554 



Captain Spratt on the Undercurrent [June 15, 



than the former, and therefore of less resistance to the drag of the boat by 

 the wind, sea, and surface-current, even if both kettle and chip-log had 

 reached the region of perfectly still water. 



But if, as may have been probable, the kettle was still in a portion of 

 the surface-current, and the chip-log in about 50 fathoms lower down was 

 in the still regions, or even a more diminished rate of the surface-current, 

 the float of the latter would more rapidly separate from the boat in 

 the opposite direction to the surface-current, and thus appear to be 

 marvellously dragged by an undercurrent against wind and sea and surface- 

 current — that is, against or opposite to the boat's natural drift. Now, as 

 Lieut. Walsh notices that the weather was too rough for attempting deep 

 soundings, except on the 14th of May, we must infer that there was 

 sufficient wind and sea to cause considerable drift to the boat ; but he does 

 not notice the direction of the wind. 



Therefore, as there was no fixed object as a point of comparison suffi- 

 ciently exact in the last experiment, when the kettle was used, much less in 

 the others, when only the chip-log was used, and with a compass bearing 

 from a drifting boat for ascertaining the presumed direction of the under- 

 current, even the true direction of the surface-current cannot be depended 

 upon by reversing the direction he has given for the undercurrent (this is, 

 by assuming that the surface-current ran E.N.E. 1| knot when he gives 

 the undercurrent as setting W.S.W. 1^ knot), since there were so many 

 sources of error vitiating the results. A fixed object for reference can 

 always be obtained in any depth by a 20-lb. lead and sufficient twine, and 

 a light float attached when it has reached the bottom, as I have long since 

 shown and recommended as a necessity in all such delicate experiments 

 in mid-ocean or elsewhere. The following are the results at the six posi- 

 tions given by Lieut. Walsh : — 



Date. 



Lat. 



Long. 



Depth. 



Eate. 



Direction. 



Temperature, 

 surface. 



Fathoms, 

 100. 









fins. 



knot. 





o 





May 11th ... 



24 43 



65 2 



126 



1 



W.S.W. 



77-3 



73-5 



May 12th ... 



24 55 



64 43 



126 



If 



S.E. 



75 



69 



May 13th ... 



26 42 



64 4 



128 



14 



w.s.w. 



77-5 



74-5 



May 14th ... 



26 46 



63 53 



126 



1* 



N. by E. 



77 





Mav 18th ... 



30 06 



67 56 



126 





JNT.E. 



70 



65 



May 29th ... 



33 58 



72 00 



126 



l 



W.N.W. 



71 



67 



It is therefore only the rate as given above for the undercurrents that can 

 be relied upon as the rates of a surface-current that really existed in those 

 positions, and which from the high mean temperature for the whole six, of 

 74°*6, and of 77°'3 at three of the positions, as the surface-temperature for 

 the month of May, and of 73*5 and 74*5 in a depth of 100 fathoms at two 

 of them, would seem to show it to have been a continuation of a portion of 

 the trade-wind or equatorial current, its easterly portion running outside of 

 the West-India Islands, but somewhat checked, and even perhaps at times 



