Figure 44. A buoy with a drogue used by the CHALLENGER Expedition in the Gulf
Stream for current studies in 1873. The drogue acts like an underwater sail and
is pushed by the current. The buoy allows visual tracking of the motion of the
drogue. It was first tested off Bermuda in in 1873 in separate tests at 50
fathoms, 100 fathoms, and 600 fathoms.