Mr. J. Croll on the Physical Cause of Ocean-currents. 177 



evidence that all the water flowing through the Straits of Florida 

 passes through the section examined by the officers of the ' Chal- 

 lenger/ Be this, however, as it may, the observations made 

 between St. Thomas and Sandy Hook reveal the existence of an 

 immense flow of warm water, 2300 feet deep, entirely distinct 

 from the water included in the above section of the Gulf-stream 

 proper. As the thickest portion of this immense body of water 

 joins the warm water of the Gulf-stream, Captain Nares con- 

 siders that " it is evidently connected with it, and probably as 

 an offshoot/'' At Sandy Hook, according to him, it extends 

 1200 feet deeper than the Gulf-stream itself, but off Charleston, 

 600 miles nearer the source^ the same temperature is found at 

 the same depth. But whether it be an offshoot of the Gulf- 

 stream or not, one thing is certain, it can only come from the 

 Gulf of Mexico or from the Caribbean Sea ; and that it is a 

 moving stream is proved by the fact that at some places its ve- 

 locity was found to be as great as 18 miles per day. This mass 

 of water, after flowing northwards for about 1000 miles, turns 

 to the right and crosses the Atlantic in the direction of the 

 Azores, where it appears to thin out. 



If, therefore, we take into account the combined heat con- 

 veyed by both streams, my estimate of the heat transferred from 

 intertropical regions into the North Atlantic will be found 

 rather under the truth than above it. The quantity of heat 

 thus borne into the Atlantic is enormous compared with that 

 which can possibly be conveyed by a " General Vertical Oceanic 

 Circulation." It follows, therefore, that, so far as the distribu- 

 tion of heat is concerned, it is a matter of perfect indifference 

 whether such a circulation does or does not exist. 



The Wind Theory of Oceanic Circulation. 



Ocean-currents not due alone to the Trade- Winds. — The gene- 

 rally received opinion amongst the advocates of the wind theory 

 of oceanic circulation is that the Gulf-stream and other currents 

 of the ocean are due to the impulse of the trade-winds. The 

 tendency of the trade-winds is to impel the intertropical waters 

 along the line of the equator from east to west ; and were those 

 regions not occupied in some places by land, this equatorial cur- 

 rent would flow directly round the globe. Its westward progress, 

 however, is arrested by the two great continents, the old and 

 the new. On approaching the land the current bifurcates, one 



annual rate of the Gulf-stream in the " Narrows ; " but in the Admiralty's 

 Current-chart, published October 1872, the minimum rate is stated to be 

 32, and the maximum rate 120 nautical miles per day. This gives 87 sta- 

 tute miles per day, or fully 3| miles per hour, as the mean rate. 

 Phil. Mag. S. 4. Vol 47. No. 311. March 1874. N 



