bubbles or nuclei. It seems likely that the large excess 

 attenuations following stirring are caused by very fine 

 bubbles, when the water is supersaturated, but the attenua- 

 tion remaining after periods without disturbance may be 

 caused by nuclei. These nuclei may either be stabilized 

 by dead particulate matter, or perhaps inside of living 

 phytoplankton. There is evidence that plankton, at least 

 under certain conditions, do contain nuclei. 



The importance of the above considerations in the 

 evaluation of the transmission anomalies encountered in 

 the ocean remains to be seen. Vertical currents are 

 known to exist in nearshore areas, and the pressure 

 changes created may well result in bubble growth when a 

 water mass is brought close to the surface. It was ob- 

 served at two occasions that bubbles could be stabilized 

 by particulate matter, and there is reason to believe that 

 this phenomenon also takes place in nature. Bubbles 

 stabilized in this manner may again be carried down and 

 cause some of the anomalies observed. 



SUGGESTIONS FOR FUTURE STUDIES 



Little is known of the acoustic properties of water 

 containing a large number of air bubbles much smaller 

 than resonance size. Future work must therefore include 

 an investigation of small bubbles and nuclei, before a full 

 understanding of sea water measurements can be expected. 

 Special attention must be given to the influence of plankton, 

 since nuclei inside a living cell may have an especially 

 marked effect due to the surrounding organic matter. Only 

 when these problems have been answered satisfactorily is 

 it to be expected that the acoustic properties of plankton 

 itself can be ascertained. 



Cavity methods are well suited for most phases of 

 this work. Single bubbles may be created in the center of 

 a cavity around a fine platinum wire by means of electroly- 

 sis. This method has been utilized in the past for 



98 



