Page:Physical Geography of the Sea and its Meteorology.djvu/154

128 circulation to that part of the coast, though it be as heavily charged with moisture as at Patagonia, has a greater extent of country over which to deposit its rain, and, consequently, the fall to the square inch will not be as great. In like manner, we should be enabled to say in what part of the world the most equable climates are to be found. They are to be found in the equatorial calms, where the north-east and south-east trades meet fresh from the ocean, and keep the temperature uniform under a canopy of perpetual clouds.

300. Amount of evaporation greatest from the Indian Ocean.—The mean annual fall of lain on the entire surface of the earth is estimated at about five feet. To evaporate water enough annually from the ocean to cover the earth, on the average, five feet deep with rain; to transport it from one zone to another; and to precipitate it in the right places, at suitable times, and in the proportions due, is one of the offices of the grand atmospherical machine. All this evaporation, however, does not take place from the sea, for the water that falls on the land is re-evaporated from the land again and again. But in the first instance it is evaporated principally from the torrid zone. Supposing it all to be evaporated thence, we shall have, encircling the earth, a belt of ocean three thousand miles in breadth, from which this atmosphere hoists up a layer of water annually sixteen feet in depth. And to hoist up as high as the clouds, and lower down again all the water in a lake sixteen feet deep, and three thousand miles broad, and twenty-four thousand long, is the yearly business of this invisible machinery. What a powerful engine is the atmosphere and how nicely adjusted must be all the cogs, and wheels, and springs, and compensations of this exquisite piece of machinery, that it never wears out nor breaks down, nor fails to do its work at the right time and in the right way. The abstract logs at the Observatory in Washington show that the water of the Indian Ocean is warmer than that of any other sea; therefore it may be inferred that the evaporation from it is also greater. The North Indian Ocean contains about 4,500,000 square miles, while its Asiatic water-shed contains an area of 2,500,000. Supposing all the rivers of this water-shed to discharge annually into the sea four times as much water as the Mississippi (§ 274) discharges into the Gulf, we shall have annually on the average an effective evaporation (§ 282) from the North Indian Ocean of 6.O inches, or 0.0165 per day.