Gravity

¹⁵ These bubbles, these glasses, are variously inserted, figured, and massed together to produce such a great apparatus of things, of which we shall soon speak in the origin of species.

Gravity

Gravity is the affection of the whole system.

Gravity is the cause of most extraordinary motions in our globe, or certainly the key to them, even those exerted privately in species.

Physical work must be given so that all things are reduced, as much as possible, to mechanical reasons, as they are the simplest.

¹⁶ Gravity arises from the circulation of the aether:

• around the earth
• in the earth
• through the earth

The cause of which was discussed above in sections 9 and 10.

This aether penetrates water and air most greatly, since they are more porous.

Hence, the earth descends in water (unless it contains more superficial aether than the water itself), and water descends in air.

¹⁷ Whatever has been separated from the Homogeneous (for things joined together do not sensibly gravitate because of an insensible gravity mutually lifting itself in all parts) and is placed in a location having more aether and less earth, now impedes the circulation of the aether and disturbs it.

The more it is elevated, the more it disturbs it, because the entire circum-terrestrial aether is homogeneous in itself, like an ocean or air, running through various rivers, borders, lakes, straits, and channels.

Every heterogeneous thing disturbs the circulation of a homogeneous liquid, because even if one part attempts to follow a part carrying the liquid away, the other part cannot follow due to a different reason of consistency or divisibility.

This is also why:

• things dissolved in liquids are gradually cast down and shine forth into crystals
• things enclosed and digested gradually ferment.

¹⁸ This is why air, water, and earth gravitate in the aether.

For they are cast down by its circulation.

Since they disturb the circulation, they are expelled—not upwards, for they would disturb it more there (because spherical surfaces increase in the duplicate ratio of diameters, not in the same ratio as the diameters.

Thus, a greater inequality happens in sections acting on the same body)—therefore downwards, that is, descending.

Hence, furthermore, comes the increment of impetus due to a new impression everywhere during the descent in any free or freer aether, as the ratio of that thing allows.

From here, other mechanical and static phenomena are deduced in a common manner and mode.

¹⁹ There are 2 Mechanical Potencies:

1. Impetus from falling
2. Distance from the line of direction.

The third is physical, which I am accustomed to call Nisus [striving/effort], such as comes from the motion of muscles, of which we speak below in section 50.

Distance from the line of direction increases potency…

Every power in bodies depends on velocity, since things, however continuous, can be moved by any motion of theirs, however small.

…axis in the wheel and axle (peritrochio), the screw, and the pulley; it has been discovered that in balancing things, a weight always ascends as much more quickly as the load descends, and conversely, as much as the load is greater than the weight;

This is the ratio of linear [paths] completed in the same time, which is the ratio of distances from the line of direction.

²⁰ However, there remain some things even in common motions and phenomena that, at first glance, appear contemptible but are difficult to solve if you look into them more sharply.

For example:

• Why do hard things struck against hard things spring back?
• Why do certain things, when bent, restore themselves with such great force?
• Why, according to the most ingenious experiments of men like Huygens and Wren (which are universal), does a body striking a resting one—as if a permutation were made—remain in its place, while transferring its own motion to the other?

A similar ratio exists for two [bodies] running together.

Such things and many others of this kind are not consistent with the abstract reasons of motions unless the “economy” of our globe is added.