Gravity, force together with material motion, and percussion are the four accidental powers by which the human species, in its marvellous and varied operations, seems in this world to show itself a second nature. For by such powers all the visible works of mortals have their being and their death.

Material motion together with gravity, force and percussion are the four accidental powers by which all the works of mortals have their being and their death.

Motion.

Among these powers, motion is the cause whereby each of them can generate itself, and likewise each of the others, and motion itself as well; for without it none of these can be generated.

Gravity.

Gravity is a power created by motion, which by means of force carries one element into another; and this gravity has as much life as the element takes to return to its own place.

Force.

Force and gravity have much likeness in all their powers, but they differ only in the motions of their birth and death. For simple weight moves in one direction only, that is towards the centre, whereas force is born and dies through every kind of motion.

Many philosophers, by common opinion, affirm force to be an accidental weight—saying 'accidental' as if gravity too were not itself accidental.

Here it is asked whether weight is generated of itself or by force, or force of itself or by weight.

The spirit of sentient animals moves to the limbs of their bodies, and, entering equally into the muscles found there, disposes them to swell; and as soon as they swell they shorten, and in shortening they draw back the sinews joined with them. From this arises the force and motion of the human limbs. Therefore material motion is born of the spiritual.

It is possible to know the motion of an arrow, knowing first its weight, and likewise the weight that loads the crossbow together with the motion of its string.

The same proportion that the weight of the arrow has to a weight that would draw the crossbow's string onto its nut, such will the motion of the string in loosing the arrow have to the motion made by the arrow itself—provided the arrow is proportionate to the force of that crossbow.

Here one must set out the crossbow short and thick; long and thick; short, thin and broad; broad, thin and long; the square Syrian bow, or a crossbow of like nature.

If you take the arrow shot from the crossbow and measure its whole course, and divide it by 3 and 1/7, and imagine a wheel of that diameter; and likewise divide the travel of the crossbow's string by 3 and 1/7, and imagine a similar circle; and set the arrow upon that wheel as a lever, and let the counter-lever be the crossbow's string—you will see that by the weight of that arrow the string will be drawn onto the nut.

If the arrow ran 300 braccia, make the wheel's circle 300 braccia; and if the string moved 1/3 braccio, make the lesser circle 1/3 of a braccio. Now note that the semidiameter of the wheel of 300 is 50 braccia, and likewise the semidiameter of 1/3 of a braccio is 1/10 of a braccio. Now say: 50 times 18 makes 900; now you have 900 of leverage against 1.