TRACT 37.

Or GUNNERY.

279

strikes an immoveable obstacle, it either recoils, or pene-trates into it, until its whole motion is destroyed by the re-sistance. Now the. recoil takes place when the bodies areendued with elasticity, and will always happen when theelasticity is perfect, that is, when they possess a power ofrestoring themselves completely to the same shape, whichthey had before the stroke, after having their forms chargedby the said stroke; and a recoil will often happen when theelasticity is imperfect, or only partial, and the velocity andforce cf the striking body may not be sufficient to overcomethe resisting strength of the obstacle, thereby preventing thepenetration and lodging of the striking body. When thebody and obstacle are quite destitute of this faculty of elas-ticity and restoring themselves, the body will penetrate intothe obstacle till all its motion be destroyed, and remain atrest, as in the case of a body falling down into a mass of softclay, 8tc. In both cases the ball will make an impression;in the former case there will be a restoration to the formershape ; in the other the impression will remain ; and in thisconsists the difference between clastic and unelastic bodies.

135. But, between these two kinds of bodies, there maybe an infinite number of other sorts, endued with differentdegrees of this power of restoring themselves. Indeed, itmay with certainty be affirmed, that there is not in the wholeworld any body that is perfectly elastic, nor yet quite devoidof elasticity ; but that all possess this quality in some degree.However elastic a body may seem to be, yet it will still re-tain some small mark, where an impression has been made;and no body has yet been discovered which, having receivedan impression, will not restore itself in some small degree.To explain this proposition, we need only consider the cavitywhich is made by the collision of the two bodies, and whichis always made, whether the body is elastic or not; and, inthis view, it is the same thing, whether the cavity remainunaltered, or restore itself, either wholly or in part, to itsformer condition. And it is clear from experiment, that if aball be impelled against such an obstacle as a bank of earth,