104 THE OCEAN. [Book IV. 



manner as that in which astral gravitation opposes 

 terrestrial gravitation it must also oppose the action of 

 solar and lunar gravitation, causing a counter-move- 

 ment for each movement caused by the latter forces. 

 The movements and the counter-movements in each 

 case equally result from the effort of vis-inertia to 

 maintam the equilibrium of the ocean. 



In the same manner as vis-inerti^e creates the 

 movements and counter-movements which circulate 

 the ocean, as described in Book II., in consequence 

 of inequalities in the action of terrestrial and astral 

 gravitation, it must also, as shown in Book III., 

 create movements and counter- movements in conse- 

 quence of inequalities in the action of solar or lunar 

 and astral gravitation. The consideration of the 

 action of vis-inertise thus leads us to a considera- 

 tion of the effects of solar and lunar gravitation on 

 the ocean, as forming a part of that action of vis- 

 inertise. 



Let us now consider the effect of the gravitation 

 of the sun and moon in their actual positions. 



For this purpose let s, in Plate XIIL, represent the 

 sun, E the earth, and m the moon. Then, first, as 

 regards the solar tides : if the sun and earth be at 

 rest, the sun's gravitation raises a tide (cd in Fig. 1) 

 on that part of the earth unmediately under the sun, 

 and astral gravitation raises a tide (f g) on exactly 

 the opposite side of the earth. 



The sun, however, rotates eastwards on its axis, 



