63 



The solution of this last problem becomes so 

 much the more important, as the latitudes which 

 we have just indicated are traversed by the 

 greater part of the vessels, which return to Eu- 

 rope from the West India islands, or the Cape 

 of Good Hope. Beside the direction and swift- 

 ness of the currents, this expedition would serve 

 to discover the temperature of the sea at it's 

 surface, the lines without variation, the dip of 

 the needle, and the intensity of the magnetic 

 forces. Observations of this kind become ex- 

 tremely valuable, when the position of the place 

 where they were made has been determined by 

 astronomical means. In the seas most frequent- 

 ed by the Europeans, far out of sight of land, an 

 able navigator may still devote his time to im- 

 portant labours. The discovery of a group of 

 uninhabited islands is less interesting than the 

 knowledge of those laws, which link together a 

 considerable number of insulated facts. 



In reflecting on the causes of the currents, we 

 find, that they are much more numerous than is 

 generally believed ; for the waters of the sea 

 may be put in motion by an external impulse, 

 by a difference in heat and saltness, by the peri- 

 odical melting of the polar ice, or by the inequa- 

 lity of the evaporation, which takes place in 

 different latitudes. Sometimes several of these 

 causes concur to the same effect, and sometimes 

 they produce effects that are contrary. Winds 



