Hydrography^ and the Art of Navigation. 53 



mits ; and, lastly, by the diagonal directions, that is to say, by 

 going alternately from a northern summit to a southern one. 

 The following is the result of these various combinations : — 



The mean difference, 0™,73 (2 feet 3 inches), is too small, 

 particularly when we recollect the extent of the line which was 

 levelled, to prevent us from concluding that, when in a state of 

 repose, the waters of the ocean, and those of the Mediterranean, 

 form a surface of the same level. At all events, U can scarcely 

 be doubted that if any difference in this respect exist, it is too 

 small to be appreciated. 



In this article I wish merely to prove that the subject of cur- 

 rents is far from being exhausted ; that differences of level, to 

 which hydrographers have recourse for an explanation of them, 

 are either completely nugatory, or insignificant; that there is 

 still room for further investigation. This object I conceive m}'- 

 self to have attained. I shall still, however, add a few reflec- 

 tions. 



The theory of currents has made little progress hitherto, be- 

 cause those phenomena have chiefly been considered which ap- 

 pear at the surface of the sea. Currents produced by difference 

 of saltness and temperature exist at all depths. There are cur- 

 rents, for example, in contact with the very bed of the sea, 

 which transport the cold waters of the polar zones as far as the 

 equator. Near the poles these waters move like the solid part 

 of the earth which supports them, at a very slow rate, from west 

 to east. As they pass by degrees to temperate and warm re- 

 gions, they arrive at greater parallels of the earth, which hence- 

 forth move quicker than they ; hence relative currents directed 

 from the east to the west, the size of which is equal to that of 

 the polar currents. 



