A Study in the Nattiral History of Ice 289 



flank receives very little, and that only in the afternoon. On 

 the other hand, it is fully exposed to all the reflection and 

 reverberation from the steep rocky slope which rises directly 

 from the bottom of the valley and close to the ice-foot. It is, 

 however, difficult to admit that this heat of low intensity can 

 really be so much more efficacious in melting ice than the 

 direct rays of the summer sun. A glance at the two photo- 

 graphs shows that the ice-flank is very steep and faces 

 westward. This position not only limits the amount of 

 insolation which it can receive, it also affords it considerable 

 protection against loss of heat by radiation into space. Every 

 one knows that even on a hot summer day when the surface 

 of the glacier is seamed by countless rills of running water 

 and the sun-weathered ice-grains crumble beneath the boot, 

 if the sun goes behind the mountains the fountain of the 

 waters is closed almost immediately and the ice becomes 

 hard and unyielding. Surface melting has ceased for the 

 day. It is not impossible, but I have not checked this by 

 observation, that the protection from cold which the flank 

 enjoys may increase the amount of ice which it loses by 

 melting in the year. But whatever may be the explanation, 

 the distance, 3 5 '6 metres, is exact. It depends on measure- 

 ments made with the same 1 5-metre tape and involves no 

 greater possible error than two or three centimetres. In 

 September 1906 I fixed very exactly the position of the 

 northernmost extremity of the glacier. In September 1909 



1 placed myself again exactly on the same spot, and found 

 the ice-face only 5 metres distant. At the end of the glacier 

 the rate of apparent annual retreat of the ice was less than 



2 metres, while on its western flank it was nearly 9 metres. 



Dependence of the Melting Temperature of Ice on the Nature 

 of the Medium in which it Melts. 



From observations which I made on sea-ice in the Ant- 

 arctic Ocean in the " Challenger," which were followed up by 

 researches in the laboratory after returning home, and from 

 the study of much interesting work on the grain of the glacier 



B. 19 



