THE ATMOSPHERE. 99 



207. Amoimt of Eva])oration. — The mean annual fall of rain 

 on the enth'e surface of the earth is estimated at about five feet. 



208. To evaporate water enough annually from the ocean to 

 cover the earth, on the average, five feet deep with rain ; to trans- 

 port 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. This water is evap- 

 orated principally from the torrid zone. Supposing 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 atmosphere ! and how nicely ad- 

 justed must be all the cogs, and wheels, and springs, and compen- 

 sations of this exquisite piece of machinery, that it never wears 

 out nor breaks down, nor falls to do its work at the right time and 

 in the right way ! 



209. In his annual report to the Society ( Transactions of the 

 Bombay Geographical Society from May, 1849, to August, 1850, 

 vol. ix.). Dr. Buist, the secretary, states, on the authority of Mr. 

 Laidly, the evaporation at Calcutta to be " about fifteen feet an- 

 nually ; that between the Cape and Calcutta it averages, in Octo- 

 ber and November, nearly three fourths of an inch daily ; between 

 10° and 20^ in the Bay of Bengal, it was found to exceed an inch 

 daily. Supposing this to be double the average throughout the 

 year, we should," continues the doctor, "have eighteen feet of 

 evaporation annually." 



210. If, in considering the direct observations upon the daily 

 rate of evaporation in India, it be remembered that the seasons 

 there are divided into wet and dry ; that in the dry season, evap- 

 oration in the Indian Ocean, because of its high temperatui'e, and 

 also of the high temperature and dry state of the wind, probably 

 goes on as rapidly as it does any where else in the world ; if, 

 moreover, we remember that the regular trade- wind regions pi:oper 

 at sea are regions of small precipitation (§ 179) ; that evaporation 



