Chap. 7.] Caufes which impede tbedftion of Machines. 8 5 



the .fame power Q^would produce a double effect, 

 that is, ic would raife 5,700 Ibs. 



But if, without changing the number of teeth in the 

 wheel C b y or the length of the radius E M of the han- 

 dle, another perpetual fcrew is placed upon the axis 

 of the wheel inftead of the windlafs T, the thread of 

 which fhould catch with the teeth of a fecond wheel 

 of the fame number of teeth with the fi ft, and to 

 which fhould be annexed the windlafs T, which is to 

 fuftain the weight P, then the fame power Qjvould 

 be capable of raifmg a weight 19 times as great ; in 

 other words, this power, intrinfically only 30 Ibs. would 

 be able to raife the weight of 54,150^5. 



Such are the fimple machines which have been ge- 

 nerally confidered as the bafes of all others. 



If all the materials of which machines are compofed 

 were perfectly hard, perfectly polifhed, and if the ropes 

 which are often ufed to tranfmit the motive force from 

 one part of the machine to another had a perfect flexi- 

 bility, the theory of equilibrity, of which we have 

 hitherto fpoken, would be fufficient to determine, in 

 every cafe, the force requifite to counterbalance a 

 given refiflance ; and this force once found, it would 

 be clear, that by augmenting it ever fo little, the equi- 

 librium would be deftroyed and the refiftance over- 

 come -, but the friction of furfaces one againft an- 

 other, and the refiftance which cords produce by being 

 wrapped round pullies and cylinders, necefTarily impede 

 the motion of machines ; and it is extremely difficult 

 to eftimate, with even tolerable exactnefs, the amount 

 of the refiftance which may in different circum- " 

 fauces proceed from thefe caufes, 



G 3 BOOK 



