

716 DYNAMICAL GEOLOGY. 



was in aphelion in the northern winter, the circumstances would be 

 reversed, and the warm waters would thus be " wholly withdrawn from 

 the northern hemisphere," causing snow and ice to accumulate in the 

 north. Further, since the cycle of the seasons is 21,000 years, and 

 therefore short compared with a time of maximum eccentricity, there 

 may be two glacial eras in one maximum with an era of unusual 

 warmth in the interval between ; and, besides, the glacial era of the 

 southern hemisphere would occur 10,500 years after the northern, 

 that is, when aphelion came in the southern winter. 



While it is certain that a period of maximum eccentricity would be 

 favorable for glacial conditions, it is not admitted by all physicists that 

 facts sustain the conclusion respecting the winds and oceanic currents 

 set forth by Mr. Croll, or make the cold sufficient for so extreme re- 

 sults. 



4. With the Changes from Maximum to Minimum in the Spots on the 

 Sun's Surface. — These changes have a cycle of about 11 years, the 

 minimum occurring on the year 1 of the century, and the year 1878 be- 

 ing therefore at the minimum. How far this cycle is a cycle of chang- 

 ing temperature* to the earth is not ascertained. 



5. Owing to a progressing Dimunition in the Sun's Heat, or to 

 Changes in the condition of the Sun's Surface. — As the Sun, like 

 other heated spheres, has been losing heat by radiation through all 

 time, and probably without any corresponding supply, the earth must 

 receive less heat now than in Archaean time ; and its greater heat dur- 

 ing the early geological ages may be a chief cause of the warmer cli- 

 mate then over the globe. 



6. Owing to Changes in the Density of the Earth's Atmosphere. — 

 The atmosphere absorbs and retains heat, and the amount absorbed 

 increases with its density. In early geological time the earth had a 

 denser atmosphere than now, through the presence of more carbonic 

 acid and more moisture, and hence it would have absorbed more of the 

 sun's rays as they passed through it. 



For a like reason, the earth's lower plains are warmer than its ele- 

 vated regions, the atmosphere being rarer above. The lowest places 

 should thus have the warmest climate ; and accordingly the basin of 

 the Dead Sea, 1,308 feet below the sea-level, has the heat of the tor- 

 rid zone. 



2. Internal Heat A. The proofs of the existence of a source of 



heat within the earth, usually appealed to, are the following : — 



(1.) The spheroidal form of the earth (p. 9). 



(2.) Borings for Artesian wells and shafts in mines have afforded a 

 means of taking the temperature of the earth at different depths. It 

 has thus been found that, after passing the limit of surface-action, the 



