1875.1 179 [Shaler. 



and depressions of the land on the total rainfall, will, in the main, 

 arise from the following causes : — 



1. The narrowing or widening of the evaporation area. 



2. The restrictions put upon the passage of marine currents by 

 the form of the land. 



I am inclined to think that the first of these actions may have con- 

 siderable value. For instance, if the lands of the tropical regions 

 have been steadily increasing in area ever since an early geological 

 period, a proposition which could find a good deal of support, 

 then the total evaporation area and the consequent rainfall must have 

 been diminishing. Even supposing the Lyellian hypothesis to be 

 true, and the amount of land and water to remain the same, the total 

 evaporation would be greatly effected by changes which should ac- 

 cumulate the water area about the poles, or about the equator. 



The effects of the obstruction of oceanic currents are not less import- 

 ant than those just suggested. If, by any cause, as, for instance, from 

 the barring of the currents in their northward course, as the Japan 

 current is now barred, only on a more extensive scale, the oceanic 

 streams were kept more within the equatorial belt than at present, 

 the result would probably be a slight diminution of the total amount of 

 rainfall. 



These causes, though greatly affecting the distribution of rain, must 

 on the whole, have comparatively little effect upon the aggregate rain- 

 fall. I am inclined to think that the main cause must be sought in the 

 alterations in the heat which comes to the earth from the sun. Al- 

 though some importance has been attached to the accession of heat 

 from extensive and prolonged volcanic eruption, it does not appear 

 that this can be a great cause, for Mayer's computation shows that, at 

 present, the quantity of heat received from the earth's interior cannot 

 amount to more than one sixtieth of the total heat that comes to its sur- 

 face. But the variation in the supply from the sun is a possible cause 

 that has been but little considered. The fact that the limited time 

 that has elapsed since star maps have been made has shown us very 

 many variable stars of exceedingly different periods of variation, 

 some passing regularly through a cycle of change, and some varying 

 in what seems to be a paroxysmal manner, may well make us ques- 

 tion whether it, is not in the nature of stars to be variable, and 

 whether this variability does not belong to our sun as well. It should 

 be noticed that slight variations are probably more likely to occur 

 than great changes, and that to bring about great alterations in the 



