Translations of M ATHEMATICAL M ONOGRAPHS ΑΓΕΩΜΕ ΕΙΣΙΤΩ ΤΡΗΤΟΣ ΜΗ F O U N D E D 1 8 8 8 A M E R I C A N M A T H E M A T I C A L S O C I E T Y American Mathematical Society Providence, Rhode Island Volume 15 Intrinsic Geometry of Surfaces A. D. Aleksandrov V. A Zalgaller