G FOKMS OF THE PLACETS. SECT. I. 



The gravitation of matter directed to a centre, and attracting 

 directly as the mass and inversely as the square of the distance, 

 does not belong to it when considered 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 centre of the earth, 

 without attracting each of its particles, the tides would be very 

 much greater than they now are, and would also, in other respects, 

 te 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 ratio 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 centre 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 (X. 11), in con- 

 sequence 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. There- 

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

 distances from the centre of a sphere, the equatorial particles 

 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 flattened 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, sphe- 

 roidal, 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 



