226 THE GREEK ASTRONOMY. 



of Chalid ben Abdolmalic, and the other having 

 at its head Alis ben Isa. These two parties pro- 

 ceeded, the one north, the other south, determining 

 the distance by the actual application of their mea- 

 suring-rods to the ground, till each was found, 

 by astronomical observation, to be a degree from 

 the place at which they started. It then appeared 

 that these terrestrial degrees were respectively 

 56 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 in- 

 formed that the cubit is that called the black 

 cubit, which consists of 27 inches, each inch being 

 the thickness of six grains of barley. 



Sect. 6. Ptolemy s Discovery of Evection. 



BY referring, in this place, to the last-mentioned 

 measure of the earth, we include the labours of 

 the Arabian as well as the Alexandrian astrono- 

 mers, 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 inter- 

 vening discovery marks a new epoch, and begins 

 a new period; because no great revolution in 



