580 TEE POPULAR SCIENCE MONTHLY 



fluctuations in the quantity of this gas may go far to explain the 

 noteworthy glacial episodes of later and earlier geological times, and 

 those warm climates which at other periods have spread so widely over 

 the earth. 



Thus we have geographic hypotheses, and astronomic hypotheses, 

 so it seems appropriate that we should have atmospheric hypotheses, 

 in the laudable effort to understand and explain that great series of 

 geologic climates, which may indeed seem remote, but with the latest 

 of which, we should remember, we ourselves have to do. 



Our interest in the evolution of the atmosphere and of climate is 

 of no theoretical sort. "We are not in the grip of forces which are in a 

 despotic way our masters. We have a large control of organic life on 

 the earth and of the disposition and character of all land waters. 

 Through these means we also largely regulate the processes of denuda- 

 tion and we may thus in some measure modify the very constitution 

 of the atmosphere. Van Hise, on what he regards as a moderate esti- 

 mate of the coal the human race will burn per annum during the 

 present century, estimates that in 812 years the amount of carbon 

 dioxide in the atmosphere would be doubled. 5 According to the 

 view of Arrhenius such a change would greatly ameliorate the 

 climate of the world. This view of the heat-holding effects of an in- 

 crease of COo is not undisputed, but so large a change in the constitu- 

 tion of the atmosphere, by the hand of man himself, may well cause 

 him to investigate, with serious persistence, the terrestrial conse- 

 quences of his own deeds. 



Van Hise has impressively set forth the work of man in lowering 

 the level of the ground water. We do this by deforestation, by cultiva- 

 tion, by irrigation, by the sinking of artesian wells and by mining. 

 For so many purposes man needs water, or needs to get rid of water, 

 that actual and serious lowering of the water table has taken place, 

 and will be brought about more and more with growing density of 

 population. Lowering the ground water, as we have seen, increases the 

 contact of the atmosphere with the rocks, and sets in motion a chain 

 of actions which may have consequences for good or ill, quite outside 

 our present knowledge and inviting expert investigation for many 

 years to come. These considerations have a bearing, possibly an immi- 

 nent bearing, upon all our conservation enterprises in the United 

 States. 



Scientific truth often gives us no inkling of astonishing practical 

 results which are about to flow from it. Thus meteorology is no re- 

 stricted theme for the curious. It is not merely a science of climate, 

 though this would give it the highest interest for science and for life: 

 it is profoundly related to the history of our planet and it is an essen- 

 tial part of physical, biological and human geography. 



6 " Treatise on Metamorphism," p. 464. 



