262 ATTEACTION OF BODIES. 



the opposite action of some other particle : and P is not at 

 all attracted by any part of the spherical surface ; or the 

 sum of all the attractions upon P is equal to nothing. So of a 

 hollow sphere ; for every such sphere may be considered as 

 composed of innumerable concentric spherical surfaces, to 

 each of which the foregoing reasoning applies; and conse- 

 quently to their sum. 



We may here stop to observe upon a remarkable inference 

 which may be drawn from this theorem. Suppose that in the 

 centre of any planet, as of the earth, there is a large vacant 

 spherical space, or that the globe is a hollow sphere ; if any 

 particle or mass of matter is at any moment of time in any 

 point of this hollow sphere, it must, as far as the globe is con- 

 cerned, remain for ever at rest there, and suffer no attraction 

 from the globe itself. Then the force of any other heavenly 

 body, as the moon, will attract it, and so will the force of the 

 sun. Suppose these two bodies in opposition, it will be 

 drawn to the side of the sun with a force equal to the dif- 

 ference of their attractions, and this force will vary with 

 the relative position (configuration) of the three bodies ; but 

 from the greater attraction of the sun, the particle, or 

 body, will always be on the side of the hollow globe next 

 to the sun. Now the earth's attraction will exert no influ- 

 ence over the internal body, even when in contact with the 

 internal surface of the hollow sphere ; for the theorem which 

 we have just demonstrated is quite general, and applies to 

 particles wherever situated within the sphere. Therefore, 

 although the earth moves round its axis, the body will always 

 continue moving so as to shift its place every instant and 

 retain its position towards the sun. In like manner, if any 

 quantity of movable particles, thrown off, for example, by the 

 rotatory motion of the earth, are in the hollow, they will not 

 be attracted by the earth, but only towards the sun, and will 

 all accumulate towards the side of the hollow sphere next the 

 sun. So of any fluid, whether water or melted matter in the 

 hollow, provided it do not wholly fill up the space, the whole 

 of it will be accumulated towards the sun. Suppose it only 



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