CONDITIONS AFFECTING PRECIPITATION. 193 



however, that this decrease of the ice-masses has been attended with any of 

 those floods which we are told by geologists must have been synchronous 

 with the meUing of the great glacier. On the contrar}-, complaints, not 

 unsupported by statistical data, have been going on all this time that the 

 rivers fed by this region of disappearing glaciers were themselves, if not 

 disappearing, at least diminishing in volume. 



Much more might be added in illustration of the position here taken, that 

 the diminution of the lakes and rivers and the general dryiug-up of the 

 earth, described in the preceding sections of this chapter, are not to be ex- 

 plained by a simple reference to the melting of a " great glacier." But it 

 is thought that, if enough has not already been said on this sulyect, the next 

 chapter will furnish sufficient additional matter in refutation of this idea, 

 shown to be so commonly held by geologists, — at least in this country, — 

 and that we may now proceed with the discussion of the subject of desicca- 

 tion, with a willingness on the part of our readers to hear what there is to 

 be said in regard to causes of climatic chancre far more general in their 

 action and important in the results they have brought about than any im- 

 plied in the melting of the ice over certain areas of very limited extent 

 compared with the entire land-surface of the globe. 



Sectiox IV. — Examination of the Conditions favoring, or tending to diminish, 

 Precipitation upon the Earth's Surface. 



As a preparation for what is to follow in this chapter and the succeeding 

 one, it will be desirable, at the present stage of this discussion, to consider 

 — although necessarily in a somewhat brief manner — the causes influencing 

 precipitation on the earth's surface, both as to its entire quantity, and the 

 manner in which it is locally distributed, whether in the form of rain or 

 snow. What is now to be said will also have a bearing on, and be an assist- 

 ance to, the understanding of portions of that which has been already set 

 forth in the preceding pages of this volume. 



Precipitation is the indirect result of evaporation, and the amount of water 

 which falls as rain or snow is entirely dependent on that taken up from the 

 surface of running or standing water by the agency of the sun's heat. From 

 all moist surfaces evaporation is continually abstracting the water, but with 

 very different degrees of rapidity according to varying conditions. The 

 principal agent accelerating evaporation is increase of temperature. " By 



