22 THE OCEAN WORLD. 



character of its waters would be completely changed if they were 

 fresh in place of salt. " Let us imagine," says M. Julien, " that the 

 sea, now entirely composed of fresh water, of one uniform tempera- 

 ture from the Pole to the Equator, and from the surface to its 

 greatest depths ; the solar heat would penetrate the liquid beds 

 nearest to the Equator ; it would dilate them, so as to raise them 

 above their primitive level ; by the single effect of gravitation, they 

 would glide on the surface towards the Polar zones. The absence of 

 all solar radiation would tend, on the contrary, to cool and contract 

 them without this tendency. An exchange would be established 

 from the extremities towards the centre ; in other words, a counter 

 current of cold and heavy water, calculated to replace the losses 

 occasioned by the action of solar radiation, would descend from 

 the Poles, but quite maintaining itself beneath the light and warm 

 current from the Equator." 



In a like system of general circulation, the physical properties of 

 pure water, which attains its maximum of density 7 2" F. below 

 zero, would produce the most singular consequences. As its tem- 

 perature rose above that point, the water would become lighter, 

 having consequently a tendency to ascend towards the upper beds. 

 After this, the equatorial current, meeting in its progress towards the 

 Poles the cold water, would itself be cooled down ; and when its 

 temperature had reached 4 below zero, being now heavier than the 

 polar current, would change places with it, descending until it reached 

 water equally dense, while the polar current would ascend. Hence 

 would arise a sort of confusion of currents, which would give to a 

 fresh-water ocean the strangest results, disarranging every instant the 

 regular circulation of its waters. It could not be so, however, in an 

 ocean of salt water, which attains its maximum specific gravity at 

 4 8" below zero. By evaporation at the surface it is concentrated 

 and precipitated, and thus rendered denser than that immediately 

 below the surface. It consequently sinks, while the lower beds come 

 up to replace, in order to modify it, and in turn to be precipitated in 

 the same manner. " In this manner we find established a continually 

 ascending and descending movement, which carries down into the 

 depths of ocean the water warmed at the surface by the solar rays of 

 the Torrid zone. This double vertical current facilitates and pre- 

 pares the grand horizontal current which puts these submarine 

 reservoirs of heat in communication with the lower beds of the 

 glacial sea. In the Arctic basin the clouds, the melted snow, and 

 the great rivers, which have their mouths on the north of both con- 

 tinents, produce considerable quantities of fresh water, which, mixing 



