GENEEAL CAUSES OF CUEEENTS. 61 



water, whetlier salt or fresh, ever seeks its level and incessantly flows 

 from tlie more elevated places towards tlie depressions. Every atmo- 

 spheric variation has, for result, a displacement in one direction or 

 another of the superficial water. But the great currents which flow 

 with a regidar movement around the basins of the ocean, between the 

 polar and the equatorial zones, are determined by general causes 

 acting at the same time on the entire planet. These causes are the 

 sun's heat and the rotation of the earth on its axis. 



The equatorial basin, incessantly heated by the solar rays, loses a 

 great quantity of water,, which is transformed into vapour, and rises 

 into the higher strata of the atmosphere to be condensed into 

 clouds. Admitting that the annual evaporation is about 14 feet,* 

 which is probably below the reality, the quantity of fluid raised 

 from the Atlantic in the tropical zone would be nearly 120 trillions 

 of cubic yards, and would consequently represent the same value 

 as a cubic mass of water nearly 30 miles in extent. It is true that 

 a considerable part of this vapour, the half perhaps, falls as rain 

 into the sea from which it was taken, yet a great proportion of the 

 clouds are carried by the trade- winds f and other aerial currents, 

 into seas situated beyond the tropics, and over the neighbouring 

 continents. I^ear the equator therefore much more water is drawn 

 from the ocean by evaporation, than is returned to it by the clouds, 

 and in consequence an immense void is formed which can only 

 be filled by the waters from the polar basins, where the contribu- 

 tions of snow, rain, and ice exceed the loss in vapour. This super- 

 abundant mass of fluid continually flows towards the basin of the 

 torrid zone, and forms the two great currents, which meetj one 

 another from the opposite poles in the Atlantic and tlie Pacific, 

 incessantly describing a regular orbit like the celestial bodies. But 

 the excess of evaporation which occurs in tropical waters is not 

 the only reason of this great movement of the polar seas towards 

 the torrid zone. The trade-winds, attracted by the force of equa- 

 torial heat, blow incessantly in the same direction, and always 

 driving the waves before them thus accelerate the march of the 

 oceanic current. 



If the mass of water which continually flows from the poles to the 

 equator were exactly equal in quantity to that which is evaporated 

 by the sun's heat, the arctic currents would be arrested under the 

 tropics, and no return movement would be produced towards the 



* Maury's Geography of the Sea. 

 t See below, the chapter entitled, The Air and the Winds. 



