Xll 



INTRODUCTION TO MECHANICS. 



it will be found after the contact to have spread, and will thus" shew that 

 there has been compression. Soft bodies, which easily retain impres- 

 sions, such as clay, wax, tallow, butter, &c. have very little elasticity. 



The cause of elasticity is not well ascertained. Elasticity implies sus- 

 ceptibility of compression, and the susceptibility of compression depends 

 upon the porosity of bodies ; for were there no pores or spaces between 

 the particles of matter of which a body is composed, it could not be com- 

 pressed. But you must not hence infer, that bodies whose particles are 

 most distant from each other are most elastic. Elasticity implies not 

 only susceptibility of compression, but the power of resuming its former 

 state after compression. The pores of such bodies as ivory and metals 

 are invisible to the naked eye ; but it is well ascertained, that gold, one of 

 the most dense of all bodies, is extremely porous, and that its pores are 

 sufficiently large to admit water, under great pressure, to pass through 

 them. In cork, sponge, and bread, the pores form considerable cavities ; 

 in wood and many kinds of stone, when not polished, they are perceptible to 

 the naked eye ; whilst in ivory, metals, and most varnished and polished 

 bodies, they cannot be discerned. To give an idea of the extreme 

 porosity of bodies, Sir Isaac Newton conjectured that if the earth were so 

 compressed as to be absolutely without pores, its dimensions might pos- 

 sibly not be more than a cubic inch. 



The elasticity of ivory is very perfect ; that is to say, it restores itself, after 

 compression, with a force very nearly equal to that exerted in compressing it. 

 If two ivory balls of equal weight be suspended by threads (fig. 1), and one 

 of them A be drawn a little on one side and then let go, it will strike against 

 the other ball B, and drive it off to a distance equal to that through which the 



Fig. 1. Fig. 2. Fig. 3. 



first ball fell ; but the motion of A will be stopped, because, when it strikes 

 B, it receives in return a blow equal to that it gave, and its motion is con- 

 sequently destroyed. Therefore, when one body strikes against another, 

 the quantity of motion communicated to the second body is lost by the 

 first, but this loss proceeds not from the blow given by the striking 

 body, but from the re- act ion of the body which it struck. 



If six ivory balls of equal weight be hung in a row (Jig. 2), and the first 

 be drawn out of the perpendicular, and let fall against the second, none of 

 the balls will appear to move except the last, which will fly off as far as the 

 first ball fell. For when the first ball strikes the second, it receives a blow in 

 return, which destroys its motion. The second ball, although it does not 

 appear to move, strikes against the third ; the re-action of which sets it 

 at rest : the action of the third ball is destroyed by the re-action of the 

 fourth, and so on, till motion is communicated to the last ball, which, not 

 being re-acted upon, flies off. This effect takes place accurately only in 

 the case of perfectly elastic bodies. 



If two balls of clay (fig. 3), which are not elastic, be suspended, and one 

 of them, D, be raised out of the perpendicular and let fall against the 



