342 THE REVOLUTIONS OP THE CRUST OF THE EARTH. 



and raises the temperature of the regions situated to the north of this 

 line, and that, consequently, notwithstanding the astronomical compen- 

 sation of J. Herschel, one of the hemispheres is heated at the expense of 

 the other, by the intervention of the fluid envelope of the globe. 



During the glacial period, the existence of which is no longer dis- 

 puted, the climatic state was reversed ; the aerial and oceanic currents 

 (we will see in what condition) must have been stronger on the northern 

 coast. The principal motor of the Gulf-Stream must not only have been 

 less powerful, bat the winds from the north being also more violent, the 

 current had to contend against the efiect of contrary winds. Instead, 

 then, of dividing at Cape Saint Eoque, the equatorial current could 

 pass entire into the South Sea, and along the eastern coast of South 

 America, to heat the Antarctic pole. 



But, excepting the periodical influence of the refrigerating causes 

 upon the climatic conditions of the earth and upon the distribution of 

 currents, it is impossible to know to how remote a period the action of 

 the Gulf-Stream extends ; for a modification of the continents might 

 have here opened a new passage for the marine waters or closed another 

 which had for a long time existed, creating thus the great variety in 

 climatic conditions, manifested to us by paleontology and geology 

 Thus the Madagascan flora seems to indicate, by its Indian character, 

 notwithstanding its j)roximity to Africa, that at a period quite recent, 

 an oceanic current starting from India diverged toward Madagascar,* 

 the reverse of what takes place to-day. 



If the Gulf-Stream were reduced only one-half, this alone would pro- 

 duce a lowering of the mean temperature of all the northern part of 

 Europe, and the glaciers would descend to the level of the sea. The 

 temperature, which would be lowered enough for such an extension of 

 the glaciers, need not fall more than G'^ centigrade. 



Not only cold but also humidity is necessary to the formation of large 

 glaciers, and also to the lowering of the mean temperature of the year; 

 for it is pfincipally the snow accumulated during the winter which, by 

 forming, so to say, large stores of cold, prevents the solar heat from 

 raising the mean temperature of the summer. It is, as we have said, 

 the upper currents, contrary to the trade-winds, which carry the humid- 

 ity with which they are charged, to the polar regions, where it is precip- 

 itated in the form of snow. 



The enormous evaporation from the tropical seas, estimated at more 

 than five meters a year, produces oceanic currents analogous to the 

 aerial currents. The ambient and subjacent strata replace without ceas- 

 ing the evaporated water. This produces a general movement of the 

 cold water toward the equator of heat, which varies with the seasons. 

 The waters may be either favored or impeded in this movement by the 

 trade- winds, and the latter are generally impelled toward the hemis- 

 phere in which the mean temperature is highest. 



• ' Richard, Elemenin de Botanique, ltfH4, p. o47. 



