INTEECHANGE OE WATEE AND SALT. 91 



shores of countries most diverse in appearance and geological forma- 

 tion, have a tendency to resemble each other in their composition, salt- 

 ness, and in most of the other phenomena of their waters. The currents 

 are the great agents in producing this equilibrium in the seas ; but 

 by their very mobility, their dependence on the seasons, winds, con- 

 figuration of the coasts, and finally, by reason of the submarine part 

 of their course, they are exceedingly difficult to observe in a sys- 

 tematic manner, — and among the numerous general and partial cur- 

 rents, there is not a single one, not even the Gulf- stream, whose 

 normal course can be traced with complete precision. Happily, 

 scientific observations are now being multiplied over all the seas ; 

 they add to and unite with one another ; and, little by little, approach 

 the truth by approximations which result from tlie comparison of 

 facts. Every new sounding, every new thermometrical reading, is 

 an acquisition to science, and allows us to follow with a clearer eye 

 the complicated circulation of the waters in the immense labyrinth 

 of the ocean. 



