Page:Physical Geography of the Sea and its Meteorology.djvu/136

110 meeting the south-east trade-wind, the two form clouds which 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 30° 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 8½ millions of square miles that are embraced 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 8½ 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—observations wanted.—The state of our knowledge concerning the actual amount of evaporation that is daily going on at sea has, notwithstanding the activity in the fields of physical research, been but