264 DEPTHS OF THE OCEAN 



is a propeller which is moved by the current, the velocity- 

 determining the number of revolutions in a certain period. 

 The propeller works some small cog-wheels connected with 

 hands showing on a dial the number of revolutions. The 

 mechanism for indicating the direction of the current is very 

 ingenious. Some small shot are inserted into a tube leading 

 to one of the cog-wheels, which is provided with notches each 

 holding one little ball. The balls are carried round by the 

 wheel, and after half a revolution are discharged through 

 another tube into the centre of a metal box, in which is poised 

 a magnetic needle with a groove along the top of one branch. 

 As the shot fall, they roll along the needle and drop off its point 

 into the box. Their path may be traced in the figure. The 

 bottom of the box is divided into thirty-six small partitions, and 

 the balls fall into one or other of these according to the position 

 of the needle. The position of a ball in the box thus indicates 

 the angle between the axis of the apparatus and the magnetic 

 meridian, that is, the direction of the current. When the 

 apparatus is lowered into the water, the propeller is set and 

 fixed, and is subsequently released by a small messenger so 

 as to spin with the current ; when desired, a larger messenger 

 is sent down to arrest the propeller before hauling up. With 

 this current-meter a great number of observations have now 

 been made, many of which have given very important results. 



In order to obtain good results it is necessary that the 

 apparatus should hang practically still, without being carried 

 along by the ship or the water, or — if this be unavoidable — 

 that the drift should be perfectly well known. The boat 

 from which the work is done must be very firmly anchored. 

 In the Norwegian investigations we have, as a rule, worked 

 from a small boat with anchors fore and aft, and it was possible 

 in this way to hold the boat, even when more than 500 metres 

 over the bottom, the most exact bearings showing that the 

 boat did not drift sufficiently to influence the current-meter ; 

 one anchor alone is usually not sufficient, for the boat may 

 swing, thus affecting the apparatus. When measuring the 

 currents in the Straits of Gibraltar, we tried double staying 

 with the life-boat, using a strong hemp line about one inch in 

 circumference, but the current was so strong that the line broke 

 again and again, and we had to give it up. When the current 

 (or the wind) is very strong, good results may be obtained by 

 means of a single anchor forward, so we dropped one of the 

 large anchors of the "Michael Sars," and the steamer lay so 



