46 OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



drifting buoy from a ship that remains as close as possible to the anchored 

 buoy. 



Flow Methods. From an anchored vessel or float the currents can 

 be measured by stationary instruments past which the current flows, 

 turning a propeller of some type or exerting a pressure which can be 

 determined by different methods. The advantage of these methods is 

 that observations need not be limited to the currents of the surface layers 

 but can be extended to any depth. The obvious difficulty is to retain the 

 instrument in a fixed locality so that the absolute flow of the water may be 

 measured, and not merely the flow relative to a moving instrument. In 

 shallow water a vessel can be anchored so that the motion of the vessel is 

 small enough to be insignificant or of such nature that it can be elimi- 

 nated. In deep water, current measurements were first made from 

 anchored boats, but in recent years the technique of deep-sea anchoring 

 has been advanced to such an extent that vessels like the Meteor, the 

 Armauer Hansen, and the Atlantis have remained anchored in depths 

 from 4000 to 5500 m for days and weeks. In other instances, relative 

 currents have been measured from slowly drifting vessels. 



Maintaining a vessel at anchor for a long time is expensive, and devices 

 have therefore been developed for anchoring automatic recording current 

 meters that can be left for weeks at a time. Measurements of currents 

 very close to the sea bottom cannot be made safely from an anchored 

 vessel, no matter how securely it is kept in position, because an instru- 

 ment suspended from the vessel cannot be retained at a constant distance 

 from the bottom owing to the motion due to swells and tides. The diffi- 

 culty was first overcome by Nansen, who lowered a tripod to the sea 

 bottom and suspended a current meter from the top of the tripod. This 

 method has more recently been used by Stetson, by Revelle and Fleming, 

 and by Revelle and Shepard. The latter suspended three current meters 

 from the top of the tripod and were thus able to obtain simultaneous 

 measurements of currents at three levels within less than 2 m of the 

 bottom. 



Numerous types of current meters have been designed, each having 

 special advantages and disadvantages. Owing to its simplicity and 

 reliability, the Ekman current meter is widely used (fig. 7). The essential 

 parts of the instrument are the propeller, the revolutions of which are 

 recorded on a set of dials, the compass box with the device for recording 

 the orientation of the meter, and the vane that orients the instrument so 

 that the propeller faces the current. The free swing of the instrument 

 is ensured by mounting it in ball bearings on a vertical axis. The wire for 

 lowering is fastened to the upper end of this axis, and a suitable weight 

 is attached below the axis. The instrument is balanced in water so that 

 the axis is vertical. The carefully balanced propeller, with four to eight 

 thin, light blades, runs inside a strong protective ring, but can easily be 



