21 



APPENDIX: CURRENT MEASUREMENT* 



WIND-DRIVEN SURFACE CURRENTS 



To study the transfer of momentum between the air and the sea it seems 

 necessary to be prepared to investigate both very large and very small scale 

 phenomena. Apparatus for small scale studies of motion or the gradients of 

 motions, temperature, and other properties through the sea surface and in waves 

 are, to nny knowledge, entirely lacking. However, some existing equipment 

 miay be useful in connection with studies of the larger scale surface motions and 

 the vertical shear in the water phase. These are a pitot-type current meter 

 recently devised by Willem Malkus, the current cross as used by Pritchard and 

 Burt, and the G . E.K. 



The Malkus bathypitotmeter (1953) has the form of a large bathythermo- 

 graph and responds to currents in the range 50 to 250 cm/sec. It is necessary 

 to move the ship ahead slowly if the current speeds do not fall in this range and 

 then allow the instrument to come to equilibrium at each of a succession of 

 depths to obtain readings. Readings on two headings must be made and com- 

 posed to evaluate the resultant velocity at each level if this is not known at the 

 start or if the current turns with depth. As far as I know there is no corre- 

 sponding instrument for measuring the wind shear immediately above the sea 

 surface although stereophotographs of vertical smoke trails have been con- 

 sidered. 



Pritchard and Burt (1951) have found that for Reynolds numbers above 

 1000 the drag coefficient of a suitably weighted current cross remains constant 

 over a usefully wide range of velocities, hence the angle of a wire supporting it 

 is a simple function of the speed of the passing water. This convenient property 

 of so familiar a device may have many applications as, for example, measure- 

 ment of the vertical shear from a fixed or drifting station. If a recording in- 

 clinometer can be devised to accompany the current cross on its descent, it 

 should be possible to measure vertical shear through great depths as well as 

 small ones. Edmond Watson has constructed a device for measuring wire angle 

 as a function of depth. This consists of a smoked glass slide moved in one di- 

 rection by the action of pressure on a sylphon and a pendulum carrying a stylus 

 to write the record of depth and incliniation. The pendulum is damped by the 

 water which floods the instrument case. This might be adapted to cover a num- 

 ber of ranges of depth and might also house a recording thermograph so that the 

 current shear and thermal gradients may be compared. Means for recording 

 direction at great depths remain to be considered. 



* - Mr. von Arx has suggested that his remarks made in the course of the 

 discussions following his paper be presented in these proceedings as an appen- 

 dix to the main body of the paper. 



