The Tides. 347 



The surface of the earth and of the sea is so 

 nearly spherical, that it may for the present be 

 regarded as such. This being granted, if we ima- 

 gine the moon A (PI. XXIX, fig. 121) situated 

 in any part above the surface of the sea at E, it 

 is evident that the water E will be attracted by 

 her more in that point than any other in the 

 whole hemisphere PEH ; there will of course 

 be a tide at E. 



For the same reason, the water at G will be 

 less attracted by the moon than any part of the 

 sea in the hemisphere PGH. The water then at 

 this part will be less affected by the moon than 

 at any other ; it will be therefore elevated on the 

 opposite side, and this will make a tide at G. 



By these means the surface of the whole ocean 

 will assume an oval form, the longest diameter 

 of which is EG, and the shortest PH. As the 

 moon then changes her position, by the earth's 

 diurnal motion, this oval figure will follow the 

 apparent place of the moon ; this therefore will 

 produce two tides in the course of 25 hours, as 

 before established. 



Such is the general theory of the tides. But 

 to explain it more fully, let us suppose the moon 

 to be at rest, and let us imagine the earth to be a 

 solid globe also at rest, covered however to a 

 certain depth with a homogeneous fluid, the 

 surface of which shall also be spherical.- Suppose 

 the particles of this fluid to gravitate, as in fact 

 they do, towards the centre of the earth, at the 



