CHAP. V.] THE GULF-STREAM. 337 
of hammered iron four feet high and four feet across (Fig. 89), 
supporting four vertical fans of canvas. The iron cross-pieces 
are fastened by a bolt in the centre so that they can be folded 
away when not in use; and they are kept at right angles by 
a lanyard attached to their ends. To the bottom of the frame 
a half-cwt. lead is slung to sink it rapidly in the water, and the 
current-line—a sufficient length of ordinary service “ cod-line” 
—is fastened to the top. When the drag has been let down to 
the desired depth, the line is attached on the surface to the 
watch-buoy, a spindle-shaped iron buoy five feet long by one 
wide in the centre, so fashioned as to expose as little surface as 
possible to the drift or the wind, while it has sufficient buoy- 
ancy to sustain a weight of seventy pounds in the water. 
In using this instrument, the direction and force of the sur- 
face-current are first ascertained in the manner already de- 
scribed, and the boat then frees itself by letting go the dredge- 
rope, which is hauled in by the ship. The current-drag is low- 
ered to say 50 fathoms from the boat, and the watch-buoy at- 
tached. The boat with the observer then follows the watch- 
buoy closely, without interfering with its movements; and the 
surface-log is again dropped from the boat, allowed to run for a 
given fraction of an hour and checked, when its bearing and the 
length of line run out give the direction and rate of the surface- 
current from the boat. But the boat is no longer a fixed point 
—it is keeping with the watch-buoy ; that is to say, it is moving 
in the direction and with the rate of a current at a depth of 50 
fathoms. As the log-ship is free to move with the surface-cur- 
rent, all divergence, whether in rate or direction, between it and 
the watch-buoy must be due to influences acting upon the lat- 
ter, for they would otherwise be drifted along together ; and the 
rate and direction of the surface-current being already known, 
the deep-water movement can be readily caleulated from the rel- 
ative positions of the watch-buoy and the log-ship; the actual 
movement of the watch-buoy with reference to a fixed point 
