64 



that are light, but which, like the trade winds^ 

 are continually acting on the whole of a zone, 

 cause a real movement of transition, which we 

 do not observe in the heaviest tempests, because 

 these last are circumscribed within a small 

 space. When, in a great mass of water, the par- 

 ticles placed at the surface acquire a different 

 specific gravity, a superficial current is formed, 

 which takes it's direction towards the point 

 where the water is coldest, or that which is most 

 saturated with muriat of soda, sulphat of lime, 

 and with muriat or sulphat of magnesia. In 

 the seas of the tropics we find, that at great 

 depths the thermometer marks 7 or 8 centesimal 

 degrees. Such is the result of the numerous 

 experiments of Commodore Ellis and of Mr. 

 Peron. The temperature of the air in those la- 

 titudes being never below 19 or 20 degrees, it is 

 not at the surface that the waters can have ac- 

 quired a degree of cold so near the point of con- 

 gelation, and of the maximum of the density of 

 water. The existence of this cold strata in the 

 low latitudes is an evident proof of the existence 

 of an inferior current, which runs from the poles 

 towards the equator : it also proves, that the sa- 

 line substances, which alter the specific gravity 

 of the water, are distributed in the ocean, so as 

 not to annihilate the effect produced by the dif- 

 ferences of temperature *. 



* In fact, if the mean saltness of the sea was 0 005 greater 



