122 THE PHYSICAL GEOGRAPHY OF THE SEA. 



tent 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 northeast and southeast trades meet 

 fresh from the ocean, and keep the temperature uniform under a 

 canopy of perpetual clouds. 



800. Amount of Evaporation. — The mean annual fall of rain on 

 Amount of evapo- the entire surface of the earth is estimated at about 



RATION greatest from n n m in 



the Indian Ocean, live lect. 1 o cvaporatc watcr cuough 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 pre- 

 cipitate it in the right places, at suitable times, and in the propor- 

 tions due, is one of the ofiices of the grand atmospherical machine. 

 This water is evaporated principally from the torrid zone. Sup- 

 posing it all to come thence, we shall have, encircling the earth, 

 a belt of ocean three thousand miles in breadth, from which this 

 atmosphere evaporates 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 at- 

 mosphere! 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 ; there- 

 fore 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. Sup- 

 posing 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 an average annual evaporation (§ 282) 

 from the North Indian Ocean of 6.0 inches, or 0.0165 per day. 



801. The rivers of India are fed by the monsoons, which have 

 The rivers of India, "^^ clo their work of distributing their moisture in 

 "^^tfect^^t^voi a^o^* ^^^e® months. Thus we obtain 0.065 inch 

 tion from that ocean, ^g ^|^q avcragc daily rate of effective (§ 282) evapo- 



