Mr. J. Croll on the Physical Cause of Ocean- currents. 181 



would modify the equatorial current of the Indian Ocean, a mo- 

 dification which in like manner would affect the Agulhas current 

 and the Southern Atlantic current — this last leading in turn to a 

 modification of the equatorial current of the Atlantic, and conse- 

 quently of the Brazilian current and the Gulf- stream. Further- 

 more, since a current impelled by the winds, as Mr. Laughton in 

 his exceent paper on ocean-currents justly remarks, tends to leave 

 a vacancy behind, it follows that a decrease or increase in the Gulf- 

 stream would affect the equatorial current, the Agulhas current, 

 and all the other currents back to the Antarctic currents. Again, a 

 large modification in the Great Antarctic drift-current would in 

 like manner affect all the currents of the Pacific. On the other 

 hand, any great change in the currents of the Pacific would 

 ultimately affect the currents of the Atlantic and Indian Oceans, 

 through its influence on the Cape-Horn current, the South- 

 Australian current, and the current passing through the Asiatic 

 archipelago ; and vice versa, any changes in the currents of the 

 Atlantic or Indian Oceans would modify the currents of the Pacific. 



I may now consider the cause of the Gibraltar current. There 

 can be little doubt that this current owes its origin (as Mr. 

 Laughton points out) to the Gulf-stream. " I conceive," that 

 author remarks, " that the Gibraltar current is distinctly a stream 

 formed by easterly drift of the North Atlantic, which, although 

 it forms a southerly current on the coast of Portugal, is still 

 strongly pressed to the eastward and seeks the first escape it 

 can find. So great indeed does this pressure seem to be, that 

 more water is forced through the Straits than the Mediterranean 

 can receive, and a part of it is ejected in reverse currents, some 

 as lateral currents on the surface, some, it appears, as an under 

 current at a considerable depth" *. The funnel-shaped nature 

 of the strait through which the water is impelled helps to ex- 

 plain the existence of the under current. The water being- 

 pressed into the narrow neck of the channel tends to produce 

 a slight banking up ; and as the pressure urging the water for- 

 ward is greatest at the surface and diminishes rapidly down- 

 wards, the tendency to the restoration of level will cause an 

 underflow towards the Atlantic, because below the surface the 

 water will find the path of least resistance. It is evident indeed 

 that this underflow will not take place toward the Mediterra- 

 nean, from the fact that that sea is already filled to overflowing 

 by the current received from the outside ocean. 



If we examine the Current-chart published by the Hydro- 

 graphic Department of the Admiralty, we shall find the Gibraltar 

 current represented as merely a continuation of the S.E. flow 

 of Gulf-stream water. Now, if the arrows shown upon this 

 * Journal of Royal United-Service Institute, vol. xv. 



