74 Advantage of large Wheels to Carriages. [Book I. 



But if it is impOiTible to overcome thefe difficulties 

 entirely, they may be prevented in part by employing 

 large ir.ftead of fmall wheels. For it is certain, that 

 fmall wheels entangle themfelves more than great ones 

 in the ruts and hollows in roads, as may be feen by 

 the fig. 5, where the radius c q of the fmall wheel, 

 which bears againft the ground, in riling out of a hol- 

 low in the road, is much more oblique to the direc- 

 tion of the power cp than the radius C q of the great 

 wheel to the direction C P. As the circumference alfo 

 of a great wheel meafures, in rolling, more of the road 

 than that of the fmall one, it turns fwifter, or makes 

 fewer revolutions, in pafiing over a given fpace, which 

 faves no inconfiderable part of the friction. 



III. The WHEEL and AXLE * or windlafs, one of the 

 fix fimple machines, is a cylinder which turns upon its 

 own axis, by means of which, with a fmall force, a great 

 burden may be elevated by a rope which wraps round 

 the cylinder by the aid of a handle, or by means of cogs 

 or bars ufed as levers, acting on the circumference. 



It is the common practice to fix at one of the 

 extremities of the cylinder AB (fig. 6.) levers, fuch 

 as E, F, G, H, by means of which the cylinder is 

 turned upon its axis C D, while the cord which fuf- 

 tains the weight a, is wrapped or wound about it. It 

 is eafy to fee that the effect of the wheel and axle is 

 analogous to that of a lever of the firft order. For, 

 fuppofe that bg (fig. 7.) reprefents the radius of a 

 cylinder, and that b P reprefents the arm of a lever, 

 by which the power P acts : if the length of b P is to 

 that of bg as 3 to i, a power of 100 pounds at P, 

 acting in a perpendicular direction at P b, will balancq 

 a weight G of 300 pounds. 



* By fome called the axis in peritrochio. 



Hense 



