80 MECHANICS. 



touch of a child's finger will move as much as two, 

 horses can draw. 



The lever in mechanics compensates power by 

 space, and what is lost in power is gained in time. 

 In engines that act by the mere force of gravity, 

 what we gain in power we lose in velocity, and the 

 reverse. 



The Wheel and Axle. 



This is only a lever moving round an axis, and 

 always retaining the effect gained during every part 

 of the motion, by means of a rope wound round the 

 but end of the axle; the spoke of the wheel being 

 the long arm of the lever, and half the diameter of 

 the axle the short arm. By a combination of levers, 

 wheels, and pullies a great increase of force is ob- 

 tained. , 



Percussion on one Point. 



If an ivory ball strike against another of the same 

 weight, there should, according to common theory, 

 be an equal transfer of motion. But if the velocity 

 of the impinging ball be very considerable, so far 

 from stopping suddenly, it will recoil back again with 

 the same force, while the ball which is struck will re- 

 main at rest. 



The reason of this is, that the shock is so momen- 

 tary, as not to permit the communication of impulse 

 to the whole mass of the second ball. A small spot 

 only is affected, and the consequence is therefore the 

 same as if the ball had impinged against an immove- 

 able wall. 



It is on a similar principle, that a bullet, fired 

 against a door hanging freely on its hinges, will per- 

 forate the same, without in the least agitating it. 

 Nay, a pellet of clay, a bit of tallow, or even a small 

 bag of w r ater, discharged from a pistol, will produce 

 the same effect. 



In all these instances the impression of the stroke 



