What is force. I say that force is a spiritual power, an invisible potency, which by accidental, external violence is caused by motion and placed and infused into bodies that are drawn back and bent out of their natural use, giving them an active life of marvelous power. It compels all created things to change form and position. It runs with fury toward its desired death and varies itself according to its causes. Slowness makes it great, and quickness makes it weak. It is born of violence and dies of liberty. And the greater it is, the more quickly it consumes itself. It drives away with fury whatever opposes its undoing. It desires to overcome its opponent, to slay its cause, and in conquering it slays itself. It becomes more powerful where it finds greater resistance. Every thing willingly flees its death, yet, being constrained, every thing constrains. Nothing moves without it. The body where it is born grows neither in weight nor in form. No motion made by it will be lasting. It grows through effort and fails through rest. The body in which it is confined is deprived of liberty, and often, by means of motion, it generates new force.

Force is caused by motion and infused into weight; and likewise the blow is caused by motion and infused into weight.

Force is the cause of motion, and motion is the cause of force; and motion infuses force and the blow into weight by means of the object.

Force, in some effect, when it undoes itself, transfers into that body which flees before it, and by means of movement generates a blow of greater efficacy, and after itself makes ruin, as appears in the motion of the ball driven by the force of the bombard.

Force extends into only three offices, which contain infinite ones within them; these three offices are to pull, to push, and to stop. And this force can arise in two different ways. The first is by the sudden expansion of a rarefied body within a dense one, as with the multiplication of fire in the bombard, which, finding no empty vessel to receive its expansion, runs with fury to a wider place, driving away every obstacle that opposes its desire. And the course of water and of wind does the same, driving away every obstacle that opposes it. The second is that which is created in bodies bent and twisted out of their nature, such as the crossbow or other similar machines, which are unwilling to be bent, and once they are loaded, they desire to straighten themselves; and as soon as they are given liberty, they drive away with fury the thing that opposed their course.

The blow is born in the dying of motion, and motion in the death of force.