60 The Sea 
knowledge of the time and place at which they were re- 
leased, and at which they were recovered, a rough estimate 
of the speed and direction of the current may be obtained. 
Other studies can be made from the drifts of derelicts and 
wreckage, which may travel enormous distances and stay 
afloat for years before finally being destroyed or sunk. 
Modern methods for measuring currents include measure- 
ments at all depths with an Ekman current meter. The instru- 
ment is lowered to any depth, left there for a definite length 
of time. It is then hauled in and the number of revolutions of 
its little propeller read from a dial. The direction of the cur- 
rent is obtained by a self-registering compass attachment. 
Another means of determining currents at sea is by com- 
parison of a large number of measurements of salinity and 
temperature at different depths. From their data the cur- 
rents may be computed from hydrodynamic theory. The in- 
terested reader may refer to such a study made by C. OD. 
Iselin of the Woods Hole Oceanographic Institution. (A 
Study of the Circulation of the Western North Atlantic, 
Contribution No. 108, Price $1.00, obtainable by writing the 
Woods Hole Oceanographic Institution, Woods Hole, Massa- 
chusetts, U.S.A. ) 
Ocean currents arise between bodies of lighter (less 
dense) and heavier water. A simple rule for obtaining the di- 
rection of such a current is: 
In Northern Hemisphere: Stand so that the lighter body of 
water lies on your right, the heavier on your left. You are 
then looking in the direction of the current. The Gulf Stream, 
which skirts the warm Sargasso Sea, is a good example. 
In the Southern Hemisphere: Reverse the words “right” 
and “left.” 
