PHYSICAL ASTRONOMY. 333 



The force drawing the water horizontally is not 

 included here ; its tendency is to increase the ef- 

 fect just calculated. 



The force , by which the solar force everywhere 



increases gravity, need not be taken into account, 

 and does not affect the equilibrium of the water. 



The height to which the sea would thus rise at high- 

 water, above the level which it would stand at if 

 acted on by gravity alone, is twice as great as its 

 depression under that level at low-water. This is 

 easily demonstrated from the content of the sphe- 

 roid remaining always the same. 



337. The preceding is sufficient to shew, that 

 the phenomena of the tides are effects that might 

 be expected from the principle of gravitation. 

 It is, however, an approximation from which ex- 

 act results cannot be obtained, since a material 

 element has been left out, namely, the motion 

 of the water, on which the forces of the Sun 

 and Moon are exerted. 



338. The rapid motion of the waters, in 

 sequence of the diurnal rotation, prevents them 

 from assuming, at every instant, the figure 



which 



