134 A TEXTBOOK OF OCEANOGRAPHY 



which rotâtes by the action of the current. The rotation is 

 registered and recorded, and gives a measiire of the velocity 

 of the current. The propeller is made fast by means of a hook 

 which can be released by the messenger sent down the wire.. 

 The number of révolutions of the propeller being recorded, 

 the velocity of the stream or current can be calculated from a 

 prepared table. The direction of the current is ascertained by 

 means of an apparatus attached to the revolving arm which 

 leads off from the propeller. This apparatus consists of a 

 rotating wheel divided into compartments, each of which 

 contains a separate shot. Every thirty révolutions of the 

 propeller libérâtes a shot, which falls into a box containing 

 a compass needle which is suitably grooved. Below the needle 

 is a box divided into thirty-six compartments, each of which 

 therefore corresponds to io° of the compass. The shot rolls 

 down the compass into one of thèse compartments. From 

 the distribution of the shot in the varions compartments the 

 average direction of the current can be ascertained. Ekman's 

 current-meter can be used in great depths, but owing to its 

 complicated construction it requires careful handling. Owing 

 to friction, the meter will not register currents of less velocity 

 than 3 centimètres per second. In actual practice the instru- 

 ment is rather difficult to use, since it is necessary to moor the 

 boat or ship very securely, to prevent moving or swinging by 

 surface currents. 



Floating and drifting bodies in the océan give a valuable 

 indication of the direction and velocity of océan currents. 

 When the floating objects are fully submerged they give an 

 indication of the océan currents only ; when a part of the 

 object projects from the water, the influence of the wind has 

 to be taken into calculation. 



The plankton serves, in a way, as an indicator of océan 

 currents, but the extent of our knowledge of the distribution 

 of planktonic organisms is not sufliciently wide to justify the 

 extensive use of this method at présent. The researches of 

 the International Commission hâve, however, given us much 



