THE REVOLUTIONS OF THE CRUST OF THE EARTH. 349 



must have been four times greater, and this inequality of heat received 

 and lost bj radiation, combined with the < ther atmospheric causes of 

 which we have spoken, would produce a considerable cooliug of one 

 hemisphere, which would be followed, as a consequence, by great 

 extension of the polar glacier. The humidity necessary for the accumu- 

 lation of the ice is furnished, as we have seen, by the upper aerial cur- 

 rents contrary to the trade- winds. The precipitation of this humidity 

 necessitates renewed currents, and the entire glacier, when it attains 

 certain dimensions, acts as a drain upon the neighboring atmosphere 

 with force in proportion to its size. The long-continued precipitations 

 of the humidity in the form of snow, unless a considerable portion melt, 

 must at last increase the glacier to such a degree that it will influence 

 the center of gravity of the earth, and produce a change in the position 

 of the fluid matter of the globe. 



The height of the accumulation necessary to i>roduce this change 

 might be calculated, were the volume of the waters known, their eccen- 

 tricity, the extension of the two glaciers, the center of gravity of the 

 earth, and that of the fluid mass. But as only a very uncertain esti- 

 mate can be made of the mean depth of the seas, the result of such an 

 attempt can only approximate reality. M. Adh6mar compares the vol- 

 ume of the austral glacier to a stratum of ice, 20 leagues in height and 

 extending uniformly like a cap over the sphere, having for its base the 

 seventieth parallel. As the glacier at the outside edges is rarely less 

 than a thousand yards in height, we may suppose that near the pole it 

 is double that thickness. This enormous stratum of ice, which eflaces 

 the flattening of the earth at the poles, would be represented upon a 

 globe, of the radius of a meter, by an elevati!»n of 0°^.0214. 



Were we to reduce this mass one-half, one-quarter, or even more, its 

 action would still be sufficient to produce a submersion of a large part 

 of the continents of the boreal hemisphere, if on the return of the con- 

 ditions of cold the north pole was covered with a cupola of ice anal- 

 ogous to that of the south pole. As these conditions of temperature 

 change every 10,500 years, by virtue of the precession of the equinoxes, 

 it must be at the end of this period of time that the waters are removed 

 from one hemisphere to the other. This removal commences with the 

 melting of the greater glacier and the increase of the small one. This 

 action is at first very slow, but it progressively acquires great rapidity, 

 and when, in the general breaking up of the ice, the lower stratum which 

 rests upon the bottom of the ocean is carried away, the waters are dis- 

 persed with such velocity that the sedimentary surface is deeply fur- 

 rowed and large blocks are transported to a great distance. It is easy 

 to see that thus great currents must be produced twice from the north 

 to the south, and twice from the south to the north, in the period of 

 21,000 years. 



In all the hypotheses of M. Adh^mar, the only conjectural point is the 

 thickness of the austral glacier; the other facts are so evident that they 



