Summary
Having explained the structure of the entire visible universe, Descartes now lets his principles of physics loose on earth. He begins with an account of how terrestrial phenomena could have come into existence. Once again, his explanation makes heavy use of the three elements of matter. The Earth, according to his picture, is divided into three regions, each with different sorts of particles, deriving from the three elements. The formation of various bodies from these particles arises from four forces: the general motion of celestial globules, gravity, light, and heat.
Descartes' discussion of gravity, or the "second force," is generally the only section translated into English from the first two thirds of Part IV. It is, without a doubt, his most interesting treatment of a terrestrial phenomenon. Descartes, a pre-Newtonian, does not think of gravity as a universal attractive force. Rather, for Descartes as for the Scholastics, gravity was literally "heaviness." The Scholastics used the Latin term "gravitas" to refer to the supposedly inherent tendency of terrestrial bodies to move downward. Descartes, of course, wants to give a completely mechanistic account of the tendency of earth's bodies to head downward.
Given that the earth moves around the sun, Descartes asks, why do earthly bodies not fly off the planet like sand flying off a spinning top? The reason is that celestial matter surrounds all terrestrial matter. Earth does not move of its own motion but is moved by the terrestrial matter in which it is embedded. Earth, therefore, behaves like a body at rest.
This surrounding celestial matter not only accounts for the fact that terrestrial bodies do not go flying off into the heavens, it also accounts for the fact that all terrestrial bodies tend downward. It is not that terrestrial bodies in themselves have any property of heaviness or some tendency toward the ground (a property that Descartes would not be able to explain as the result of extension). Instead, what causes terrestrial bodies to move downward toward the center of earth is that celestial bodies, which are in motion, are constantly trying to move upward, away from the center of earth.
Since the natural motion of body is rectilinear, when a body is prevented from rectilinear motion, it is constantly trying to regain its rectilinear course and, therefore, tends away from the center of the spiral it has been temporarily forced into. Celestial particles are prevented in their rectilinear motion by encountering the earth. They are therefore, moving away from the center of the earth. In order to move upward, they must displace the particles that are in their way. They cannot displace other celestial particles because these have an equal tendency to move away from the center of the earth. They can, however, displace terrestrial particles because terrestrial particles do not have this tendency to the same extent (though they must have it to some extent since they too need to move in rectilinear motion). The force the celestial globules exert in order to displace terrestrial particles above them is what forces all terrestrial bodies toward the center of the earth.
After explaining the force of gravity, Descartes goes on to attempt to account for every conceivable terrestrial phenomena: earthquakes, the nature of various metals and minerals, the tides, flammability and the nature of fire, the nature of glass, and magnetism.
