80 



THE VOYAGE OF H.M.S. CHALLENGER. 



to it a float on the surface, it is evident that exact observation of the motions of 

 the apparatus could not be ascertained, as these motions were liable to be retarded 

 or accelerated by the friction of the surface water on the float, as well as by the 



friction of the water on the line connecting the float 

 and apparatus. A fair approximation, however, of 

 the movement of the sunken apparatus may b.e made 

 if that apparatus be constructed in such a manner 

 as to expose as large an area as possible to the 

 influence of such forces as may be at work where it 

 happens to be, while the float be constructed to present 

 as small an area as possible to the surface current. 

 The lower apparatus must be of sufficient weight to 

 sink readily, and to keep the line between it and the 

 float as nearly perpendicular as possible, otherwise 

 there would be no certainty as to the depth at which 

 it was ; and the float must be of sufficient size to 

 support the weight of the sunken apparatus and 

 the connecting line together with the strain caused 

 by the difference between the lower and surface 

 currents. 



It will thus be seen that the current apparatus 

 consisted of three parts, one called the " current drag," 

 which was lowered down to such depths as were 

 deemed requisite, another the " watch buoy," which 

 pointed out on the surface the movements of the 

 current drag, and a third the " current line," which 

 connected the drag with the buoy. The current drag 

 (see fig. 25) was made of two cross-pieces of iron at the 

 top and bottom A A, A A, with canvas spread between 

 them ; the iron cross-pieces were each 4 feet in length, 

 and were joined together by a bolt in the centre, so that 

 they might be folded up when not required ; they were 

 kept at right angles w r hen in use by a laniard fastened 

 to their extremities. The canvas between the cross- 

 pieces was 4 feet in depth. On the lower part of the 

 drag was a \ cwt. lead B to sink it readily, and the 

 current line was fastened to the upper part ; this was the ordinary service cod line. The 

 current drag was lowered to the required depth, and the line was then fastened to the 

 watch buoy C, which was like a large anchor buoy, being 5 feet in length and 



Fig. 25. -The Current Drag. 



