EVAPORATION FROM THE SEA, 155 



proportiouate to the area, the temperature, the time, the tension, and 

 much besides. 



Even from the land the evaporation is great. Maury, in his 

 treatise on the '' Physical Geography of the Sea," (p. 274), estimates 

 the annual amount of precipitation in the valley of the Mississippi at 

 620 cubic miles of water, and the discharge of that river into the sea 

 at 107 cubic miles; and he concludes that "this would leave 513 

 cubic miles of water to be evaporated from the river basin annually." 

 This is but one basin — a furrow on the dry land ; and then there is 

 the sea ! 



The evaporation from the Mediterranean, the Black Sea, and the 

 Sea of Azov, is reckoned at 50 inches on the whole extent of their 

 surface ; this is about 1,500,000 square miles, and a simple calcula- 

 tion shows that from these seas alone must be evaporated upwards 

 of 500 (508) cubic miles of fresh water. 



A corresponding estimate and calculation shows that the annual 

 evaporation from the Red Sea must amount to 165 cubic miles of 

 water. 



Such are the quantities with which we have to deal in speaking of 

 evaporation from inland seas alone. 



Of the whole surface of the earth, three-fourths are covered by 

 sea ; that surface measures in round numbers 196 millions of square 

 miles, of which nearly 150 millions are covered by water. What 

 startles me is the consideration of the quantity of water which this im- 

 plies, and the question arises. What can have been its source 1 But it is 

 the evaporation from it with which alone we have at present to do. 

 We should probably err were we to reckon the evaporation, over the 

 whole area of the ocean, at the ratio of the evaporation which takes 

 place from the surface of the Red Sea and the Mediterranean. But 

 it is going on everywhere — in an increasing ratio, probably, towards 

 the equator, in a decreasing, towards the poles ; and apparently it is 

 not confined there to what is called the open sea. I have already 

 had occasion to refer to the statement by M. Hayes, in a work entitled 

 La mer liber du Pole, published in 1868, that thin wet linen exposed 

 to the air dries at the lowest temperature, and that a sheet of ice 

 suspended by a cord evaporates by degrees. He adds, " that evapora- 

 tion is more restricted near the coast than in the interior districts of 

 a country, and more restricted under an equatorial wind from the 

 south-west, than under a polar wind from the north-east, the former 

 being saturated with moisture, while the latter has been dei>rived of 

 its humidity." 



