W. J. McGee — Geology of the Mississippi Valley. 419 



feet thick over the whole surface of the earth, provided it were 

 all employed in that direction and were uniformly distributed. 

 But the heat received by the frigid zones annually, per square foot, 

 is less than the mean received by the whole earth in the ratio of 

 5-45 to 9*83, according to Meech. 1 Therefore only 55 - 44 feet would 

 be melted from the frigid zones in a year, if all the solar heat 

 reaching them were utilized for that purpose. But Prof. Tyndall 

 tells us in the same paragraph that four-tenths of the solar heat is 

 absorbed by the terrestrial atmosphere, on an average. Now it is 

 universally conceded that the atmospheric absorption is greater in 

 polar than tropical regions ; but to be sure of erring on the safe 

 side, if at all, the mean mentioned by Tyndall may be taken as 

 representing the atmospheric absorption of the frigid zones. 

 Badiation and direct reflection from snow-covered regions may also 

 be disregarded, though important factors. Diminishing, therefore, 

 the solar power in arctic regions by four-tenths, we find that only 

 38-26 feet of ice could be melted annually from the frigid zones, if 

 all the heat were so employed. 



But normally all the solar heat is expended in raising the 

 temperature of the earth's surface to a certain point, which is too 

 low to remove all of the ice from polar regions. To keep the 

 temperature as at present, and at the same time remove 33'26 feet 

 of ice from the frigid zones, the solar action would have to be 

 doubled ; and, similarly, to remove any less quantity of ice, the 

 heat would have to be proportionally increased. The same effect 

 could be produced, however, by lengthening the summer and 

 shortening the winter. The whole amount of heat reaching the 

 earth, or any part of it, may be conveniently represented as 365 

 days. Each hemisphere may receive 365 days of heat ; but, owing 

 to telluric changes, the amount received by one may be augmented, 

 and that received by the other diminished. This is argued by Dr. 

 Croll, who, while admitting that a lengthening of summer is exactly 

 counterbalanced by the greater relative distance from the sun, 

 insists that a chain of physical agencies is brought into operation, 

 by which the heat actually rendered effective is made to about 

 equal, proportionally, the lengthening of the summer. It would be 

 absurd to consider it more ; and in truth it is probably less ; but it 

 may be assumed that it is proportional. During the last period of 

 great excentricity in the terrestrial orbit, which occurred about 

 210,000 years ago, there was a time, according to Croll, when the 

 northern summer was nearly 27 days longer than its winter ; and 

 it may be admitted that the whole northern hemisphere received 

 27 days more heat than the southern, or 13-5 days more than the 

 normal. This is the maximum for a period of about 12,000 years, 

 and the two minima are each zero, when the distribution of heat 

 over the two hemispheres is equal, as it is when the apsides coincide 

 with the equinoxes. The mean excess for the period of say 12,000 

 years would thus be only 6-75 days, or f-^f- of the total accession 

 annually. 



1 " Eelative Intensity of the Sun's Light and Heat," Smithsonian Contrihutions to 

 Knowledge, 1855, p. 35. 



