HEAT. 255 



the earth's history, though not making it the sole cause of glacial conditions, 

 or holding that such conditions would necessarily ensue. The heat received 

 during the summer and winter intervals being as 5 to 3, and the winter inter- 

 val 199 days long in an extreme case, the severe and prolonged cold of the 

 winters might, other things favoring, accumulate more snow than the short 

 summers could melt. This theory makes the Glacial period of the northern 

 hemisphere follow or precede that of the southern by 10,500 years ; that is, by 

 half of a revolution of the seasons (21,000 years). Moreover, the condition 

 of maximum eccentricity is so slow in passing that, according to this theory, 

 two or more glacial periods might occur in the course of one maximum. 



This subject and CroU's theory have been ably discussed in a volume of 180 pages, 

 entitled The Cause of an Ice-Age, by the Astronomer Royal of Ireland, Sir Robert Ball, 

 1891. The conclusion is reached that the conditions of a period of maximum eccentricity 

 are fully adequate to cause glacial periods in geological history. See also a notice of the 

 work by G. H. Darwin in Nature for January 28, 1892. 



Geology has no evidence in favor of the idea that the latest of Glacial periods occurred 

 in the southern hemisphere 10,500 years after, or before, the northern, and it has prob- 

 able evidence that the time of the Glacial period was not over 10,000 years since, and 

 therefore not nearly as far back as the maximum of 210,000 years since, or that of 

 100,000. Further, it has discovered no satisfactory traces of a second Glacial period, 

 corresponding to the extreme maximum 850,000 years since ; for it has good proof that none 

 occurred between the Glacial period and the epoch closing the Cretaceous period, some 

 millions of years since. It is admitted, however, that the calculation of the time to the 

 extreme maximum (850,000 years) is not wholly trustworthy. 



4. Progressing diminution in the sun''s heat. — Since the sun has been 

 radiating heat through all past ages, the earth must receive less heat now 

 than in Archgean time ; and the greater heat of the early geological ages may 

 have this as a chief cause. 



5. Changes in the condition of the suri's surface. — The changes from 

 maximum to minimum in the spots on the sun's surface have a cycle of 

 about 11 years, the minimum occurring in the year 1 of the century, and 

 the year 1889 being therefore at the minimum. How far this cycle is one of 

 changing temperature to the earth is not known. Other cyclical changes are 

 possible, and are conveniently assumed at times, though not proved. 



6. Changes in the position of the earth'' s axis of rotation. — ■ Mathematical 

 investigations by Lord Kelvin (Sir William Thomson), S. Haughton, G. H. 

 Darwin, and others, have shown this hypothesis to be of no geological value. 

 Darwin has demonstrated that a displacement of the pole of merely 1° 46' 

 would require that a twentieth of the whole earth's surface should be 

 elevated to a height of 10,000 feet, with a corresponding subsidence in 

 another quadrant ; and for one of 3° 17', that double the surface should have 

 undergone these great changes. Kelvin concludes from his discussion of the 

 subject that "there is no evidence in geological climate throughout those 

 parts of the world which geological investigation has reached to give any 

 indication of the poles having been anywhere but where they are, at any 

 period of geological time." 



