EAINS AKD RIVERS. HI 



little improved. Eecords as to the amount of water daily 

 evaporated from a plate or dish on shore afford iis no means of 

 judging as to what is going on even in the same latitude at sea. 

 Sea- water is salt, and does not throw off its vapour as freely as 

 fresh water. Moreover, the wind that blows over the evaporat- 

 ing dish on shore is often dry and fresh. It comes from the 

 mountains, or over the plains where it found little or no water 

 to drink up ; therefore it reaches the observer's dish as thirsty 

 wind, and drinks up vapour 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 obsei-vations of 

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



281. Mivers are gauges for the amount of effective evaporation. — 

 There is no physical question of the day which is more worthj^ 

 of attention than the amount of effective evaporation that is daily 

 going on in the sea. By effective I mean the amount of water 

 that, in the shape of vapour, is daily transferred from the sea to 

 the land. The volume discharged by the rivers into the sea 

 expresses (§ 270) that quantity ; and it may be ascertained with 

 considerable accuracy by gauging the other great rivers as I 

 procured the Mississippi to be ganged at Memphis in 1849. 



282. Importance of rain and river gauges. — The monsoons 

 supply rains to feed the rivers of India, as the north-east and 

 south-east trade-winds of the Atlantic supply rains to feed 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, especially 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 

 Eiver, it would not make a difference 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. Hypsometry in the North Atlantic peculiar. — That part of 

 the extra-tropical North Atlantic under consideration is j)eculiar 



