304 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



every part of them, and all the materials of which they are composed, and to 

 which they are attached, have a degree, or more properly speaking, a certain 

 compass of elasticity, which, as such, is perfect, and no motion thereby lost. 

 We must not therefore expect the two compound bodies after the stroke to stick 

 together without separating, as would be the case with bodies truly non-elastic 

 and soft ; but that from the elasticity they are posses-ed of, they will by rebound- 

 ing be separated ; but that elasticity being perfect, can occasion no loss of mo- 

 tion to the sum of the two bodies ; so that if the body c ascends as much above 

 its mark c as the body d falls short of its mark d, then it will follow, that their 

 medium ascent will still be to the mark z, as it ought to have been, had they 

 been truly non-elastic soft bodies ; and this, in reality, is truly the case in the 

 experiment, as nearly as it can be discerned. 



After a few vibrations, by the rubbing of the springs against each other, they 

 are soon brought to rest ; and here they would always rest had they been truly 

 and properlv perfect non-elastic soft bodies ; but here, as in the case of these 

 bodies, by a change of the figure and situation of the component parts, there is 

 expended one half of the mechanical power of the first mover, yet in this case the 

 other half is not lost, but suspended, ready to be re-exerted whenever it is set 

 at liberty; and that it is really and bona fide one half and neither more nor less, 

 appears from this uncontroverted simple principle, that the power of restitution 

 of a perfect spring is exactly equal to the power that bends it. And this may, 

 in a certain degree, be shown to be fact by experiment, if there were any need of 

 such a proof; for if, when the bodies are at rest after the last experiment, the 

 two rods are lashed together near the bottom with a bit of thread, and then the 

 ratchets unpinned and removed ; on cutting the thread with a pair of scissars 

 they will each of them rebound, c towards m, and d towards n ; and if they re- 

 bounded respectively to z and y, the mechanical power exerted would be the 

 same as it was after the stroke, when the mean of their two ascents was up to 

 the mark z ; but here it is not to be expected, because not only the motion lost 

 by the friction of the ratchets is to be deducted, because it had the effect of real 

 non-elasticity ; but also the elasticity that separated them in the stroke, which 

 was lost in the vibrations that succeeded ; neither of which hindered the mean 

 ascent to be to z , but yet, under all these disadvantages in the machine, if not 

 unreasonably ill made, the rod ef will ascend to d, and gh to a : and hence I 

 infer, as a positive truth, that in the collision of non-elastic soft bodies, one half 

 of the mechanic power residing in the striking body is lost in the stroke.* 



* Here Mr. S. puzzles himself again about his mechanical power. There is no real force or mo- 

 mentum lost by striking bodies, if they adhere and move together on the stroke ; as he might easily 

 and directly have convinced himself of, by discharging a leaden bullet into a pendulous block of 

 wood ; being both, in some degree, soft and non-elastic bodies. 



