THE SURFACE OF THE EARTH. 



part of it will be converted into vapour at the surface in contact 

 with, the air ; and the rate of such evaporation, other things being 

 the same, will be proportionate to the extent of the surface, the 

 temperature of the air in contact with it, and the superficial 

 temperature of the soltftion. 



The water of the ocean is a solution of certain salts and alkaline 

 substances. The evaporation which takes place from its surface 

 affects only the pure water, leaving the saline and other similar 

 constituents still dissolved in it. The pure aqueous vapour thus 

 taken into the atmosphere, rising to the more elevated strata, is 

 transported by atmospheric currents, and attracted by the mountain 

 summits and other elevated parts of the land, upon which it is 

 precipitated in the form of rain or snow, from which streams flow 

 down the declivities, discharging the functions of irrigation, and 

 thus contributing to the maintenance of animal and vegetable life 

 upon the land. 



201. Climatic effects Independently of the obvious advan- 

 tages which the ocean affords, as supplying the means of intercom- 

 munication by commerce between distant parts of the earth, it 

 also serves an infinite variety of purposes in the climatic economy 

 of the globe. It has been already shown that from the uniformity 

 of its physical qualities, it has a tendency to equalise and regularise 

 climate, so as to bring the isothermal lines into closer proximity 

 with the parallels of latitude. 



202. Ocean currents. Its liquid properties, however, com- 

 bined with the effects of temperature, render it further sub- 

 servient to the general equalisation of the temperatures of the 

 extreme zones, moderating the heat of the torrid and the cold of 

 the frigid. This is accomplished by the great ocean currents, 

 the existence, directions, and limits of which have been ascertained 

 by modern navigators. 



These currents are classed as constant, periodical, and variable, 

 the two latter classes being determined chiefly by the influence of 

 the winds and tides. 



203. Antarctic drift current The constant currents which 

 are by far the most important, have their origin chiefly in the 

 southern frigid zone, from which a vast stream called the antarctic 

 drift current, pours its cold waters first northwards into the 

 Pacific and then eastwards towards the eastern coast of South 

 America. Striking upon the shores of Chili, opposite the island 

 of Juan Fernandez, it is diverted to the north, following the coast 

 of the South American continent, until it encounters the jutting 

 shores of Peru, by which it is turned westward, where it takes 

 the name of the equatorial current. 



204. Its equatorial course. From that point, following the 

 190 



