342 Mr. Herapath on the Causes, Laws, and principal [May 



momentum in the path it has aheady described. It will, there- 

 fore, being acted on still by the soliciting force, retrace this path, 

 until, having- ascended to the point whence it set out, it has lost 

 all its velocity, and begins to descend again as before. As soon 

 as it has arrived again to the place where it before was met by 

 the other body, let it receive a second equal shock, and again 

 it will begin to ascend ; and so on as long as we please. Thus 

 it might be continually kept oscillating between these two 

 points by impulses properly regulated. And if the distance 

 between the points be diminished, the number of oscillations 

 will be increased, and the intervals between the shocks dimi- 

 nished, and the body consequently approach nearer to a state of 

 apparent rest. 



This is how things will happen when the affected body comes 

 in contact v»ith only one other body, and receives the strokes in 

 the direction of the track of its centre of gravity. But the same 

 would evidently take place, if, instead of one body striking it in 

 this identical manner, it were struck by several in different parts 

 of its surface, either all at once, or in any order ; the aggregate 

 direction and intensity of whose strokes, however, should be 

 similar to those of the single body under similar circumstances. 

 Then might the body acted on by the force remain in a state of 

 rest, if not absolute, at least so nearly so, as to differ from it 

 insensibly. Now the tendency of this body to move in the 

 direction in which it is solicited is precisely the same thing as 

 mathematicians understand by the force, which they call pres- 

 sure ; and, therefore, the opposition of the other smaller bodies 

 is also equivalent ; and, as to effect, the same as this force of 

 pressure. And because the value of this 'pressure might be 

 increased or diminished ; by increasing or diminishing the soli- 

 citing force, or the pressing body ; so also might the value of the 

 resisting force of the smaller bodies be increased or diminished, 

 by increasing or diminishing their momentum, the number of 

 their collisions, or the number of the bodies. 



From these considerations it follows, that if a number of 

 small bodies be inclosed in any hollow body, and be continually 

 impinging on one another, and on the sides of the enclosing 

 body ; and if the motions of the bodies be conserved by an equi- 

 valent action in the sides of the containing body, then will these 

 small bodies compose a medium, whose elastic force will be like 

 that of our air and other gaseous bodies ; for if the bodies be 

 exceedi^igly small, the medium might, like any aeriform body, 

 be compressed into a very small space ; and yet, if it had no 

 other tendency than what would arise from the internal collision 

 of its atoms, it would, if left to itself, extend to the occupation 

 of a space of almost indefinite greatness. And its temperature 

 remaining the same, its elasticity would also be greater when 

 occupying a less, and less when occupying a greater space ; for 

 in a condensed state the number of atoms striking against a 



