1870.] 



Deep-sea Researches. 



165 



its upper end, and by stretching a piece of sail-cloth between each arm 

 and the side of the basket ; which device 

 caused a uniform resisting surface to be 

 presented to the current, whatever the 

 manner in which the sails might meet it. 

 To the lower part of this "drag" a couple 

 of sinkers, of 1 12 lbs. each, were attached ; 

 and the whole apparatus -was supported 

 by cords meeting in a ring above it, to 

 which the suspending line was secured. 



38. This "current-drag" having been 

 transferred to a boat, was lowered down by 

 a couple of men placed in her, to the de- 

 sired depth ; and the boat was then left 

 entirely free to move, being lightened by 

 the return of the men into the ship. The 

 motion of the boat would be the composite 

 result of (1) the action of wind (if any) 

 upon the transverse section of the part of 

 of the boat above the water; (2) the action of the surface-current upon 

 the transverse section of the immersed part of the boat ; (3) the action of 

 the upper current upon the suspending line ; and (4) the action of the 

 current in which the "drag" is suspended upon the drag itself. Putting 

 aside the first of these agencies, which will be of very little account if (as 

 in the experiment now narrated) the boat be small and the breeze be light, 

 it is obvious that the relative influence of the second and third to that of 

 the fourth will depend upon the proportion between the surfaces presented 

 by the boat, the line, and the "drag" respectively, and the strength of 

 the current acting upon each. The surface given to the "drag" being 

 larger than that of the boat and line taken together, the force acting on 

 the "drag" will dominate, if it hang in an opposing current superior, 

 equal, or even somewhat inferior in rate to that which acts on the boat 

 and line ; so that the boat would be carried along by the drag against the 

 surface-stream, at a rate proportioned to the excess. — If, again, the rate of 

 the undercurrent should be greatly inferior to that of the surface, its 

 action upon the "drag" might still be sufficient to neutralize that of the 

 surface-current upon the boat and line, and the boat would then remain 

 stationary or nearly so. — A still further reduction in the rate of the op- 

 posing undercurrent would make its action upon the " drag " less power- 

 ful than that of the surface-current upon the boat and suspending line ; 

 and the boat would then move with the surface-current, but at a rate of 

 which the great retardation would indicate an antagonistic force beneath. — 

 Supposing, again, the water of the stratum in which the " drag " is sus- 

 pended to be stationary, the action of the surface-current upon the boat 

 and line would be opposed by the resistance offered by the deeper water 



Current-drag. 



