-but Prof. Franceschini was not able to use the excellent Prague MS. My text differs slightly from his, but not enough to make any significant difference in the following summary. Grosseteste's authorship has been denied by F. M. Henquinet, Archivum Franciscanum Historicum, 1932, 25: 553 and

D. A. Callus, ed., Robert Grosseteste: Scholar and Bishop, p. 22, because of the ascription in the Assisi MS to "magistro A. Oxon. in scolis suis determinata" (corrected from Pelster's original reading (art. cit.) "magistro R. Exon

."): they attribute the work to Adam Marsh. However, the great similarity of this work to Grosseteste's other works (the "Light Metaphysics," the observation that it is colder on mountaintops than in valleys, the assertion that heat is produced by scattering [disgre- gacio] and that light rays are incorporated in a dense medium, and the uncertainty about how the moon could act on the sea when it was on the other side of the earth) makes it almost certain that this is indeed Grosseteste's work. The Florence and Prague MSS both ascribe the work to "Lynconiensis."

387


parts: (1) the material and efficient causes of the motion of the sea; (2) the causes of the increase and decrease of the rise of the sea; (3) bodies of fresh water which do not have tides, which do have tides but do not seem to, and which have tides and seem to.

In section (1) he begins by discussing the efficient cause. It is, he says, a power of the sky or a power of a star in the sky, since an element is not moved by itself or by another element, but rather by a material form. He then cites Alpetragius' explanation of the tides:15 By a power of the furthermost sky, which is above the sphere of the fixed stars, all lower spheres are moved from east to west as far as the sphere of water. But each sphere receives as much less of the ultimate power as it is lower, since this power is the power of a body, and undergoes diminution at a distance. The earth, however, because of its maximum distance from the farthest sphere, remains completely im- mobile. Therefore the water of the sea is moved by a power of the furthest sky from east to west, and from this he says a crushing together of the waters occurs, and a rise. But it approaches its former place because of its heaviness and when the reversion is completed it again begins to be moved and to rise as far as its heaviness allows. And then consequently it reflows. And these two rises and falls take a time more than one day and its night.

Grosseteste refutes this explanation by showing that the observed times of the tides would not result from it nor would the sea behave as it does.

In addition to this refutation, however, some positive assertions result from the criticism of Alpetragius. In the first place, says Grosseteste, we perceive by experience that the rise and fall of the sea is from a certain rarefaction and condensation of it. This is evident from the fact that ships in the sea are more elevated in the time of rise than of fall,16 since the fall comes about because of a subtlety of the parts and the rise because of condensation. In the second place, experience also tells us that in the time of rise, the water is found to be hotter than in the time of fall, again because of the lesser subtlety of the parts. And in the third place, no planet or fixed star could cause the tides, because the motion of the sea follows the motion of no heavenly body more than that of the moon, as will be proved below.

The rules of the astronomers, he continues, lead to the same conclusion: there are two great luminaries, the sun and the moon, which are the first principles of all generation and corruption. The sun principally affects changes in the air, but the moon, being wet and cold, principally affects the water. It is thus sufficiently evident, he says, that the moon is the efficient cause of the tides. We must now investigate in what manner it is the cause.

When the moon rises on the horizon of any sea, it first sends forth its luminous rays into the midst of the sea and, strongly impressing its power, it moves this sea and increases it. This motion is increased until the moon arrives at the meridional circle. However, when it crosses over the meridional circle, the

15Kitab al-hai'a, translated into Latin by geles, 1956), pp. 165-166. Michael Scot in 1217; cf. G. Sarton, Introduc- 16 If this means what it seems to-that ships tion to the History of Science (Baltimore, draw less water at high tide than at low tide 1931) II, pt. i, pp. 400-401 and F. J. Carmody, -it (and the following assertion that water is Arabic Astronomical and Astrological Science hotter at high tide) is an example of flagrantly in Latin Translation (Berkeley and Los An- inaccurate observation.




effective power is lessened and the sea recedes to its former place until the moon arrives at the east. When again it passes over from the east to the middle of the sky under the earth, the sea is increased; and when it passes from the middle of the sky under the earth the sea diminishes until the moon again arrives at its rise. And thus in one revolution of the moon from rise to rise there are accomplished two rises of the sea in the same place above whose horizon is the rise of the moon; wherefore the rises and falls are thus divided into four quarters.

At this point, Grosseteste is somewhat dissatisfied with his own explanation. It is clear enough, he says, how the moon acts on the sea while it is in the sky above the earth, for while it is rising it impresses its power more strongly than when it is setting. But when the moon exists in the two quarters which are under the horizon of the sea, it is not present nor is it illuminating that sea. And since celestial bodies do not act on lower bodies except by their light, it is doubtful how the moon can be the principle of the motion of the sea. To this, he notes, the astronomers answer that opposite quarters in the sky have similar effects. But one must ask whether this holds true, and the matter is in need of further speculation.

Despite his mild dissatisfaction with his scheme, Grosseteste proceeds to correlate the observed times of the tides with the position of the moon and its relation to the sun with the results that should follow from his explanation. Just as the time of the revolution of the moon from rise to rise exceeds the time of a day and night, he says, so the time of two diverse and complete rises and falls of the sea exceeds the time of a day and night. "I seek to know, therefore," Grosseteste says, "by how many hours the rising of the moon precedes the rising of the sun." It is known by how many hours the rising of the sea follows or precedes the beginning of the day, and that one lunation contains twenty-nine days and some few minutes. Therefore, in seven and one-quarter days and a few minutes, the moon will be a quarter of the circle toward the horizon away from the sun; whence in a third of this time, when the sun is in ortu, the moon will be in the middle of the sky under the earth, and than will be the beginning of a revolution, since in the beginning of this time will have been the start of the sea's rise.

He then deduces the consequences of other relative positions of the sun and moon, finds that they are verified by "what sailors say," and closes his discussion of the efficient cause of the sea's motion.

And so, the consequences of this explanation having been drawn and verified by observation, it now remains to discuss in detail the causa materialis propria. It is the property of waters to be congregated in a deep and broad place; in such waters is much matter of vapors and winds. Whence the moon, rising and impressing her power, generates in these waters many vapors and stirs up the winds. But that water, being incapable of division or expurgation because of the grossness and viscosity of its parts, passes through itself and excites itself because of the enclosed vapors.

In order to make clear his meaning here, Grosseteste discusses how this contrasts with the behavior of sweet water. Sweet water, he says, being subtle and penetrable, allows the winds and vapors to be withdrawn and so does not

389


undergo an augmentation. He goes on to assert that a celestial body acts on lower bodies only through its luminous rays, and that these luminous rays are incorporated in some way with the elements and by reflecting they intersect themselves at one point and thus by scattering the parts of matter they generate heat. The more subtle the matter, the less heat will be generated. This is apparently the first time in his scientific works that Grosseteste has used the notions of incorporation and scattering and the variation of heat with the density of matter. This same explanation, refined by the use of the mathematical principles developed in De lineis and De natura locorum, is incorporated into De calore solis. To verify his assertion that the more subtle the matter the less the heat, Grosseteste again mentions that it is colder on mountain tops than in valleys because of the greater subtlety of the air on mountain tops.

With this, he concludes his discussion of the material cause of the tides and turns his attention to the second major division of his treatise, namely why at some times the rise of the sea is increased and becomes stronger, and at other times is diminished and becomes weak. This section consists simply of a list of eight reasons. It is much inferior to the first part both in method and in sharpness of thought. A rather brief summary of its contents will suffice.

One, in the conjunction of the moon with the sun the power of the moon is increased because of the increased light in it, and the rise of the sea becomes great; and in the recession of the moon from the sun its power is lessened. Two concerns diverse comparisons between diverse motions of the moon and the mean motion of the moon. Three, the rise of the moon toward a longitude farther from earth lessens its power on account of the increased distance; its approach to a longitude nearer the earth has the opposite effect. Four and five also have to do with the moon's position. Six is a very vague and confused reference to "days which by the ancients were called 'Egyptian'."'7 Seven again involves the help of the sun: from the spring equinox to the summer solstice it increases the rise; from then until the autumnal equinox it is diminishing; and from then to the winter solstice it is increasing again; and from then to the spring equinox it is diminishing. The eighth and final reason is that the wind, when it is blowing in the direction of the rise, increases it, but when blowing in the other direction lessens it.

The third section may similarly be disposed of briefly. Certain rivers and springs undergo a rise and fall not because of their own nature, but because they are continuous with marine water either on the surface of the earth or by subterranean channels. There are three kinds of these: those which do not have tides; those which have tides but do not seem to; and those which have tides and seem to. The first kind is accounted for by a defect of the material or efficient causes discussed above; the second by the extenuating circumstances of the great distance from the motion of the moon or the great subtlety of the water; and the third by the expulsion of vapors from underground caverns, into which flow the waters made subtle by the moon.

17 "Sexta causa sunt dies qui ab antiquis Assisi, Comun. 138, fol. 262A; Florence, Ma- vocantur Egyptii quare et ipsi primo invenirent rucel. c. 163, fol. 19A; Prague, Nat. Mus. XII, eos, quam causam obmittamus ad presens eo E 5, fol. 42A. quod multum latent effectus in eis." MSS.




This work should apparently be dated'8 after De impressionibus elementorum because in it, for the first time, Grosseteste uses the concepts of incorporation of rays in matter and the scattering of parts of matter producing heat when rays intersect in a point. But that it is definitely earlier than De natura locorum (1230-31) is evident in two places.

First, in De fluxu et refluxu maris Grosseteste assumes that the moon's rays increase in strength as the moon is rising and decrease as it is declining, but he sees no reason to explain why this is so. In De natura locorum, however, he presents a more detailed presentation of this part of his explanation of the tides, giving mathematical reasons for the phenomenon:

Because the lunar rays rising above the sea of any region have longer lines and pyramids and are less straight and fall less at equal angles and are less reflected on themselves and are more broken, therefore they are weaker than when the moon rises toward the center of the sky. For then all the rays have shorter lines and straighter pyramids and fall more at equal angles and are more perpendicular and more returning on themselves and less broken... and therefore they are of stronger operation. Therefore when the moon rises, its rays because of their weakness are only able to loosen the vapors from the depth of the sea and they are not able to consume them or to draw them out completely to the air.... But when the moon rises to the middle of the sky, on account of the power of the rays it is able both to consume these resolute vapors and elevate them to the air. And when it arrives at the meridian, then it completely consumes and extracts them, and because when the cause ceases the effect also ceases, therefore the waters of the sea then naturally run again into their own place lest a vacuum be made.19

A similar explanation is utilized concerning the sun's rays in De calore solis. Second, Grosseteste's puzzlement in De fluxu et refluxu maris as to why

opposite quarters of the world should have similar effects is solved by him in De natura locorum:

And therefore the reflection of rays solves this, since the lunar rays are multi- plied toward the sky of the stars, which is a dense body. And therefore, through its medium we are not able to see the heaven, which is scarcely luminous, just as Alpetragius and Messalahe say. And other reflected rays fall on the opposite quarter at equal angles.20

3. De cometis et causis ipsarum21 Grosseteste's work on comets was probably written after De fluxu maris

because of its use of the concept of the incorporation of rays in a dense medium and the more extensive knowledge of optics shown in the beginning of the

18 There is other evidence for the date of 19 Baur, op. cit., pp. 69-70. this work. The earliest MS is dated by Thom- 20 Ibid., p. 70. son ca. 1225 (Assisi, Comun. 138). But we 21 S. H. Thomson, "The Text of Grosse- must allow at least ten years leeway in dating teste's De Cometis," Isis, 1933, 19: 19-25; ad- on paleographical grounds along. The ascrip- denda S. H. Thomson, "Grosseteste's Questio tion "De fluxu et refluxu maris a magistro A. de Calore, De Cometis and De Operacionibus (?) Oxon. in scolis suis determinata," if it is Solis," Medievalia et Humanistica, 1957, 11: intended to refer to Grosseteste, would estab- 36-37; cf. Thomson, Writings of Robert Gros- lish its date as before 1229; cf. also Pelster, seteste, p. 110; English paraphrase in Crombie, art. crit., p. 573 and Franceschini, art. cit., op. cit., pp. 88-90; cites no author or work by p. 12. name.

391


RICHARD C. DALES

work. But, on the other hand, the strongly qualitative nature of Grosseteste's study and its extensive use of the principle of sublimation would seem to place it in the middle period, before his predilection for mathematics transformed the nature of his scientific works (as in De colore, De calore solis and De iride). We may therefore date De cometis with some confidence between 1228 and 1230. It is consequently the last work of the middle period.

De cometis begins with a refutation of four current theories about the nature of comets and provides us with an excellent example of Grosseteste's method of falsification. He feels that these false theories result from loose analogies with things known and from haphazard experiments not systematically employed, made by people who do not adequately consider the laws of the special sciences.

The first theory which he tests is that the tail which a comet draws behind it is a radiosity of the sun reflected by a star. This is a conclusion jumped to by people who have noted that the visible rays of the sun are reflected by a mirror with visible radiation and that the stars are mirrors reflecting rays which fall upon themselves; but it can be refuted in two ways. In the first place, a radiosity is not reflected visibly except when the reflected rays are mixed with a transparent medium having a terrestrial rather than a celestial nature. And secondly, the tail of a comet is not always extended opposite the Sun as it should be, since all reflected rays go in opposite directions at equal angles.

A second theory is held by certain people who, knowing that from a concurrence of rays an inflammable object is ignited, think that many rays come together in the highest air by which combustible fumes are elevated, and there by means of a concurrence of rays the fumes are set on fire; and this very inflammation appears to be the tailed star. This theory is also refuted in two ways. First, if there were a concourse of rays descending straight from the planets, it could not burn for very long since the planets change their positions swiftly; but comets are seen to have lasted for six months. (On the other hand, if the concourse of rays descended straight from the stars it would be stable and permanent.) Second, if this were a concourse of ascending reflected rays such as from a concave mirror or a congregation of rays such as would be brought about by their passing through a transparent sphere, then the occur- rence here of a concourse of rays would be a natural sublunary body the duration of which could not be long, nor would its motion necessarily follow the motion of the heavens. But, as mentioned above, the duration of comets is sometimes very long, and their motion does follow the daily motion of the heavens.

There is a third theory held by those who, knowing that many things close to each other appear from a distance to be continuous, and knowing that a galaxy is a congregation of nearby stars "run together" in the judgment of the sight, think that a comet is an aggregation of nearby stars and that a comet appears when a number of erratic stars run together. This can easily be refuted by noting that comets do not always appear in the paths of erratic stars, but more often outside them.

The fourth and final theory to be refuted here is that held by certain people who, knowing that it is possible that one thing might appear to be of some

392



other sort of figure because of the characteristics of a transparent object placed between the observer and the thing seen, think that a comet is an apparent star. This can not be true because the figure of the comet which is seen and the figure of the object which would appear because of the interposed vapor are not proportionately large. This theory would also run counter to what has already been said, namely that the elevated vapor does not retain one shape for a very long time, nor does it necessarily follow the motion of the heavens.

After disposing of these theories, Grosseteste undertakes the resolutio of the phenomenon into its principles. Since in the superlunary region nothing is renewed except position and those things which result from a change of position - such as eclipses and visible projections of rays and the waxing and waning of the moon - it is evident that a comet is not a new star; therefore a comet is sublunary. Because of the light and gleaming of the comet and its tail, it is clear that a comet is nothing other than fire; because under the moon and above us, nothing gives off light and glistens except fire. But fire is of two kinds: that which is lasting and does not cease as soon as it is generated (for example, the element fire in its own sphere); and an ebullition of ignited fumes ceasing at the same time it is generated (for example, flame originating among us). The matter of a comet can not possibly be the second kind: first because it would not have matter continually helping it in its long duration; and second because matter does not follow the motion of the heaven for a long time, since its nature is of the earth, not completely sublimated. For we do not see any fire at all of terrestrial matter generated in the air to be of a very long duration nor following the motion of the heaven. However, it is not possible for the sphere of the element fire to descend into the region of the air, but only its power will descend with the rays of the stars in the sphere of its burning, nor will the sphere be touched by fire generated from matter coming downward, but from matter coming upward, because it is not possible that there be generated fire having a permanent duration except when it were sublimated matter separated from terrestrial matter and assimilated to the nature of heavenly matter. This concludes the resolutio. Grosseteste then presents his explanation of comets.

Therefore, it is clear, he says, that a comet is sublimated fire separated from terrestrial nature and assimilated to celestial nature. Since the agent and patient are assimilated by a completed action when before they were dissimilar, the natural cause of a comet is necessarily a power of the sky, sc. a fixed or erratic star. Similarly, for each and every comet there is an effective cause, a special star from whose direction it appears to the senses to be moved. How- ever, since comets are moved by the daily motion of the heavens, it is evident that the power of the first heaven is in itself the moving cause.

The only reason the location of a comet is from the direction of one star more than from another is because of the greater assimilation of that star from whose direction it is located, when on account of the similitude it has with that star whose power has sublimated it, it is drawn by that star just as iron is by a magnet.

In every thing connected with earth, there are spiritual bodily things as-

393


similated to celestial natures, incorporated by the connected things themselves; these spiritual things are separable from things connected through the action of celestial bodies. When this separation occurs, the connected thing is re- leased by the celestial nature and is weakened or corrupted; and the sensible parts of this world are easier and swifter of resolution than would be the parts of connected things.

"Therefore from these things," he concludes, "it appears that a comet, which is fire sublimated by a part of the sensible world, is a sign of a preceding sublimation of a separation of an uncorrupted spiritual nature by things connected and assimilated to earth in a spiritual nature; thus it is a sign of weakness or corruption of connected things which dominate the planet to whose nature the seen comet is assimilated."

Grosseteste then illustrates this difficult sentence by an example: "If there were a star drawing a comet of the nature of the sun, and the power of this star sublimated the comet and separated it from its terrestrial nature, it would separate the spirits which are in complex bodies from the assimilated nature of the sun, and there will be an infirmity or corruption in men, animals and plants over which the sun principally rules."

The remainder of the work concerns the way in which planets affect men and earthly things and permit some men to "sense complete what is yet inchoate."

C. The Late Period It is apparent from what has already been said that Robert Grosseteste had

long been interested in mathematics and optics. Probably about the year 1231, however, he completed a very important work on these two subjects and from this year on, he displays in his physical works a strong mathematical emphasis along with a greater knowledge and more careful use of mathematics and optics in the investigation of natural phenomena. Many points established in his earlier works are incorporated in more detailed and comprehensive studies; some notions he had employed before are slightly modified and sharpened; and others he discards without comment as evidently mistaken. In this period he perfects his use of the method of resolutio and compositio and for the first time employs the principle of the subordination of sciences. The works from this period give evidence of their author's mastery of his techniques and con- fident (if sometimes mistaken) use of his basic concepts and factual data. They are therefore more difficult to date in relation to each other than the earlier works, where a progression was observable, but that they all belong to the period 1231-1235 seems certain.

De lineis, angulis et figuris22 stresses the importance of mathematics in understanding the world of nature and establishes certain rules which may be employed in physical inquiries: the shorter and straighter the line, the greater the power; when lines fall on curved or straight surfaces, the smaller the incident angle, the greater the power; there is greater action from a concave surface than from a plane or convex surface; every agent multiplies its power spherically; and it discusses the laws of reflection and refraction by bodies of

22 Pub. Baur, op. cit., pp. 59-65.




different densities and the properties of the pyramid and cone. De natura locorum,28 evidently a continuation of De lineis, in addition to expanding the optical discussions of De lineis, applies some of these to the natural world in order to illustrate points and to illustrate the limitations of a purely mathematical approach to nature. Thus Grosseteste mentions that according to the rules about short straight lines and short pyramids, mountains should be hotter than valleys, and indeed they are, essencialiter; but because of high-blowing winds or the coldness of the second interstice of the air they are accidentaliter colder. Then, after explaining why the region of the equator is perfectly temperate rather than burning, as the mathematical rules would indicate, he develops an explanation of the sun's heat which he later incorporates in De calore solis.

.. It is impossible that upon the regions of the world outside the tropics a ray of the sun (or of any other planet) should fall directly, neither at equal angles, nor perpendicularly, nor reflected back on itself, nor not broken; but upon the regions between the tropics, the rays of the planets fall directly and at equal angles and perpendicularly and they are reflected on themselves and they fall not broken, since bodies of the world are spherical and concentric. Wherefore only those rays which fall between the center of the world and the center of that body, if they were concentric, would be direct and at equal angles and perpendicularly returning on themselves and not broken. But no ray coming outside the tropics falls on the center of the world, but more toward the circumferences of mundane bodies. Nevertheless, between the tropics they are well able to fall on the center of the world.24

Then follows the explanation of how the moon acts on the sea, which has already been quoted in connection with De fluxu et refluxu marns.

This is perhaps the place to make some observations on the date of Grosse- teste's commentary on the Posterior Analytics. As Dr. Crombie has said, "The problem of the date of the commentary will be solved only when a thorough examination has been made of all the manuscript evidence."25 Since this commentary is among the most important of Grosseteste's works, and since much of it is so intimately bound up with his scientific work, determination of its composition date is of paramount importance. If the commentary was indeed written in 1220, as Dr. Crombie has tentatively proposed, then the assumptions underlying this article are false, since Grosseteste shows in his commentary much knowledge, especially of optics, which I have assumed he only acquired during the second half of the 1220's. Without pretending to solve the problem here, I should like to put forth several reasons for moving the commentary's probable date of composition from 1220 to ca. 1228.

28 Pub. ibid., pp. 65 if. make the date of the commentary itself after 24 Ibid., p. 67. 1217-20; a supposed reference to the Comm. 25 Crombie, op. cit., p. 47, n. 1. Dr. Crombie Post. Anal. in the commentary on the Physics

bases his dating (pp. 4647) on: 1. Nicolas (undoubtedly a gloss); and the frequent cita- Trivet's statement that Grosseteste "wrote tions of Euclid and absence of references to compendiously" on the Post. Anal. while he Averroes. This definitely places the work be- was still a Master of Arts, i.e. before 1209 tween 1217-1231, but I do not see that Dr. (cf. D. A. Callus, "The Oxford Career of Crombie has presented any compelling reasons Robert Grosseteste," Oxoniensia, 1945, 10: for preferring the beginning of this period to 45); 2. the use of Michael Scots translation the end. of Aristotle's De animalibus, which would

395


In commenting on the text: Horum autem quodam genere sunt eadem quecunque habent differeintias ex quibus aliorum aut aliter sunt, ut propter quod eccho, aut propter quod apparet, et propter quod iris. Omnia enim hec idem propositum sunt genere. Omnia enim repercussio sunt, sed specie altera, Grosseteste attempts to illustrate how these are all forms of repercussion. (Note that it is Aristotle, not Grosseteste, who makes this suggestion.) He had already written a work on sound (ca. 1221-1222), so he incorporated this almost verbatim into his commentary. His knowledge of the rainbow was not nearly so precise, so he simply says that a rainbow is a "repercussio vel fractio radiorum solis in nube concava aquosa."26 An image in a mirror "is a repercussion of the visual ray from the surface of a mirror." But in amplifying this last remark, Grosseteste shows that he is already aware of refraction: "Si obstaculum sit corpus perspicuum, [radius] generat se revertendo, et etiam non directe, sed angulariter perspicuum penetrando; sicut radius solis cadens super aquam perspicuam revertitur a superficie aque sicut a speculo et penetrat aquam faciendo angulum in ipsa superficie aque, et hoc proprie vocatur fractio radii."27 It is inconceivable that this was written before De impressionibus elementorum, which shows no knowledge of refraction although such knowledge would have been very useful in the inquiry.

Immediately below, commenting on "... ut propter quod nilus finiente mense magis fluit...", he gives an account of the monthly rising of the Nile28 completely consistent with the final section of De fluxu et refluxu maris. Crombie has rightly pointed out29 that this is a more primitive theory than that put forth in De natura locorum, but so indeed is all of De fluxu et refluxu maris.

There is still another troublesome question. If Grosseteste had fully eluci- dated his principle of subordination of sciences in 1220, why did he neither use it nor mention it again until nearly a decade later (in Book II of his commentary on the Physics, ca. 1229; in De lineis, angulis et figuris, ca. 1230; and De iride, between 1232 and 1235)?

We may approximate the date of this commentary by noting that it was probably done after De generacione sonorum and De impressionibus elementorum, certainly before De natura locorum, and about the same time as or shortly after De fluxu et refluxu maris. But much basic work on the manuscripts remains to be done.

1. De colore80

There is considerable disagreement and confusion concerning the date of De colore. S. H. Thomson has noted that this work was used by Bartholomeus Anglicus in his De proprietatibus rerum (Bk. XIX, ch. 7), begun in Paris after 1230. Then he says that "this work of Grosseteste falls in the group on which his early fame was founded. A date shortly before 1220 seems likely."32 A. C. Crombie, however, feels that it is the last in a series of optical works and

26Roberti Lincolniensis Commentaria in 29 Crombie, op. cit., p. 112, n. 6. Libros Posteriorfum Aristotelis, cum textt 30 Pub. Baur, op. cit., pp. 78-79; cf. S. H. seriatim inserto (Venetiis, 1494), f. 29D. Thomson, Writings, pp. 93-94; cites Aristotle

27 Loc. cit. and possibly Averroes. 28 Ibid., ff. 29D-30A. 31 Thomson, op. cit., p. 93.




assigns it to "the last period of Grosseteste's scholastic career at Oxford." His reasons are: that it contains a reference to Averroes and so must be after 1230; and that "it resumes a discussion of the nature of colors which forms the last part of De iride ... [and] may have been a continuation of this work."32 Although Dr. Crombie is apparently correct in putting this work at the end of Grosseteste's scholastic career at Oxford, his reasons for so doing and his assertion that De colore is a continuation of De iride must be questioned.

First, it is not even certain from the manuscripts that Grosseteste did cite Averroes. In Baur's printed edition, the reading "sermo Aristotelis et Aver- rois, qui ponunt" is accepted in the text.33 Of the nine manuscripts of De colore listed by Thomson,34 Baur used six in preparing his edition. Among these six, V reads "auctor et Aristotelis qui ponunt;" Q, which was written in the mid-thirteenth century, has Aveur where Baur reads Averrois, as has another thirteenth century manuscript, Madrid, B. N., 3314, fol. 91A, which Baur did not use. The other two, manuscripts which Baur did not use, Florence, Marucelliana, C. 163, fols. 5D-6A and Prague, Nat. Mus., XII E 5, ff. 42B-, both read "Augustinus et Aristoteles." So although "Averroes" may be a possible, or even probable, reading, it is by no means certain. In any case, the sentence containing what may be the citation of Averroes has the appearance of a gloss. It has nothing to do with the development of the essay and is really an extra- neous comment (see below in summary of De colore, n. 35). In this case, the reading of manuscript V, "auctor et Aristoteles" would be acceptable.

It remains then to investigate whether De colore "resumes" the last paragraph of De iride, as Crombie says on page 51, or whether it is presupposed by De iride, as he seems to imply on page 126 ("The colors of the rainbow [in De iride] Grosseteste tried to explain by his general theory of color."). This problem does not admit of definite solution, but it seems quite clear that Grosseteste had already worked out his theory of color very carefully before writing the concluding paragraph of De iride and that he assumes the con- clusions of De colore in his attempt to account for the colors of the rainbow. We have noted before how general notions entertained by Grosseteste under- went a clarification and amplification in the course of his scientific works. However, the theory of color used in De iride is as precise as that in De colore.

In a beautifully concise resolutio, Grosseteste isolates the principles of the complex phenomenon, color. Color, he says, is light incorporated by a transparent medium. Transparent media are either pure or impure. Light can be bright, dim, much or little.

Then by combining the elements discovered through this analysis, he asserts that white is much, bright light in a pure medium; and black is little, dim light in an impure medium.35 There are seven colors descending from white. For since three things constitute the essence of white, sc. a multitude of light, the brightness of the same, and the purity of the transparent medium, a remission of any one of these three can be made by the other two, and there

32 Crombie, op. cit., p. 51. explanatus est sermo Aristotelis et Averroes, 33 Baur, op. cit., p. 78. qui ponunt nigredinem privationem et albedi- 34 Writings, pp. 93-94. nem habitum sive formam." Baur, op. cit., 35 It is at this point that the possible cita- p. 78.

tion of Averroes occurs: "Et in hoc sermone

397


will be in this manner a generation of three colors; or by the remaining one of the three there will be a remission of the two which were relinquished, and thus a triple generation will be made of other colors by the three prior ones; or there will be a remission of all three at once; and thus there will be an immediate progression of seven colors from white. Similarly, there are seven colors ascending from black. These two series meet in the middle. So there are in all sixteen colors: Black, the seven colors ascending from it, white, and the seven colors descending from it. However, the degrees of extension and remission possible in the middle colors (all but black and white) will be in- finite. "That the essence of color and a multitude of the same behaves in the aforesaid manner," he concludes, "is manifest not only by reason but also by experiment, to those who know the principles of optics deeply and surely.... They can produce all the modes of colors which they wish visibly, by art."

2. De calore solis36 De calore solis is one of the most beautifully executed of Grosseteste's sci-

entific works. That it belongs to the late period is sufficiently evident from its advances over De impressionibus elementorum, De fluxu et refiuxu maris, and De cometis, and the influence of De lineis and De natura locorum; but just where in this period it should be placed is difficult to say.

The question being investigated is "In what way does the sun generate heat?" The resolutio begins by asserting that there are three principles of the generation of heat, a hot body, motion, and a collection of rays. In all of these, the proximate cause of heat is "scattering." How a hot body produces heat by scattering is evident, but how local motion and a collection of rays do so is difficult to see. In local motion, violent motion by conflicting with natural motion produces a scattering of parts and thus heat. And even in natural motion some heat is generated because there is a degree of violent motion in every part of a naturally moving object except those parts whose motion is along a line with the center of the earth. Scattering takes place through a concentration of rays because these rays are incorporated in a transparent medium; in a dense medium the incorporation is greater and in a subtle medium it is less. Hence the rays draw with them parts of the air in which they are incorporated and when they are collected in one point a great scattering and consequently great heat result.

So if the sun generates heat, it will do so as a hot body, as local motion, or as a collection of rays. It does not generate heat as a hot body does, for it is not in immediate contact with the heated thing. Neither does it generate heat by its motion, because its motion is circular and circular motion does not produce heat. Therefore, the sun must generate heat by a concentration of rays. The problem has now been reduced to a matter which can be investigated both mathematically and experimentally, and Grosseteste proceeds with the compositio.

The sun's rays, he says, are to some extent incorporated in the transparent 36 Ibid., pp. 79-84; Eng. translation by A. C. Aristotle (Physica, vii and De caelo et mundo,

Crombie in Callus, op. cit., pp. 116-120, after ii) and Euclid (Catoptrica). Baur's text; cf. Thomson, op. cit., p. 93; cites




medium of the air, a dense body. When these rays fall on the earth's surface, they are reflected at equal angles; so if they fall perpendicularly (as they will between the tropics of Cancer and Capricorn when the sun is at the zenith), the incident and reflected rays go along the same line in opposite directions, and there is a maximum of scattering and thus of heat. A similarly violent scattering and great heat can be produced by the concentration of rays refracted through a spherical body or reflected from a concave mirror.

North and south of the tropics, however, the sun's rays must always fall at less than right angles, so the paths of incidence and reflection will not be the same. It follows from this hypothesis that the farther a place is from the equator, the more obtuse will be the angle at which the sun's rays fall and are reflected, and so the scattering and heat will be proportionately less. This accords with observation.

There are, however, variations in temperature which cannot be accounted for solely by the hypothesis suggested and verified above. Consequently, Grosseteste turns his attention from the rays themselves to the medium in which they are partially incorporated, i.e., the air. He proposes that the density of the transparent medium and the corresponding degree of incorporation of the sun's rays are also directly proportional to the amount of heat generated. Then once again he deduces the consequences of his hypothesis and tests them either by reason or by experience.

In the fifth element, he says, even if the sun's rays did intersect (and they do not), no heat would be generated because there is no dense nature; hence there is no incorporation and no scattering is possible. In the upper layer of air-on mountain tops, for instance-where the air is thinnest and the degree of incorporation slight, the least amount of heat is generated, as observation shows. But in a valley, where the air is more dense, there is a greater incorporation of rays and therefore more scattering and more heat.

3. De iride37

Perhaps the last of Grosseteste's scientific works is De iride. It alone among his physical investigations explicitly asserts and clearly employs the principle of the subordination of sciences. It also assumes and uses the laws of compound refraction developed in De lineis.

In the lengthy and carefully-reasoned resolutio, which occupies over half of the entire work, he introduces his principle of subordination by noting that speculation concerning the rainbow is the province of both optics and physics; the physicist is concerned with quid, the experienced fact, and the student of optics with propter quid, or the reason for the fact.

The science of optics may be divided into three parts, according to the method of transition of the visual ray to the seen thing. In the first part, called de visu, the transit of the visual ray is straight, through the medium of

37 Baur, op. cit., pp. 72-78; partial transla- and De generatione aninmalium) and Euclid tion and paraphrase in A. C. Crombie, Robert (Catoptrica); may have used Ptolemy's Optica Grosseteste and the Origins of Experimental and Averroes' commentary on the Meteoro- Science, pp. 110-112; 117-119; cf. Thomson, logica (cf. Crombie, op. cit., pp. 116-117). op. cit., p. 105; cites Aristotle (Meteorologica

399


a transparent body placed between the viewer and the seen thing. In the second, called de speculis, its transit is along a line straight toward a body which acts as a mirror and reflects the ray toward the seen thing. In the third part, "which," he remarks, "has remained unknown and untouched among us to the present time," the transit of the ray is through many transparent bodies of diverse kinds in which the visual ray is bent at their points of contact and makes an angle. This is clear from the following experiment described in de speculis:38 if something is placed in a vessel, and one then steps back to a point from which the thing can not be seen, and then water is poured in, the thing which was put into the vessel will be seen. This third part, perfectly known, shows us how to make distant things appear near, large things appear small, etc. "It is perfectly evident in geometrical demonstrations how by means of a transparent medium of known size and shape placed at a known distance from the eye, a thing of known distance and size will appear according to place, size, and position."

It must now be decided to which of these three parts of optics the science of the rainbow belongs. A rainbow could not be made by solar rays by a direct approach from the sun falling on a concave cloud, for they would not form a bow. Nor is it possible that a rainbow be made by the reflection of the sun's rays upon the convexity of the mist descending from the cloud as upon a convex mirror in such a way that the concavity of the cloud should receive the reflected rays and thus a rainbow appear. If this were so, the rainbow would not always be in the shape of a bow, and when the sun were high the rainbow would appear high and large and when the sun were low the bow would be less. Just the contrary of this is evident to the senses.

Therefore it is to the third part of optics that the science of the rainbow belongs, and the task now remaining is to deduce a rainbow from the principles established above. Having proved that a rainbow could not arise from the sun's rays falling directly on a concave cloud or by the simple reflection of these rays, Grosseteste sees as the only remaining possibility that the rainbow must be made by the compound refraction of the sun's rays in the mist of a convex cloud.

The exterior of a cloud, he says, is convex, its interior concave, and that part of a cloud which we can see must be less than a hemisphere. When moisture descends from the concavity of a cloud, the moisture must be pyramid- ally convex at the top, descending to the ground, and therefore more condensed near the earth than in the upper part. There are consequently four different transparent media through which the rays of the sun penetrate: the pure air containing the cloud; the cloud itself: the higher and rarer part of the moisture descending from the cloud; and finally the lower and denser part of this moisture.

From this hypothetical figure, Grosseteste attempts to deduce geometrically the phenomenon of the rainbow. The rays of the sun are refracted at the point of contact of air and cloud, then of cloud and moisture. Because of these refractions the rays run together in the density of the moisture and are refracted there again and spread out into a figure like the curved surface of

38 Euclid, Catoptrica, Post. 7.




a cone expanded in the direction opposite the sun. Therefore its shape is an arc and no39 rainbow is seen in the south. So far, deduction (although faulty) matches observation.

Because the apex of the above-mentioned figure is near the earth and its expansion opposite the sun, half or more of the figure must fall on the earth, the remainder on a cloud opposite the sun. Therefore, when the sun is near rising or setting the rainbow appears semicircular and is greater, and other- wise varies inversely with the elevation of the sun. Deduction and observation again match.

Grosseteste then introduces his theory of light and color developed in De colore40 to account for the variety of color in different parts of the same rainbow and from one rainbow to another, and closes his inquiry.

III From these investigations of Grosseteste's scientific works, several conclu-

sions may be drawn. In the first place, he was evidently not a great observer or experimenter. Most of the experiments to which he refers in his works are those he had read about, and if he performed them at all it was only to satisfy his curiosity about them. Others are simply appeals to every-day experience, and some of these are questionable, especially in De fluxu et refluxu maris where he says that at high tide the water is found to be hotter and ships draw less water than at low tide. There are also several instances where it seems that a simple experiment could have definitely established a hypothesis which Grosseteste accepts for inadequate reasons (for example, that the air is the medium through which sound waves are carried to the ears, in De generacione sonorum), or could have prevented his making faulty assumptions in con- structing his hypotheses (as in De iride, where he assumes there will be two refractions in a mist of varying density). In no case does he contrive an experiment for the explicit purpose of testing a hypothesis.

In the second place, it seems doubtful that Grosseteste first worked out his method in his theoretical works and then applied it in special investigations. Rather we have seen a progressive refinement of method from the rather crude De generacione stellarum to the intricate, complex, and beautifully conceived De calore solis and De iride. Indeed, his theoretical works may well have been done concurrently with his scientific investigations. It is fairly certain that his commentary on Aristotle's Physics, in which he states the principles of subordination of sciences and of resolutio and compositio, was done between 1228 and 1232.41 De lineis, angulis et figuris and De natura locorum, which powerfully influenced his later works, were written about 1230-1231. This leaves in doubt only the date of the very important commentary on Aristotle's Posterior Analytics. All in all, there seem to be more serious difficulties in- volved in insisting on a date of around 1220 than in admitting a date of 1228

39 Following Crombie's emendation, op. cit., 41 R. C. Dales, "Robert Grosseteste's Com- p. 126. mentarius in Octo Libros Physicorum Aristo-

40 This, of course, is problematical. For telis," Medievalia et Humanistica, 1957, 11: the entire section cf. Baur. op. cit., p. 79. 12-13.

401


RICHARD C. DALES

or 1229. A modern edition of this work would be a great boon to students of Grosseteste.

Finally, it will not be denied that Robert Grosseteste investigated with great thoroughness various phenomena of the natural world. He employed the double method of resolutio and compositio with considerable skill (although this method was not original with him42). He fruitfully introduced mathematical demonstrations into his researches whenever he saw an opportunity. He constructed imaginative hypotheses to try to account for phenomena. And his use of experiment and experience in analysing a problem into its constitu- ent parts, in finding suggestions for explanatory hypotheses, and in verifying or falsifying these hypotheses, although far from perfect from a modern point of view, nevertheless represents a significant step in the establishment of "modern" scientific method. It is not necessary to exaggerate his achieve- ment or his anticipation of modern developments to secure for Robert Grosse- teste a place of eminence among the shapers of our intellectual tradition.

Chronological Chart of Grosseteste's Scientific Works

Dated Landmarks 1217-20-Michael Scot's translation

of Aristotle's Historia Animalilu (cf. Sarton, Introduction to the His- tory of Science, II, pt. 2, p. 579).

1230-Introduction of Averroes to Latin Europe (cf. R. de Vaux, "La premiere entree d'Averroes chez les Latins," Revue des Sciences Phi- losophique et Theologique, 1933, 22: 193-245).

1235-Grosseteste becomes Bishop of Lincoln.

Suggested Dates for Grosseteste's Works ca. 1220-De generacione stellarum* 1221-22-De generacione sonorum 1224 -De impressionibus elementorum 1226-28-De accessu et recessu maris 1227-29-Commentarius in libros Analyti-

corum Posteriorum Aristotelis

1228-32--(never completed) Commentarius in VIII libros Physicorum Aristotelis*

1231 -De lineis, angulis et figuris* De natura locorum*

1232-35-De colore De calore solis De iride

* Indicates dates are fairly certain; for the rest, the order is probably correct, but the dates are simply approximations.

42 Cf. A. C. Crombie, op. cit., pp. 52 ff; 78 ff.

402


Article Contentsp.381p.382p.383p.384p.385p.386p.387p.388p.389p.390p.391p.392p.393p.394p.395p.396p.397p.398p.399p.400p.401p.402

Issue Table of ContentsIsis, Vol. 52, No. 3 (Sep., 1961), pp. 353-530Front Matter [pp.353-444]Nineteenth-Century State Geological Surveys: Early Government Support of Science [pp.357-371]Controversial Problems concerning the Interpretation of the Physiological Treatises of Papyrus Ebers [pp.372-380]Robert Grosseteste's Scientific Works [pp.381-402]Copernicus' Relation to Aristarchus and Pythagoras [pp.403-409]Interpretation of an Early Newton Manuscript [pp.410-416]Notes & Correspondence [pp.417-419]News of the Profession [pp.419-420]Book Reviewsuntitled [pp.421-423]untitled [pp.423-425]untitled [pp.425-426]untitled [pp.426-427]untitled [pp.427-428]untitled [p.428]untitled [pp.429-430]untitled [pp.430-431]untitled [pp.431-432]untitled [pp.432-433]untitled [pp.433-435]untitled [p.436]untitled [pp.436-437]untitled [pp.437-438]untitled [pp.439-441]

Eighty-Sixth Critical Bibliography of the History of Science and Its Cultural Influences (To 1 January 1961) [pp.445-526]Back Matter [pp.527-530]

Isis Volume 52 issue 3 1961 [doi 10.2307%2F228079] Richard C. Dales -- Robert Grosseteste's Scientific Works.pdf

Documents

scientific methodology

grossetestes methodology

theory of scientific

scientific investi gations

jstor terms

jstor archive

grossetestes trac tates

posterior analytics