U. S. COAST AND GEODETIC SUKVEY 



tional part of the distance between the curves, measured parallel 

 to the axis of specific gravities, that the point lies to the right of 

 the curve of lower salinity. Thus, if the point lies three-tenths 

 of the way from the 34 curve to the 35 curve, the salinity of the 

 sample is 34.3, and so on. 



If it should be desired to use Table 1 instead of Figure 2, it will 

 be found convenient to reduce the observed specific gravity (cor- 

 rected for graduation error) to the temperature in the table that is 

 nearest to the observed temperature. For example, suppose that the 

 observed specific gravity (corrected for graduation error) is 1.0224 

 and the temperature, 22. °3 C. It will be seen in the table that the 

 change per degree in the specific gravity is 2.5 in the fourth decimal 

 place (in the part of the table corresponding to the observed specific 

 gravity and temperature). Hence the change for 2.3 degrees will 

 be 6 in the fourth decimal place. Thus the observed specific gravity 

 and temperature are equivalent to 1.0230 at 20° C. By interpola- 

 tion between 1.02263 and 1.02340 we find the salinity of the given 

 sample to be 32.5. 



It will be seen from the table that a change of one unit in the 

 second figure of salinity corresponds to a change of T or 8 in the 

 fourth decimal place of specific gravity. Therefore, an error of 

 one in the fourth decimal place of specific gravity will produce an 

 error in the salinity scarcely greater than one in the third significant 

 figure. 



Experiments were conducted recently on board the Lydonia to 

 determine how consistently a hydrometer could be read on board 

 ship when the ship was under way and rolling and pitching gently 

 to a light chop and swell. Several officers made independent specific- 

 gravity and corresponding temperature measurements upon the same 

 sample, with the results shown in Table 2. It will be seen from the 

 table that the greatest discrepancy from the mean was one unit in 

 the fourth decimal place of specific gravity. 



Table 2. — Independent determinations of the salinity of a given sample of sea 

 toater hy the hydrometer method 



Note.— The above observations were made on board the Lydonia while she was under way and rolling 



and pitching to a light chop and swell. 



