NATURAL HISTORY. 5? 



This motion is attended with the following circum- 

 stances : 1st, It is more apparent at the full and new 

 moon than at the quadratures. It is also more violent 

 in spring and autumn than in any other season ; and is 

 weakest at the solstices. This is owing to the com- 

 bined attraction of the sun and irioon. 2d, The direc- 

 tion and quantity of this motion is often varied by the 

 winds, particularly by those which blow incessantly 

 from the same quarter. 3d, It is worthy of remark, 

 that when one part of a fluid is moved, the motion is 

 communicated to the whole. During the tides, there- 

 fore, a great part of the ocean is put in motion ; and 

 the whole ocean, from top to bottom, is agitated at 

 the same time. 



In order to throw a little more light upon this sub- 

 ject, let us attend to the causes which produce the 

 tides. We may therefore remark that the moon acts 

 upon the earth by a power called gravity or attraction. 

 This power penetrates the whole globe, and is exactly 

 proportioned to the quantity of matter, and decreases 

 as the squares of the distances increase. Let us now 

 examine what effects this power must produce upon 

 the waters when the moon comes to the meridian oi 

 any place. > The surface of the water immediately 

 under the moon is therefore nearer that planet than 

 any other part of the earth, consequently that part of 

 the sea must be elevated towards the moon, and tire 

 summit of this eminence must be opposite to the moon's 

 centre. To produce this eminence, the waters upon 

 the surface, as well as those at the bottom, contribute 

 their share, in proportion to their distances from the 

 moon, which acts upon them in the inverse ratio of 

 the squares of their distances. Hence t! surface ef 



Vol. I. G 



