METEOROLOGY. 395 



back to periodic variations in the temperature of the sea. He concludes 

 that when the winter agrees with the earth's aphelion the currents will 

 probably increase, and when the winter agrees with the perihelion the 

 currents will slightly diminish ; therefore at present northwestern Eu- 

 rope is enjoying relatively gentle rains and a large difference between 

 the winter and summer temperatures, which conditions will be reversed 

 in about 10,000 years with the change in position of the earth's axis. 

 In other regions than Europe the effect of this change may be different. 

 Thus the eastern part of North America will, with wii-ter in the aphel- 

 ion, have stronger northwest winds, and therefore a more severe climate^ 

 the same for Eastern Asia, while the western coast of North America 

 will be affected like Europe. But such changes must also affect other 

 portions of the ocean and of the world, causing periodic'changes in the 

 climate over the whole earth's surface. {Z. 0. G. M., xix, p. 413.) 



441. Dr. Penck, of Munich, in some remarks on the glacial epoch, says 

 a numerical expression for the difference between the climate of the 

 glacial epoch and that immediately before and after it is found by a 

 comparison of the limit of perpetual snow for that and the present 

 epoch. This limit was at that time decidedly lower than the present, 

 namely, at 45° N. latitude 1,300 meters lower, and at 20° K latitude, 1,000 

 lower. lu the southern hemisphere the probable depression of the 

 snow limit at 30° S. was 1,000 meters, and at 45° S. at least 800 meters 

 lower than now. During the glacial epoch the region of perpetual snow 

 in the northern hemisphere was lower than it is to-day in the southern, 

 and in the southern it was much lower than now. An interchange of 

 the climatic relations of the north and south hemispheres would not of 

 itself produce a glacial epoch. The remarkable agreement between the 

 intensity of the present glaciation and that of the glacial epoch justifies 

 the assumption that the latter occurred under general external relations 

 such as now prevail; in fact, it can be proven that the glacial epoch 

 occurred during the i^resent arrangement of land and water, and not 

 with a lower but rather a higher elevation of the land. The mass of 

 sea water that was at that time on the land in the shape of ice was 

 approximately 56,000,000 cubic kilometers. A characteristic of the gla- 

 cial epoch, the recurrence of the glaciation as recognized in northern 

 Europe and America, and which repetitions, as the Alps show, were 

 separated by epochs of mild climate whose durations were greater than 

 the time that has now elapsed since the last glaciation. The great ex- 

 tent of the glaciation is explained b}^ no other theories as to its cause 

 except Croll's theory of the alternate glaciation of both hemispheres, 

 and the theory of the existence of a sensible cold period over the whole 

 earth; but the periodicity of glaciation is explained only by Croll's 

 theory, so that this is the most probable, although great difficulties 

 stand in its way. (D. M. Z., i, p. 473.) 



