176 



EVAPORATION. 



springs in various parts of the earth, where they find a natural exit, or where 

 an artificial exit is given to them, and, in the other case, obeying the form of 

 the surface of the country through which they are carried, they wind in narrow 

 channels, first deepening and widening as they proceed, and are fed by tributary 

 streams until they form into great rivers, or spread into lakes, and at length 

 discharge their waters into the sea. 



The process of evaporation is not confined to the sea, but takes place from 

 the surface of the soil, arid from all vegetable and animal productions. The 

 showers which fall in summer, first scattered in a thin sheet of moisture over 

 the surface of the country, speedily return to the form of vapor, and carry with 

 them, in the latent form, a quantity of heat, which they take from every object 

 in contact with them, thus moderating the temperature of the earth, and re- 

 freshing the animal and vegetable creation. 



A remarkable example of evaporation on a large scale is supplied by that 

 great inland sea, the Mediterranean. That natural reservoir of water receives 

 an extraordinary number of large rivers, among which irmy be mentioned the 

 Nile, the Danube, the Dnieper, the Rhone, the Ebro, the Don, and many oth- 

 ers. It has no communication with the ocean, except by the straits of Gibral- 

 ter, and there, instead of an outward current, there is a rapid and never-ceas- 

 ing inward flow of water. We are, therefore, compelled to conclude that the 

 evaporation from the surface of this sea carries off the enormous quantity of 

 water constantly supplied from these sources. This may, in some degree, be 

 accounted for by the fact, that the Mediterranean is surrounded by vast tracts 

 of land on every side except the west. The wind, whether it blow from the 

 south, the north, or from the east, has passed over a considerable extent of 

 land, and is generally in a state, with respect to vapor, considerably below sat- 

 uration. These dry currents of wind, coming in contact with the surface of 

 the Mediterranean, draw ofT water with avidity, and, passing off, are succeeded 

 by fresh portions of air, which repeat the same process. 



