a laminar flow and by vortices being shed from the regions downstream of the 

 separation points. The vortex streets in the wake are ultimately dissipated 

 by the viscosity far downstream from the cylinder. In the transition range^ 

 encompassing Re from I50 to 300^ laminar-turbulent transition begins to 

 appear in the free-stream layer that has separated from the cylinder. For 

 Re from 300 to 10 , the flow is characterized by the shedding of vortices 

 consisting of turbulent fluid whose source seems to be the separated shear 

 layer. Therefore^ rows of hydrogen bubbles present in the cathode wire 

 wakes for these latter three Re regimes can portray very deceptive patterns 

 of the fluid velocity profiles. Accordingly^ it is most desirable to 

 operate the technique in fluid-flow velocities where the diametral Reynolds 

 n-umber is kept below UO. It is apparent that this can be done by controlling 

 the fluid velocity, by selecting a suitable wire diameter, or by altering 

 the kinematic viscosity of the fluid. 



In addition to the applicability of the hydrogen-bubble technique to 

 water flows, it has recently been established that the technique operates 

 very successfully in water-glycerine mixtures (personal correspondence and 

 Reference 6). Such mixtures are extremely useful for changing fluid viscosity 

 by means of temperature control. In this glycerine-water mixture, several 

 wire conf iguratidZB were used to obtain velocity profiles in which there 

 existed an increasing vortlcity distribution in time throughout the profile. 

 Of particular significance, however, was the higher degree of photographic 

 clarity and contrast obtainable in such mixtures . The velocities attained 

 in this study were on the order of 6 in. /sec. 



It is believed that velocities considerably in excess of this value 

 can be investigated in these mixtures without sacrificing the bubble quality. 

 The greatest percentage of glycerine used in the cited study was kO per- 

 cent . However, it is felt that higher percentages could be used and 

 acceptable bubble quality retained. 



The determination of velocity profiles in two-dimensional flows in 

 which longitudinal and transverse components of velocity are of comparable 

 magnitude should be done with great care. Figure 1 is a good illustration 

 of such a flow. When only longitudinal displacements of successive bubble 



