552 



Captain Spratt on the Undercurrent [June 15, 



From the above reasonings, it is clear that the eminent author, from the 

 supposition that a great undercurrent movement in the Atlantic had been 

 discovered as the result of the observations and experiments of Lieutenants 

 Walsh and Lee, was induced to propound his fascinating but fallacious 

 theory regarding the origin of "all the currents of the ocean 5 ' being more 

 due to temperature and density than to tides and winds. 



Now it is true in regard to tides, that is, the currents resulting from tide- 

 waves are mainly littoral and local. It is not so, however, as the result of 

 winds, which from my experience are the main sources of ocean-currents, 

 without ignoring that from the rotation of the earth, which are therefore 

 chiefly superficial, but capable of reaching a considerable depth where the 

 water is deep enough, even to 50 and more fathoms, with no greater sur- 

 face-movement than from three-tenths to five-tenths, or half a knot per 

 hour, as I have on several occasions experienced from a perfect stillness 

 of the sea from the surface down to the greatest depths in perfect calm 

 weather, but which was set in motion in the same direction as the wind to 

 that depth a few hours only after a 4- or 5-knot breeze had set in. 



To show that Capt. Maury had mainly founded his theory upon the ob- 

 servations of Lieutenants Walsh and Lee, I must quote from the Report 

 of the former as being the one most important and complete, as was sup- 

 posed, in proof of the rapid undercurrent believed in by Capt. Maury, 

 and supposed to have been confirmed by other phenomena connected with 

 the fallacious idea of the ploughing of icebergs through fields of ice in 

 Baffin's Bay, by the force of a mighty undercurrent*, instead of the fact of 

 the field of ice flowing past them, by reason of the greater strength of the 

 surface- current over the current in the depths to which the base of the 

 bergs reached, as no doubt must be the fact in that bay or strait from the 

 southerly drift of both into the Atlantic. 



"Report of Lieut. Walsh, U.S.N., to Lieut. M. F. Maury, of the Ob- 

 servatory at Washington. 



" The next subject to which I would refer is our investigation of the 

 undercurrents of the ocean. I regret we had so few opportunities for the 

 interesting experiments, but enough has been done to seem to warrant 

 the conclusion that these undercurrents are generally stronger setting in 

 various different directions than those of the surface. I am well aware 

 there is no mode of testing their exact velocity, but that practised by 

 myself, which I will describe, was certainly all-sufficient to show their 

 real velocity. There may be none so rapid as that mighty ocean-river 

 the Gulf-stream. Unfortunately the weather prevented our making 

 these investigations in that interesting region ; but in the various parts 

 of the Atlantic in which we succeeded in these experiments, on only 

 two occasions did we find the undercurrent of less velocity than that run- 

 ning in a different direction above it. 



* See Physical Geography of the Sea, pp. 162 & 163. 



