170 THE GREEK ASTRONOMY. 



till each was found, by astronomical observation, to be a degree from 

 the place at which the)- started. It then appeared that these terres- 

 trial degrees were respectively 5G miles, and 56 miles and two-thirds, 

 the mile being 4000 cubits. In order to remove all doubt concerning 

 the scale of this measure, we are informed that the cubit is that called 

 the black cubit, which consists of 27 inches, each inch being the thick- 

 ness of six grains of barley. 



Sect. 6. — Ptolemy's Discovery of Evection. 



Br referring, in this place, to the last-mentioned measure of the 

 earth, we include the labors of the Arabian as well as the Alexandrian 

 astronomers, in the period of mere detail, which forms the sequel to 

 the great astronomical revolution of the Hipparchian epoch. And this 

 period of verification is rightly extended to those later times; not 

 merely because astronomers were then still employed in determining 

 the magnitude of the earth, and the amount of other elements of the 

 theory, — for these are some of their employments to the present day, — 

 but because no great intervening discovery marks a new epoch, and 

 begins a new period ; — because no great revolution in the theory added 

 to the objects of investigation, or presented them in a new point of 

 view. Tliis being the case, it will be more instructive for our purpose 

 to consider the general character and broad intellectual features of this 

 period, than to offer a useless catalogue of obscure and worthless wri- 

 ters, and of opinions either borrowed or unsound. But before we do 

 this, there is one writer whom we cannot leave undistinguished in the 

 crowd ; since his name is more celebrated even than that of Hip- 

 parchus; his works contain ninety -nine hundredths of what we know 

 of the Greek astronomy ; and though he was not the author of a new 

 theory, he made some very remarkable steps in the verification, cor- 

 rection, and extension of the theory which he received. I speak of 

 Ptolemy, whose work, " The Mathematical Construction" (of the heav- 

 ens), contains a complete exposition of the state of astronomy in his 

 time, the reigns of Adrian and Antonine. This book is familiarly 

 known to us by a term which contains the record of our having re- 

 eei\ ed our first knowledge of it from the Arabic writers. The "Megiste 

 S\ ntaxis," or Great Construction, gave rise, among them, to the title 

 Al Magisti, or Almagest, by which the work is commonly described. 

 As a mathematical exposition of the Theory of Epicycles and Eccen- 

 trics, of the observations and calculations which were employed in 



