40 On the Tidal Retardation of the Earth's Rotation. 



subsequent movements are coerced by them. Except througn 

 the winds it has no point d'appui upon the solid earth. Now 

 the movements of the winds, however largely modified by the 

 form and rotation of our earth, mainly depend upon differences 

 of temperature produced by the sun's heat ; it would seem there- 

 fore that, so far as the kinetic energy of the ascending particle 

 of vapour is concerned, it may or may not affect, and, if at 

 all, very slightly, the horizontal motions of the winds, but can 

 have no effect upon the rotation of the earth. 



The case is different, however, as soon as the particle of vapour 

 raised by molecular forces to the level of a mountain-top is pre- 

 cipitated thereon as rain or snow, and begins to descend again 

 towards the ocean whence it came : at every foot of its descent 

 it parts with kinetic energy, which it transfers directly to the 

 earth as a whole. On the other hand, such particles of vapour 

 as assumed the form of rain or snow at greater or less elevations, 

 and fall directly as rain-drops to the sea-level, can produce no 

 effect in accelerating the earth's rotation, each drop being co- 

 erced in its movements until within a short distance of the earth 

 by the winds — that is, by the same molecular forces which raised 

 them up. 



If this speculation be admissible, then we have a source of 

 sensible acceleration to the earth's rotation in the vast volume of 

 water which is precipitated upon the dry land and runs off into 

 the ocean. Adopting Gardner's estimate of the surface of the 

 land, exclusive of the antarctic continent, and assuming a mean 

 annual rainfall for the whole earth of 60 inches per annum, and 

 that two thirds of the entire rainfall returns to the ocean by 

 streams and rivers, we have 23,891 cubic miles of water annually 

 precipitated and falling back into the ocean; and assuming the 

 mean height of the land to be about 1000 feet, this immense mass, 

 on reaching the ocean, has lost kinetic energy due to the difference 

 in velocity of rotation between the earth's mean radius at the 

 sea-level and the same plus 1000 feet, the portion of this which 

 is effective in producing acceleration depending upon the cosine 

 of the latitude. 



As respects the descent of solids from higher to lower levels, 

 there seems no room for doubt as to their tendency to produce 

 acceleration in the earth's rotation. It is true that at remote 

 epochs, when continents and mountains were originally ele- 

 vated, their uplifting tended to retard the earth's rotation, and 

 that their complete ablation could do no more than restore the 

 energy of rotation the earth had before their upheaval. But 

 the ocean-bed was depressed; and its area is four times that of 

 the land, and its mean depth probably greater than the mean 

 height of the continents ; if, therefore, we assume the present 



