23 



hemisphere, was 15 20'; so that it had a radius of half of this, 

 or 7 40', from the pole as centre. The southern cap was 

 observed in 1830, and its least diameter was 5 46', (or a radius 

 of 2° 53';*) and Schiaparelli states that it has once been seen to 

 disappear. The snow-caps grow visibly smaller, almost from 

 day to day, under the summer sun. This shows that the depth 

 of the snow cannot be very great. Their diminution, as Mr. 

 Wallace remarks, is exactly similar to the rapid melting of the 

 snow on the plains of Siberia and North America. If astro- 

 nomers in Mars were to watch the earth, they would see the 

 snow disappearing every summer from extensive tracts of land 

 round the North Pole, while the ice-cap which surrounds the 

 South Pole and covers the Antarctic Continent would be always 

 seen of the same size. This would be very perplexing, and 

 would perhaps give rise to doubts whether the white surfaces 

 that they saw around the earth's poles were of snow at all, until 

 it was seen that the southern snow-cap was surrounded by a 

 bluish expanse which was probably a vast ocean, and it would 

 be obvious that great fields of snow cannot be formed and dis- 

 appear again over water as they can over land. 



When it became known that glaciers and icebergs once 

 extended over great regions now free from them, it was an 

 obvious guess to make, that the sun must at that time have 

 been colder than at present. This however seems impossible ; 

 for no agency appears to exist in nature, or to have ever existed 

 since the first condensation of the sun out of the primary 

 nebula, whereby his stock of heat can have been greatly in- 

 creased. The idea at one time gained currency, that the sun is 

 constantly receiving supplies of heat by the falling in of meteors 

 from external space on to his surface ; — I think there is good 

 reason to believe that this is the source of a sensible though 

 small part of the heat which he is always radiating away ; — but 

 if we suppose that the sun was much colder 100,000 years ago 

 than now, we must suppose that the supply of in-falling meteors 

 has increased during that time so as to supply a large part of 

 his present heat ; and the existence of so great a quantity of 



* Edward Carpenter, in the Geological Magazine, March, 1877. 



