110 rnTSICAL GEOGKAPHY OP THE SEA, AND ITS .^lETEOROLOGY. 



meeting the Koiith-east trade-wind, the two form clouds wliich 

 give rain not only to Central America, but they drop down, also, 

 water in abundance for the Atrato, the Magdalena, the Orinoco, 

 the Amazon, and all the great rivers of intertropical America ; 

 also for the Senegal, the Niger, and the Congo of Africa. So 

 completely is the rain wrung out of these winds for these 

 American rivers by the Andes, that they become dry and rainless 

 after passing this barrier, and as such reach the western shores of 

 the continent, producing there, as in Peru, -a rainless region. 

 The place in the sea whence our rivers come, and whence Europe 

 is supplied with rains, is clearly not to be found in this part of 

 the ocean. 



278. The calm belt of Cancer furnishes little or no rain. — Between 

 the parallels of SO"" and 35° N. lies the calm belt of Cancer, 

 a region where there is no ^prevailing wind (see Diagram of the 

 winds, Plate I.). It is a belt of light airs and calms — of airs so 

 baffling that they are often insufficient to carry off the "loom," 

 or that stratum of air, which, being charged with vapour, covers 

 calm seas as with a film, as if to prevent farther evaporation. 

 This belt of the ocean can scarcely be said to furnish any vapour 

 to the land, for a rainless country, both in Africa, and Asia, and 

 America, lies within it. 



279. The North Atlantic insufficient to supply rain for so large a 

 portion of the earth as one-sixth of all the land. — All Europe is on 

 the north side of this calm belt. Let us extend our search, then, 

 to that part of the Atlantic which lies between the parallels of 

 35° and 60° N., to see if we have water surface enough there to 

 supply rains for the 8J- millions of square miles that are em 

 braced by the water-sheds under consideration. The area of 

 this part of the Atlantic is not quite 5 millions of square miles, 

 and it does not include more than one-thirtieth of the entire sea 

 surface of our planet, while the water-sheds under consideration 

 contain one-sixth part of its entire land surface. The natural 

 proportion of land and water surface is nearly as 1 to 3. 

 According to this ratio, the extent of sea surface required to give 

 rain for these 8^ millions of square miles would be a little over 

 25, instead of a little less than 5 millions of square miles. 



280. Daily rate of evaporation at sea less than on land — observa- 

 tions wanted. — The state of our knowledge concerning the actual 

 amount of evaporation that is daily going on at sea has, notwith- 

 standing the activity in the fields of physical research, been but 



