330 . Of TIDES. 



mass into a perfect sphere : now, if any power be supposed to act 

 on all the particles of this sphere with equal force, and in parallel 

 directions, the whole mass would be moved together without expe. 

 riencing any alteration in its figure. But this is not the case with 

 respect to the moon's 'action on our globe : the power of gravity 

 diminishes as the square of the distance increases, and therefore 

 the waters on the side of the earth next the moon are more attracted 

 by the moon than the central parts of the earth, and the central 

 parts more attracted than the waters on the opposite side ; and 

 therefore the distance between the earth's centre and the waters on 

 its surface under and opposite to the moon will be increased. For, 

 suppose three bodies in the same line, if they are all equally at- 

 tracted by any power, they will all move towards it with equal 

 rapidity, their mutual distances continuing the same; but, if the 

 attraction of this power is unequal, the body most forcibly at- 

 tracted will move fastest, and their reciprocal distances \vill be pro- 

 portionally increased : thus, the power of gravitation acting un- 

 equally on the three bodies, the distance of the first from the se- 

 cond, and of the second from the third, will be increased in pro. 

 portion to the difference of the gravitating power at the distance 

 of the three bodies respectively : now, suppose a number of bodies 

 placed round the centre so as to form a fluid ring, unequally at- 

 tracted by some power, the parts nearest and furthest from this 

 power will have their distance from the centre increased, while the 

 sides of this ring, being nearly equidistant from the power, the 

 centre will not recede, but rather approach the centre, and form 

 an ellipsis. To apply this reasoning to the case under consideration, 

 while the earth, by its gravity, tends towards the moon, the water 

 directly below her will swell and rise gradually ; the water on the 

 opposite side will recede from the centre (or, more properly, the 

 centre will advance), and rise, or appear to rise, while the water at 

 the sides is depressed, and falls below the former level : hence, as 

 the earth revolves on its axis from the moon to the moon again in 

 21 hours 50 minutes, there will be two tides of ebb and two of 

 flood in that period. In consequence of the earth's motion on her 

 axis, the most elevated part of the water is carried beyond the 

 moon in the direction of the rotation, and continues to rise after it 

 has passed directly under the moon, not attaining its greatest eleva- 

 tion till it has got about half a quadrant further. It continues also 



