56 THE WATER 



velocity of the current then can be read off from the 

 mean position of the shots in the compass card and the 

 number of revolutions shown on the dial. 



The wire should be as fine as possible to avoid 

 stray, and if of steel any deviation which it may cause 

 in the compass should be determined and allowed for. 

 The sinker may weigh from 10 pounds upwards, 

 according to the strength of the current. With 

 currents of a knot an hour and over, in depths greater 

 than thirty fathoms, the writer has found that it was 

 not possible to feel and time the arrival of the mes- 

 sengers on account of the vibration of the wire. In 

 such a case one must determine the time of fall of each 

 messenger for various depths and plot the result on a 

 curve. It will be seen that they soon reach a constant 

 velocity, so that the time may be calculated from the 

 depth. 



The Ekman meter, of course, demands some fixed 

 point, such as an anchored boat or ship, from which 

 to work it, and in rough weather the velocities are not 

 recorded accurately ; under suitable conditions, how- 

 ever, it is very easy to use, and the results agree well 

 among themselves. When tested at the surface from 

 a boat, especially if the current is slow, it is seen to 

 wave from side to side continually, but this appears 

 to be due to eddies ; below the surface in currents of 

 over a quarter of a knot per hour it points constantly 

 in one direction. 



The interval during which the propeller should be 

 allowed to rotate depends on the strength of the 

 current ; when this is not known approximately, four 

 or five minutes may be tried in the first instance. 



Additional apparatus has also been made by means 

 of which it is possible to work in rough weather from 

 a ship that is not anchored, but the writer has not had 



