The instrumentation on the tower permits a large num- 

 ber of oceanographic variables to be measured. It is neither 

 reasonable nor practically possible to try to correlate every 

 available parameter with the attenuation measurements. In 

 the present experiments the plankton count and oxygen con- 

 tent of the water are regarded as the primary factors. More- 

 over, these variables together with temperature and salt 

 content describe the state of the sample (at atmospheric 

 pressure) in a direct manner, independent of the source and 

 procedure of sampling. 



The viscosity coefficients of the water partially deter- 

 mine the attenuation. It is known that these coefficients are 

 functions of temperature and salinity for clean sea water, 

 but the effect is very small compared with the attenuation 

 measurable with the cavity equipment. The temperature 

 was measured for every sample, but the salinity was not 

 considered. The salinity does not change appreciably 

 around the tower, since there are no fresh-water outflows 

 close by and there is fairly easy exchange with the open 

 ocean water. 



Samples were taken anywhere from the surface to the 

 bottom. Some samples would therefore have undergone 

 considerable pressure changes before the measurements. 

 The pressure change would not have any appreciable effect 

 on the sea water itself, but it can well have a marked effect 

 on the suspended particulate matter as well as on the dis- 

 solved gases. The sampling depth was therefore recorded 

 for all samples. However, it should be borne in mind that 

 the plankton counts and oxygen content were determined for 

 the sample at atmospheric pressure. The in situ conditions 

 of these variables and the quantitative influence of pressure 

 are a separate problem and beyond the scope of the present 

 study. 



The tower is equipped with a hydrophotometer and 

 measurements are recorded as the per cent transmission 

 of that in pure distilled water. The path length of this 

 instrument is ■§ meter. Sea water samples at depths be- 

 tween surface and bottom have an up-and-down motion due 



61 



