6 FORM OF PLANETS. SBCT. I. 



each body is itself the center of an attractive force ex- 

 tending indefinitely in space, causing all the mutual dis- 

 turbances which render the celestial motions so compli- 

 cated, and their investigation so difficult. 



The gravitation of matter directed to a center, and 

 attracting directly as the mass, and inversely as the 

 square of the distance, does not belong to it when con- 

 sidered in mass only ; particle acts on particle according 

 to the same law when at sensible distances from each 

 other. If the sun acted on the center of the earth, with- 

 out attracting each of its particles, the tides would be 

 very much greater than they now are, and would also, 

 in other respects, be very different. The gravitation of 

 the earth to the sun results from the gravitation of all its 

 particles, which, in their turn, attract the sun in the ra- 

 tio of their respective masses. There is a reciprocal 

 action, likewise, between the earth and every particle 

 at its surface. The earth and a feather mutually attract 

 each other in the proportion of the mass of the earth to 

 the mass of the feather. Were this not the case, and 

 were any portion of the earth, however small, to attract 

 another portion, and not be itself attracted, the center of 

 gravity of the earth would be moved in space by this 

 action, which is impossible. 



The forms of the planets result from the reciprocal 

 attraction of their component particles. A detached fluid 

 mass, if at rest, would assume the form of a sphere, 

 from the reciprocal attraction of its particles. But if the 

 mass revolve 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 rota- 

 tion (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 

 center of a sphere, the equatorial particles would recede 

 from the center, till their increase in number balance 

 the centrifugal force by their attraction. Consequently, 

 the sphere would become an oblate, or flattened sphe- 

 roid ; and a fluid partially or entirely covering a solid, as 

 the ocean and atmosphere cover the earth, must assume 



