§ 281-283. RAINS AND RIVERS. 1()5 



little or no water to drink up ; therefore it reaches the observer's 

 dish as thirsty wind, and drinks up vapor from it greedily. Now 

 had the same dish been placed on the sea, the air would come to 

 it over the water, drinking as it comes, and arriving already quite 

 or nearly saturated with moisture ; consequently, the observations 

 of the amount of evaporation on shore give no idea of it at sea. 



281. There is no physical question of the day which is more 

 Rivers are gauges for w^orthy of attcutiou than the amount of effective 



the amount of effect- . i • n • i • • i 



ive evaporation. cvaporatiou that IS daily going on m the sea. By 

 effective I mean the amount of water that, in the shape of vapor, is 

 daily transferred from the sea to the land. The volume discharged 

 by the rivers into the sea expresses (§ 270) that quantit}^ ; and it 

 may be ascertained with considerable accuracy by gauging the 

 other great rivers as I procured the Mississippi to be gauged at 

 Memphis in 1849. 



282. The monsoons supply rains to feed the rivers of India, as 

 Importance of rain ^hc uorthcast and southcast trade- winds of the At- 

 and river gauges. Jaiitic supply raius to fccd the rivers of Central and 

 South America. Now rain-gauges which will give us the mean 

 annual rain-fall on these water-sheds, and river-gauges which 

 would give us the mean annual discharge of the principal water- 

 courses, would afford data for an excellent determination as to the 

 amount of evaporation from some parts of the ocean at least, es- 

 pecially for the trade-wind belts of the Atlantic and the monsoon 

 region of the Indian Ocean. All the rain which the monsoons 

 of India deliver to the land the rivers of India return to the sea. 

 And if, in measuring this for the whole of India, our gauges 

 should lead us into a probable error, amounting in volume to half 

 the discharge of the Mississippi Kiver, it would not make a diffet - 

 ence in the computed rate of the effective daily evaporation from 

 the North Indian Ocean exceeding the one two-thousandth part 

 of an inch (0.002 in.). 



283. That part of the extra-tropical North Atlantic under con- 

 iiypsometry in the sidcratiou is Dcculiar as to its hvpsometrv. It is 



North Atlantic pe- i i • 



cuiiar. travcrscd by large icebergs, which are more favor- 



able to the recondensation of its vapors than so many islets would 

 be. Warm waters are in the middle of it, and both the east and 

 the west winds, which waft its vapors to the land, have, before 

 reaching the shores, to cross currents of cool waters, as the in- 



