THE CAUSE OF THE GLACIAL PERIOD. 303 



mous. To melt one cubic foot of ice requires as much heat 

 as would raise the temperature of a cubic foot of water 

 176° Fahrenheit. To melt a "layer of ice only one inch 

 and a half thick would require as much heat as would raise 

 a stratum of air eight hundred feet thick from the freez- 

 ing-point to the tropical heat of 88° Fahrenheit." It is 

 the slowness with which ice melts which enables it to 

 accumulate as it does, both in winter and upon high 

 mountains and in arctic regions. Captain Scoresby re- 

 lates that when near the north pole the sun would some- 

 times be so hot as to melt the pitch on the south side of 

 his vessel, while water was freezing on the north side, in 

 the shade, owing to the cooling effect of the masses of ice 

 with which he was surrounded. 



Thus it will appear that a change in the direction of 

 the moist winds blowing from the equator towards the 

 poles might produce a Glacial epoch. If snow falls upon 

 the ocean it cools the water, but through the currents, 

 everywhere visible in the sea, the temperature in the water 

 in the different parts soon becomes equalized. If, how- 

 ever, the snow falls upon the land, it must be melted by 

 the direct action of the sun and wind upon the spot where 

 it is. If the heat furnished by these agencies is not suffi- 

 cient to do it year by year, there will soon be such an 

 accumulation that glaciers will begin to form. It is clear, 

 therefore, that the conditions producing a Glacial period 

 are likely to prove very complicated, and we need not be 

 surprised if the conclusions to which we come are incapable 

 of demonstration. 



Theories respecting the cause of the Glacial period 

 may be roughly classified as astronomical and geological. 

 Among the astronomical theories, one which has some- 

 times been adduced is that the solar system in its move- 

 ment through space is subjected to different degrees of 

 heat at different times. According to this theory, the 

 temperate climate which characterised the polar regions 



