1875.] 181 [Shaler. 



The winter of 1874-75 having been unusually severe throughout 

 the northern part of the northern hemisphere, it seemed likely that 

 we should, if the foregoing hypothesis were true, find some trace of 

 the effects arising from this temporary increase of the difference 

 between the polar and equatorial conditions, corresponding to that 

 greater change which was assumed to have taken place during the 

 glacial time. In a word, if the average of rainfall is the result of 

 the trade and counter-trade winds, and their products the sea cur- 

 rents, and if these winds are measured in their force by the difference 

 of temperature between the equatorial and polar districts, then the 

 period of very low temperature which, in the winter of 1874-5, pre- 

 vailed throughout the northern hemisphere, should have brought a 

 season of great rainfall in its train. Allowing six months for the 

 completion of the trade wind circuit, we would expect the return of 

 this rush of counter-trades, with their load of water, in the midsum- 

 mer of 1875. It may have been only a coincidence, but it is a note- 

 worthy fact that this season was one of the rainiest ever known in the 

 northern hemisphere. It will at least make it desirable to compare 

 the winter and summer temperatures and the rainfall over a conside- 

 rable time. Following this same line of conjecture one step farther, 

 I may notice that the annual rainfall during the winter seasons imme- 

 diately preceding the extremely cold season of 1874-5 had been much 

 less than usual. This gives a basis for the hypothesis that one of the 

 cycles of change in climate maybe something like this: a progressive 

 diminution of rainfall in the circumpolar region, a consequent de- 

 crease of the cloud envelop of that region, and increased loss of heat 

 by radiation leading to an intensification of cold, and that in turn 

 bringing about an increase in trade winds and consequent greater 

 rainfall. This rainfall will bring up the polar temperature, diminish 

 the difference in heat between that region and the equatorial belt, 

 whence the trade winds will slacken, and the circumpolar rainfall 

 again diminish, bringing again increase of radiation and lowering of 

 temperature. 



This is, I acknowledge, highly conjectural, but in the present 

 state of the question of climate, while too much value must not be 

 given to conjecture, it may yet have some value. As regards the geo- 

 logical effect of rainfall, there is one point of considerable importance 

 to which attention has not yet been directed. I refer to the great dif- 

 ference in the rate of wearing at different points, due to the action o 

 the different rates of rainfall. In our own country, for instance, the 



