1 2. The circular balance, called the pulley, being of such importance among machine instruments, and above all in the transmutation of forces, is not to be passed over, since by its means the power of the driver of the said machines is multiplied, as is seen in the tackles, where the power grows as much as the number of such pulleys grows. Therefore let us define its nature and power; and first we shall show how the ropes without motion feel an equal weight of the gravity sustained by them, and this we shall call natural weight; then we shall speak of motion and of how the weight that is distributed in the ropes varies, and this we shall name accidental weight, that is force, which grows the more, the faster it becomes. But the natural weight never varies; what varies is the power in the driver together with the resistance of the thing moved and the resistance of the air, which condenses and resists, as the air does to the fan of clocks.

b c d a.

1 4; 1 4; 1 4

e – d – c a - f b - g - 3 1. This balance b e, at whose ends I wish to hang two heavy bodies that are in triple proportion of weight to one another, I ask in what position this balance, having equal arms, must be set. This position of the arms is free, because the said position can be given with the arms standing beyond the site of equality at whatever degree of obliquity one wishes, provided that the potential arms and the real ones are in right-angled conjunction with their appendages, as is shown at c, the potential arm being with its real appendage c f.

8 1

The air condenses more beneath the faster-moving body, and makes that body seem lighter than it is; whereby the proportion of the weights alters the due proportion of their natural gravity.

a - b - c

S - 4 e - b a - 8 n. a b, the potential lever, sustains the real weight upon the real counter-lever a e, which in itself weighs 4, but the disposition of its motion is not worth 4, because that motion is oblique. But be it whatever weight it may, n, the counter-weight, is worth the resistance of the descent of such weight e. Without doubt one can give a power at b that will resist the descent of e, which is 4. And that this is true: either a power greater than e will be given, or a lesser one. Therefore an equal one will be given. Here it is to be considered that there are two motions disposed to move in two aspects.

The air is condensable to infinity, and this is shown to us by the very swift motion of the lightning, generator of the immense celestial thunder, which bends and twists itself the more toward diverse aspects, the more the air and the cloud before it condense it, and ... Example of the thunder. The vaporisation of water thrown upon burning embers does the like as the fire kindled in the clouds, which with fury vaporise and constrict the course of the created thunderbolt; that is, just as the water vaporises and, growing, turns into smoke, so the cloud is vaporised and converted into air, which by its increase closes in and constricts the flame that is generated within it.