306 THE ATLANTIC. [cHaP. v. 
Fra. S9.—The Current-drag and Watch-buoy. 
Various means have 
been devised for ascer- 
taining the direction and 
rate of currents of water 
at different depths below 
the surface, but none of 
these can as yet be con- 
sidered satisfactory. The 
difficulties may be best 
explained by describing 
the method employed in 
the Challenger. It is nec- 
essary in order to inves- 
tigate a deep current, to 
sink to the required depth 
some object which will be 
taken a good hold of by 
the current and carried 
along with it; and to 
measure the movement 
of this object it must be 
attached to a float on the 
surface which shall be af- 
fected as little as possible 
by the surface - drift, by 
a line which shall be af- 
fected as little as possible 
by intermediate move- 
ments. For this we use 
an instrument composed 
of the “ current-drag,” the 
“line,” and the “ watch- 
buoy.” The first of these 
consists of a light frame 
