78 GRAVITATION. 



those parts of the earth which are towards those bodies. 

 This conclusively proves that the gravitation of the earth 

 towards the sun and moon is not simply the gravitation 

 of the earth as a whole or of some nucleus in the centre, 

 but of the various parts of the earth, as separate and dis- 

 tinct portions of matter. 



There is another phenomenon, which was observed 

 long before it was understood, and which is decisive 

 proof of the same point. The earth is not exactly spheri- 

 cal but is fuller in its shape around the equator. Now 

 if the attraction of the earth to the sun is owing to some 

 peculiar substance in its centre, it is plain that this sur- 

 plus about the equatorial regions will have no effect upon 

 its motions. If, however, this surplus gravitates towards 

 the sun, as all the rest of the matter of which the earth 

 is composed, it is plain that in some peculiar circumstan- 

 ces it may cause a modification of its motions. This last 

 is found to be the fact ; and it proves that the solid parts 

 of the earth as well as the liquid and gaseous, have sepa- 

 rately a tendency towards other material bodies which 

 come into their vicinity. 



3. GRAVITATION BETWEEN THE EARTH AND THE LOOSE 

 MASSES UPON ITS SURFACE. 



It is scarcely necessary to remark that gravitation is 

 manifested in this case by the falling of bodies, and by 

 their weight. That there is no one downward direction, 

 into which all bodies tend, is evident from the fact that 

 upon different sides of the earth, bodies fall in different 

 directions towards it. A stone in America, and another 

 in Asia, falling to the ground, will move directly towards 

 each other. All these motions are evidently the result 

 of a tendency of all bodies to move towards the great 

 mass of matter constituting the earth. 



The air gravitates ; that is, is attracted by the earth, and 

 rests with weight upon it. That portion of the air which 

 is near the surface is loaded with the burden of all that is 

 above, and is compressed by it in a much smaller space 

 than it would naturally occupy. This pressure produces 



