MANUAL OF TIDE OBSERVATIONS 85 



of the Avater in the jar is 11.5° C. The nearest observed density read- 

 ing given in the table is 1.0240. Following this line, we find differ- 

 ences of —7 and —5 for temperature 11° and 12°, respectively, giving 

 an interpolated difference of —6 for a temperature of 11.5°. This 

 diflerence of —6 applied to the original hydrometer reading of 1.0244 

 gives 1.0238 as the reduced value. 



269. In the heading of form 457 the heaviest and lightest sea water 

 pertain to the reduced values. 



270. Salinity. — The salinity of sea water is defined as the number 

 of grams of salts contained in 1,000 grams of sea water. While the 

 total amount of salts contained in a given volume of sea water varies 

 in different places, the relative portions of the different kinds of salts 

 is netirly constant in all parts of the ocean. For example, sodium 

 chloride or common table salt constitutes nearly 78 percent of all the 

 salts in any locality. Chemical analysis has shown that 1,000 grams 

 of sea water contain in solution an average of 35 grams of salts of 

 various kinds, of which about 27 grams is common table salt. 



271. The salinity of sea w^ater may be determined by several differ- 

 ent methods, one of the simplest methods being based upon the density 

 of the water as obtained from the use of the hydrometer. The 

 density depends upon the salinity and the temperature of the water. 

 The table on page 88 gives the salinity corresponding to different 

 densities at the standard temperature of 15° C, to which the den- 

 sities in form 457 are reduced. This table was compiled from 

 table 2 on page 38 of Coast and Geodetic Survey Special Publica- 

 tion No. 61, Physical Laws Underlying the Scale of a Sounding Tube. 

 Through the use of this table the values for the salinity in form 457 

 may be easily obtained from the reduced densities in the preceding 

 column. 



