MEASUREMENT OF SALIlSriTY OF SEA WATER 19 



method and the sinker method, both of which arrive at the salinity of 

 the sample directly through its specific gravity. Either method, when 

 used with reasonably good equipment and with reasonable care, will 

 give salinity with an average error of about 2 in the fourth significant 

 figure. 



PYCNOMETER METHOD 



In the pycnometer method a small glass bottle with perforated 

 glass stopper is weighed — first, empty and dry; second, full of dis- 

 tilled water at a known temperature; third, full of sea water at a 

 known temperature. From the first two weighings, which can be 

 made once for all, the volume of the sample can be obtained. From 

 the first and third weighings the mass of the sample is determined. 

 From this the specific gravity of the sample can be calculated readily, 

 and its temperature having been measured, its salinity can be 

 obtained. 



Table 1 or Figure 2 might be used for this conversion from specific 

 gravity and temperature to salinity, but for precise work, such as is 

 possible with the pycnometer method and with the sinker method, it 

 is better to use Tables 16" to 19", inclusive, of Dynamic Meteorology 

 and Hydrography, Part I, Statics, by V. Bjerknes and J. W. Sand- 

 strom, which is Publication No. 88, 1910, of the Carnegie Institution 

 of Washington; or the Hydrographic Tables, by Martin Knudsen, 

 Copenhagen, 1901. 



SINKER METHOD 



The details of this method, as used by the Scripps Institution of 

 Oceanography, are contained in a paper by N. W. Cummings in 

 the Proceedings of the First Pan-Pacific Science Congress, at Hono- 

 lulu, 1921. Mr. Cummings also gives several references to papers 

 describing other good available methods for measuring the specific 

 gravity of sea water. 



A glass sinker, having a volume of about 25 c. c. and a weight of 

 about 35 g., is suspended by a fine platinum wire, 0.1 mm. in diameter, 

 from the left arm of a chemical balance of sensitivity 0.1 mg. and 

 weighed once for all suspended in distilled water of known tempera- 

 ture. The specific gravity of a given sample of sea water is obtained 

 by weighing the glass sinker in the sample and at the same time 

 determining the temperature of the sample. The portion of the 

 sample in which the sinker is immersed is " held in a specially de- 

 signed container * * *." 



"After the determination is made the sample is withdrawn through 

 a tube leading from the bottom of the containing vessel. It is 

 thus possible to introduce and remove the sample without in any way 

 disturbing the apparatus." The well-systematized procedure, both 



