There are regions of strong currents, such as 

 the Gulf Stream, in which mean surface velocities 

 rise to several feet per second (fps). The super- 

 imposed macro-turbulence has velocities with mag- 

 nitudes a large fraction of the mean and direc- 

 tions vary through the full rotation of the 

 compass. In much of the ocean, established cur- 

 rents have mean velocities of the order of a few 

 tenths of a foot per second and there are large 

 areas in which the mean may be measured in hun- 

 dredths of a foot per second. In areas of weaker 

 surface current the winds are often the chief 

 disturbing influence and, since wind-driven water 

 velocities may approach 2$> or more of the wind 

 velocity, a situation can exist in which the 

 instantaneous velocities are many times larger 

 than the mean and the direction of flow has little 

 relation to the mean direction. 



Surface waves commonly have periods in the 

 region of h to 25 seconds and heights reaching 

 50 feet or more. Heights of 6 to 12 feet are 

 common. The particle velocity in the wave is 

 quasi-sinusoidal and, at the surface, has a peak 

 velocity equal to the velocity of a particle 

 rotating uniformly in a circle of diameter equal 

 to the wave height and making one complete rota- 

 tion during one wave period. In a 12-foot wave 

 of 10 seconds period, the particle velocity is 

 then 1.2 TT, or 3-77 ft/sec. This motion may 

 affect near-surface meters directly or may intro- 

 duce stray motion into the surface-floated sup- 

 porting system. The particle motion decreases 

 rapidly with depth in proportion to e"2TT Z/L 

 where Z is the depth and L the wavelength . 

 Thus, for a decrease in depth equal to 1/9 of a 

 wavelength, the orbital diameter decreases by 

 one-half. For the 10-second wave in deep water 

 the computed wavelength is 512 feet; the particle 

 velocity would have decreased to one-half at 

 57 feet of depth and to 1/512 at 512 feet of 

 depth. Under these conditions a meter or buoy 

 even at a depth as shallow as 228 feet (k/S> of 

 the wavelength) would experience an orbital 

 velocity of only 0.23 knots. In the presence of 

 a long high swell this would not be true. 



Water velocities are generally considered to 

 decrease with depth, at least down to some depth 

 at which velocities are relatively small. This 

 is a plausible conclusion where the driving forces 

 are the winds at the surface. However, there are 

 circumstances in which the decrease with depth 

 is followed by a reversal and an increase to 

 velocities comparable to those at the surface. 

 Two such cases are the Cromwell Current in the 

 Equatorial Pacific and its counterpart the 

 Atlantic Equatorial Undercurrent. Some driving 

 forces, such as the tides, the moving atmospheric 

 pressure disturbances or the long waves of 

 tsunamis, make their relatively small contribu- 

 tions nearly equally at all depths with magni- 

 tudes of the order of 0.1 fps . There is always 

 the possibility that such deep-acting forces may 

 cause more important velocities by convergence 

 into deep narrow channels between islands or 



between seamounts. Sporadically, in some areas 

 having sufficient bottom slope, turbidity currents 

 will flow along the bottom with velocities reach- 

 ing 15 fps or more. 



Then there are the massive, but extremely slow, 

 thermohaline circulations of the deep ocean water 

 generated in the Antarctic and Arctic by the 

 sinking of surface water made more dense by cool- 

 ing and partial freezing. The velocities of these 

 flows average at most a few hundredths and perhaps 

 a few thousandths of a foot per second, but the 

 few direct measurements which have been made in 

 deep water have shown transient velocities of as 

 much as 0.7 fps and directions having little rela- 

 tion to the indirectly deduced mean. 



The Meed for Direct Current Measurements 



For many years the velocities and patterns of 

 average ocean currents have been deduced by two 

 principal methods: (l) the statistical treatment 

 of the reported deviations of naval vessels from 

 their courses and (2) indirect computation by 

 application of the geostrophic equation to the 

 experimentally (and indirectly) measured density 

 field. The first method is inaccurate and yields 

 only surface currents . The second gives some 

 information about mean velocities at all depths 

 in deep water but fails near shore. The determin- 

 ation of flows at great depth is strongly depen- 

 dent on the choice of a depth of no net motion 

 which still must be determined by methods which 

 are theoretical and not well supported by experi- 

 ment . Both methods produce means with a summing 

 period of one or more weeks and give little or 

 no information about transients of shorter period. 



A determination of mean velocities is useful 

 and the geostrophic method has advantages but it 

 is now necessary to check its validity by experi- 

 ment and supplement the findings where the method 

 is known to be weak. In the vast regions where 

 transient velocities in the sea are many times 

 greater than the mean it can be seen that many 

 processes probably have little relation to mean 

 velocity. Examples of such processes are the 

 transport of sediments, the mixing of water masses 

 and the transport of plankton and fish. The 

 understanding of these processes awaits an ade- 

 quate description of the transients. 



The Experimental Problem 



With the above outline of the complexities of 

 motion in the sea it is now possible to look at 

 the practical problems in current measurement . 

 These will be considered in two senses: (l) the 

 design of the experiment and (2) the design of 

 measuring equipment and suspensions . 



The two elements in the design of a current 

 measuring experiment which are most often poorly 

 done are: (l) design of the experiment to 



Superior numbers refer to similarly numbered references at the end of this paper. 



136 



