THE REVOLUTIONS OF THE CRUST OF THE EARTH. 337 



day, and could not, consequently, raise the temperature of the atmos- 

 phere, we may say that the modification of climate depended in part 

 upon the distribution of seas and continents, but principally upon solar 

 heat, and the geographical position of a region. Starting with these 

 facts we will endeavor to find the cause which may have changed the 

 climatic conditions and produced glacial periods. 



The geographical position of a place has great influence upon the 

 nature of its climate; and this influence would be much greater if the 

 distribution of continents and of seas was uniform. Since this is not 

 the case the mean temperature of different parts of the globe, notwith- 

 standing their geographical position, may be perfectly identical, 

 although in two distinct latitudes; and very different although in the 

 same position in regard to latitude. Proximity to the sea renders a 

 climate more uniform. Thus in Ireland, plants of Southern Europe 

 grow in the open air, which would freeze in the same latitude in Ger- 

 many. On the other hand, the summers of Ireland are rainy and even 

 cold enough sometimes for snow, when in Germany the vines ripen 

 upon the Khine, and the summers are warmer than at Home. 



The continental climates are more extreme in the two opposite seasons, 

 which is shown by the enormous difference between the two limits of tem- 

 perature at Moscow, equal to about 70° Centigrade. The winters of Pekin 

 are as severe as those of Upsal, while the summers are as burning as 

 those of Cairo. Mr. Lyell thinks the conversion of the sea into dry- 

 land, and the land into sea, the increase and diminution of the height 

 of the mountain-chains and of continents, the predominance of land or of 

 water in high and low latitudes, and, finally, a new direction given to the 

 currents of the ocean, such as the Gulf Stream, were changes of a nature 

 to modify the climates of the globe.* We cannot deny that a change in 

 the distribution of continents and of seas would be followed by an equal 

 change in climates, and Mr. Frankland has shown t that if the continents 

 were distributed about the poles, while the sea formed a belt around the 

 equator, the water, the most proper vehicle of heat, would warm the polar 

 regions at the expense of the equatorial zone ; but the periodic return 

 of the same phenomena of refrigeration appears to proceed from laws 

 more simple and constant. 



As the sun is the principal source of heat for all the surface of the globe, 

 certain causes, which modify its calorific intensity, equally affect the cli- 

 matic conditions of the earth. This modification may take place in two 

 ways. First, the intensity of the solar heat may itself vary. In fact 

 it has been proved that the solar spots are subject to a periodic return, 

 and by enfeebling the calorific power of the sun, sensibly affect the tem- 

 perature of the terrestrial surface. There is nothing, then, untenable in 

 the supposition that the sun in its quality of a variable star was formerly 

 subject to greater physical perturbations, and that, consequently, the 



* Lyel], Elements of Geology, vol. i, p. 287. 

 t Phil. Magaz., August, 1864. 

 S 22 



