y6 Mr. H. H. Howortii on 



I cannot go so far as to deny the existence of at least one 

 minor and effective cause. 



The great tropical current, which for our present 

 purpose is the most important, seems to have at least 

 one other impulse to which Dr. Croll has not given 

 due weight. As Reclus puts the case, " Every difference of 

 level which is produced on the liquid surface in consequence 

 of prolonged winds, heavy rains, or very active evaporation, 

 causes as a necessary result, the formation of a current, for 

 the water, whether salt or fresh, ever seeks its level, and 

 incessantly flows from the more elevated places towards 



the depressions 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 billions 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 seas, situated 

 beyond the tropics, and over the neighbouring continents. 

 Near the equator, therefore, much more water is drawn from 

 the ocean by evaporation, than is restored 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 contributions of snow, rain, and ice exceed the 

 loss in vapour. This superabundant mass of fluid continu- 

 ally flows towards the basin of the torrid zone, and forms 

 the two great currents which meet one another from the 

 opposite poles in the Atlantic and the 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 



