OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



47 



removed for inspection or transportation. The axis of the propeller runs 

 with tantalum points on agate bearings. Inside the protective ring is a 

 lever that can be operated by messengers. With the lever in its lowest 

 position the propeller is arrested, and in this state the instrument is 

 lowered. When the desired depth is reached, a messenger weight is 

 dropped, which pushes the lever up to its middle position, releasing the 

 propeller, the turns of which are recorded on a set of dials. After a 

 number of minutes a second messenger weight is dropped, which pushes 

 the lever up to the highest position, stopping the propeller. In later 

 types the propeller is also shielded in front when the instrument is 



Fig. 7. The Ekman current meter. Messenger, dials, and compass 

 box are seen. Propeller is hidden by protective ring. 



lowered in order to prevent fouling of the propeller by such organisms as 

 medusae. This front shield is opened by the first messenger. 



The direction of the current is recorded by an ingenious device which 

 is simple and reliable. A tube extends from above the dial box to a disk 

 on the axis of the cogwheel, which turns once when the propeller makes 

 one hundred revolutions. This tube is filled with phosphor-bronze balls 

 about 2 mm in diameter. In the disk are three or more indentations 

 corresponding to the size of the balls. When one of these indentations 

 passes below the tube containing the balls, a ball drops into it and is 

 carried around with the disk until it drops into a second tube which 

 extends downward and ends above the center of the compass box. In the 

 compass box, which can be easily removed from the bar to which it is 

 fastened, a system of magnets swings freely on a pin that runs on agate. 

 The frame to which the magnets are fastened carries a bar that is shaped 



