54 DISCUSSIONS IN CLIMATOLOGY. 



present condition of Greenland. Were it not for the 

 ice, the summers of North Greenland, owing to the con- 

 tinuance of the sun above the horizon, would be as warm 

 as those of England; but, instead of this, the Greenland 

 summers are colder than our winters, and snow during 

 that season falls more or less nine days out of ten. But 

 were the ice-covering removed, a snow-shower during 

 summer would be as great a rarity as it would be with 

 us. On the other hand, were India covered with an 

 ice-sheet, the summers of that place would be colder 

 than those of England. 



When the high grounds of Scotland and Scandinavia, 

 with those of the northern parts of America, became 

 covered with snow and ice, and the eccentricity went 

 on increasing, a diminution of the Gulf Stream, and a 

 host of other physical agencies, all tending towards a 

 glacial condition of things, would be brought into 

 operation. This would ultimately and inevitably lead 

 to a general state of glaciation, without the aid of any 

 of those additional geographical changes of land and 

 water which some have supposed. 



The Mutual Reaction of the Physical Agents. — 

 Those who think that the agencies to which I refer 

 would not by themselves bring about a glacial con- 

 dition appear to overlook a most important and 

 remarkable circumstance regarding their mode of 

 operation, to which I have frequently alluded in 

 ' Climate and Time ' (pp. 74-77) and other places. The 

 circumstance is this: — The physical agencies in ques- 

 tion not only all lead to one result, viz. an accumulation 

 of snow and ice, but their efficiency in bringing about 

 this result is actually strengthened by their mutual 

 reaction on one another. In physics the effect reacts 

 on the cause. In electricity and magnetism, for ex- 

 ample, cause and effect in almost every case mutually 



