14 



U. S, COAST AND GEODETIC SURVEY 



35 curve to the 36 curve, the salinity of the sample is 35.7. When 

 the test has been completed, the prism should be dipped into fresh 

 water and brushed dry with a clean, soft cloth. The glass cups 

 and the thermometer should also be rinsed with fresh water. 



If the weather is such that the water tends to slop out of the heat- 

 ing trough, so that the latter can not be used conveniently, the cup 

 designed for use with volatile liquids, which is supplied with the 

 instrument, can be used. The test should be performed in a closed 

 room in order that it may be possible to determine the temperature 

 of the sample with some degree of certainty. The cup should be 

 filled about half full of sample and then allowed to stand for two 

 or three minutes, after which the temperature of the sample in the 

 cup should be taken. 



The cup is then fitted to the end of the refractometer, the latter 

 pointed toward the light, and five scale readings taken. The cup 

 is then removed from the refractometer and the temperature of the 

 sample in the cup again observed. From the mean of the five scale 

 readings and the mean of the two temperature readings the salinity 

 of the sample can be obtained from the chart. The cup should be 

 rinsed out with fresh water after the test and the prism and ther- 

 mometer also cleaned as already explained. 



Experiments were conducted recently on board the Lydonm to de- 

 termine the consistency of values of the salinity of a given sample 

 obtained by means of the refractometer by several inexperienced 

 observers working independently of one another. The results are 

 given in Table 4. After the observers had a little practice with 

 the instrument their individual results would be, no doubt, in better 

 accord with one another. 



Table 4. 



-Independent determinations of the salinity of a given sample of sea 

 water tp the dipping refractometer method 



Note.— With the exception of the last observer, none of the observers had had any previous experience 

 with the method . The observations were made on board the Lydonia while she was under way and rolling 



