What was said so far is not only pleasant to know but also very useful to solve the wonderful problem concerning the infinite force and energy of percussion which must be dealt with now.

Following nature, we usually rely on a well known measure when we want to investigate some quantity. To measure the time we use the movement of the sun and the duration of the day light and of the night dark, to measure a movement we use the length of the foot and that of the step, to measure a length we use the palm of the hand and the fingers. Since these measures which we have always before the eyes are sufficiently known and tested, we use them to measure other unknown related quantities. Thus, people commonly think that the weights of bodies which can conveniently be used with a balance are suited to measure the energy of percussion. This can occur in two ways. Firstly, the forces of percussion and of a weight are compared with a third quantity. It is known that two forces which are equal to a third one are equal to each other. In the second way, the percussive force must be compared directly with the energy of a weight.

The third quantity with which we compare the forces of percussion and of gravity is the common effect produced by both. This effect is that on a wedge or a nail driven in wood or in some other hard body. It is clear that the nail can be driven in to a well-determined depth by the compressive force of some considerable weight as well as by a hammer blow. In this action, the force compressing the nail and the resistance of the hard body into which the nail is to be driven in must be compared. This resistance, as I said, results from the tenacity or from the cohesion which ties together the parts of the hard body. Adjacent and contiguous particles must be divided, displaced, stretched and expelled to enable the nail or the wedge to penetrate. The body resists driving in of the nail for all these causes, and, moreover, the movement which must agitate the devices which are called wedges, according to the laws of mechanics. This resistance is completely overcome when the stone or the wood are divided into two forth shooting pieces. Then the percussion stops and all resistance ceases. But if the nail did not so far overcome the resistance of the body, this will react by opposing further penetration of the nail. This occurs in cleavable bodies such as stones or in bodies apt to be cut such as metals. In cleavable bodies some resistance remains effective. The nail is then squeezed from both sides like in pliers or some spring. The nail would thus be expelled outwards if the rough excrescences of its surface did not prevent this. We thus observe that a wooden nail immersed in water or in quicksilver, when the compressive impulse stops, suddenly resiles and is expelled like a plum stone projected by squeezing between fingers. In other cleavable bodies such as stones, this lateral expression like that of a spring does not occur with so much energy because the driven-in nail impetuously distracts the parts of the stone and splits this in two pieces.