300 KELVIN 



the water took the figure of equilibrium. Now the water does 

 not cover the whole earth, as we have assumed in the dia- 

 grams (Pics. 152 to 155), but the surface of the water may be 

 imagined as taking the same figure, so far as there is water, 

 that it would take if there were water over the whole sur- 

 face of the earth. But here a difficult question comes in 

 namely, the attraction of the water for parts of itself. If we 

 consider the water flowing over the whole earth this attrac- 

 tion must be taken into account. If we imagine the water 

 of exceedingly small density so that its attraction on itself is 

 insensible compared with that of the earth, we have thus to 

 think of the equilibrium theory. But, on the other hand, if 

 the water had the same density as the earth, the result 

 would be that the solid nucleus would be almost ready to 

 float; and now imagine that the water is denser than the 

 earth, and we put the tides out of consideration altogether. 

 Think of the earth covered over with mercury instead of 

 water a layer of mercury a foot deep. The solid earth 

 would tend to float, and would float, and the result would be 

 that the denser liquid would run to, and cover one side up to 

 a certain depth, and the earth would be as it were floating 

 out of the sea. That explains one curious result that 

 Laplace seems to have been much struck with: the stability 

 of the ocean requires that the density of the water should 

 be less than that of the solid earth. But take the sea as 

 having the specific gravity of water, the mean density of 

 the earth is only 5.6 times that of water, and this is not 

 enough to prevent the attraction of water for water from 

 being sensible. Owing therefore to the attraction of the 

 water for parts of itself the tidal phenomena are somewhat 

 larger than they would be without it, but neglecing this, and 

 neglecting the deformation of the solid earth, we have the 

 ordinary equilibrium theory. 



Why does the water not follow the equilibrium theory? 

 Why have we tides of 20 feet or 30 feet or 40 feet in 

 some places, and only of 2 or 3 feet in others? Because 

 the water has not time in the course of 12 hours to take 

 the equilibrium figure, and because after tending towards 

 it, the water runs beyond it. 



I ask you to think of the oscillations of water in a trough. 



