BOOK IX. 



373 



they are a palm wide and two and a half digits thick, and it is necessary 

 that they should be three digits apart, as were the nindles. 



The axle should have a thickness in proportion to the spokes and the 

 segments. As it has two cams to depress each of the levers, it is necessary that 

 it should have twenty -four cams, which project beyond it a foot and a palm and 

 a digit. The cams are of almost semicircular shape, of which the widest part is 

 three palms Eind a digit wide, and they are a palm thick ; they are 

 distributed according to the four sides of the axle, on the upper, the lower 

 and the two lateral sides. The axle has twelve holes, of which the first 

 penetrates through from the upper side to the lower, the second from one 

 lateral side to the other ; the first hole is four feet two palms distant from 

 the second ; each alternate one of these holes is made in the same direc- 

 tion, and they are arranged at equal intervals. Each single cam must 

 be opposite another ; the first is inserted into the upper part of the first 

 hole, the second into the lower part of the same hole, and so fixed by 

 pegs that they do not fall out ; the third cam is inserted into that part 

 of the second hole which is on the right side, and the fourth into that 

 part on the left. In Hke manner all the cams are inserted into the consecutive 

 holes, for which reason it happens that the cams depress the levers of the 



