SECT. I. ROTATION AND TRANSLATION. 7 



that form in order to remain in equilibrio. The surface 

 of the sea is therefore spheroidal, and the surface of the 

 earth only deviates from that figure where it rises above 

 or sinks below the level of the sea. But the deviation is 

 so small, that it is unimportant when compared with the 

 magnitude of the earth ; for the mighty chain of the 

 Andes, and the yet more lofty Himalaya, bear about the 

 same proportion to the earth that a grain of sand does to 

 a globe three feet in diameter. Such is the form of the 

 earth and planets. The compression (N. 31) or flatten- 

 ing at their poles is, however, so small, that even Jupiter, 

 whose rotation is the most rapid, and therefore the most 

 elliptical of the planets, may, from his great distance, be 

 regarded as spherical. Although the planets attract 

 each other as if they were spheres, on account of their 

 distances, yet the satellites (N. 32) are near enough to 

 be sensibly affected in their motions by the forms of 

 their primaries. The moon, for example, is so near 

 the earth, that the reciprocal attraction between each of 

 her particles, and each of the particles in the prominent 

 mass at the terrestrial equator, occasions considerable 

 disturbances in the motions of both bodies ; for the ac- 

 tion of the moon on the matter at the earth's equator, 

 produces a nutation (N. 33) in the axis (N. 34) of rotation, 

 and the reaction of that matter on the moon is the cause 

 of a corresponding nutation in the lunar orbit (N. 35). 



If a sphere at rest in space receive an impulse passing 

 through its center of gravity, all its parts will move with 

 an equal velocity in a straight line ; but if the impulse 

 does not pass though the center of gravity, its particles, 

 having unequal velocities, will have a rotatory or revolv- 

 ing motion, at the same time that it is translated (N. 36) 

 in space. These motions are independent of one an- 

 other ; so that a contrary impulse, passing through its 

 center of gravity, will impede its progress, without in- 

 terfering with its rotation. As the sun rotates about an 

 axis, it seems probable, if an impulse in a contrary direc- 

 tion has not been given to his center of gravity, that he 

 moves in space, accompanied by all those bodies which 

 compose the solar system a circumstance which would 

 in no way interfere with their relative motions ; for, in 

 consequence of the principle, that force is proportional 



