OF THE MECHANICAL POWERS. 



63 



But then, just as much power or advantage as is gained 

 by the engine, so much time is lost in working it. In 

 this sort of machines it is requisite to have a ratchet- 

 wheel G on one end of the axle, with a catch H to fall 

 into its teeth ; which will at any time support the 

 weight, and keep it from descending, if the workman 

 should, through inadvertency or carelessness, quit his 

 hold whilst the weight is raising. And by this means, 

 the danger is prevented which might otherwise hap- 

 pen by the running down of the weight when left at 

 liberty. 



3. The third mechanical power or engine consists The pullty. 

 either of one moveable pulley, or a system of pulleys ; some 

 in a block or case which is fixed, and others in a block 

 which is moveable, and rises with the weight. For 

 though a single pulley that only turns 

 on its axis, and moves not out of 

 its place, may serve to change the di- 

 rection of the power, yet it can give no 

 mechanical advantage thereto ; but is 

 only as the beam of a balance, whose 

 arms are of equal length and weight. 

 Thus, if the equal weights W and Phang 

 by the cord B B upon the pulley A, whose 

 block b is fixed to a beam, they 

 will counterpoise each other, just in the 

 same manner as if the cord were cut in 

 the middle, and its two ends hung upon 

 the hooks fixed in the pulley at A and 

 A, equally distant from its center. 



66 







