of the Earth's Rotation. 39 



which is as directly derived from gravitation as is that of a 

 water-mill upon a mountain-stream. The water is raised in the 

 former case by gravitation towards the moon and by gravitation 

 falls back towards the earth ; in the latter it is raised by evapo- 

 ration, and falls back to the sea by gravitation. It is true that 

 the earth's and the moon's rotation are " inseparable accidents " 

 to the rise and then the fall of the surface of the sea at any 

 particular point; but the source of the power is derived, not 

 from the mechanism nor at the expense "of the earth's rotation," 

 but as directly from gravitation as is the case in any ordinary 

 mill-stream. If I am wrong in this I shall gladly accept cor- 

 rection. My chief object here, however, is to ask whether 

 (assuming the actuality of retardation of rotation by the tidal 

 wave acting as a brake, and be its amount more or less) there 

 may not be other forces in action upon our globe tending to 

 countervail this to a greater or less extent. It seems to me that 

 there are — though, so far as my reading goes, I have not seen 

 any notice of such on the part of physical writers. Every particle 

 of matter (rotating as part of our earth) which descends from a 

 higher to a lower elevation, must in doing so part with kinetic 

 energy proportionate to its decrease in velocity of rotation between 

 its higher and its lower positions, and the energy so lost is trans- 

 ferred at the lower point to the earth itself. Every drop of water, 

 therefore, every Hake of snow that precipitates upon the higher 

 parts of our globe, if assumed to reach these points without 

 relative velocity, must in descending to the sea-level tend to 

 accelerate the earth's rotation. So also every block of ice or of 

 stone that descends from the mountain-tops, every particle of 

 detritus carried along from higher levels towards the sea, must 

 have the same effect. With regard to the first it may be said, 

 every particle of water raised by evaporation from the surface of 

 the ocean ascends into the atmosphere with only a velocity of 

 eastward rotation due to the earth's radius at the sea-level, and 

 at the latitude at which it is taken up, and that therefore when 

 precipitated upon some much higher level it takes away from 

 the earth as much kinetic energy as it returns to it in descending 

 in streams again to the sea-level. But is this so? What 

 actually passes when a particle of sea-water at the surface of 

 the ocean, parting with its salt, rises therefrom under the in- 

 fluence of the sun's heat, and becomes an invisible vapour 

 held in suspension by the air, is to a great extent still un- 

 known. The particle of water, whatever be its physical con- 

 dition on leaving the liquid surface, undoubtedly only possesses 

 the velocity due to its low position upon the earth's surface ; 

 before it has risen even a fraction of an inch, however, it is taken 

 possession of by the air (that is to say, by the winds) j and all its 



