FOEM OF PLANETS. [SECT. i. 



about an axis, it becomes flattened at the poles, and bulges 

 at the equator (N. 11), in consequence of the centrifugal 

 force arising from the velocity of rotation (N. 30) ; for the 

 centrifugal force diminishes the gravity of the particles at 

 the equator, and equilibrium can only exist where these two 

 forces are balanced by an increase of gravity. Therefore, 

 as the attractive force is the same on all particles at equal 

 distances from the centre of a sphere, the equatorial par- 

 ticles would recede from the centre, till their increase in 

 number balance the centrifugal force by their attraction. 

 Consequently, the sphere would become an oblate, or flat- 

 tened spheroid ; and a fluid partially or entirely covering a 

 solid, as the ocean and atmosphere cover the earth, must 

 assume 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 mag- 

 nitude 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 flattening at their poles is, how- 

 ever, 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 dis- 

 turbances in the motions of both bodies ; for the action of 

 the moon on the matter at the earth's equator produces a 



