On the general 'Causes of the Ocean-Cueeents. 15 



other hand be room for two opposite streams as is the case in the 

 Straits of Gibraltar. To the causes, which give rise to that double stream, 

 I shall return farther on. 



B. CAUSES, WHICH IMMEDIATELY PRODUCE OCEAN-CURRENTS 

 BY MECHANICAL ACTION OF A MEDIUM ALREADY IN MOTION. 



Streams arising from these causes may be divided into two clas- 

 ses, first: 



lYio Streams caused hij the Winds. 



Franklin endeavoured to account for the Gulf-stream as an eftect 

 of the trade winds, and the winds were thought b}' Sir John Hekschel 

 to be the principal causes of the ocean-currents. Though Mauey could 

 not deny that they have some influence, he nevertheless seems, in con- 

 sequence of his persuasion that differences of specific gravity were the 

 main-cause of the ciirrents, to have considei'ably undervalued the effect of 

 winds. The importance of these has now of late lieen acknowledged 

 and maintained by M:r Ceoll. On this subject I will confine myself to 

 briefly expressing my opinion, especially as the limites, which I liave 

 bean able to find for the study of the ocean-currents, has not })ermitted 

 me for myself to examine M:r Croll's works, which will probably make 

 a more detailed explanation of this part of the question superfluous. 



Every wind, that sweeps over a mass of water, produces, by the 

 friction of the air-particles against the water and their impact on the 

 waves, a superficial stream in the direction of the wind and consequently 

 an elevation of level towards the coast, as also a depression on the 

 opposite coast. By this the equilibrium of the water is disturbed down 

 to the bottom and the water flows back from the elevated portion by the 

 ways, by which the reflux can most easily take place. Should the wind 

 operate uniformly over the whole surface of the water, the reflux fan 

 only take place in the form of an under-current, the velocity of which 

 must depend of the depth of the water. Should it however operate only 

 on a piirt of the surface, suppose the left, the reflux will take place also 

 in the form of a surface-stream from the right side, whereby the level 

 there will be lowered, and so on; thus at length arise on the surface side- 

 streams, which are supplied with water from the spots, where the level 

 has been raised, and which together with the original superficial stream 



