6331 HYDROGRAPHIC MANUAL PaGE 584 



and are temporarily removed from solution but these are eventually redissolved. Sea 

 water also contains gases and traces of a vast number of organic compounds. Some 

 of the gases, notably oxygen, remain in true solution, but others soon combine with 

 other substances in the sea water. Of all the dissolved substances that reach the sea, 

 only the volatile ones are removed by evaporation to the atmosphere. 



A study of a number of analyses of the composition of sea water from all regions of 

 the w^orld shows that sea water, regardless of locality, is of uniform composition and 

 varies only as to the percentage of total salts held in solution. The salinity is usually 

 liigh in regions where atmospheric temperature and evaporation are high and rainfall is 

 small, and vice versa. Ram water and water from rivers tend to float on the surface 

 of the sea and because of this the salinity near the surface is less than at greater depths. 

 For the same reason the salinity near shore is generally less than in the open ocean. 

 In some regions the salinity will vary considerably from surface to bottom and from 

 place to place. At one place in the Gulf of Mexico the salinity has been observed to 

 differ by 6 °/oo from the surface to bottom in only 8 fathoms of water. In the same 

 locahty the surface salinity has been observed to differ by 8 7oo at two places only 10 

 miles apart. 



6331. Determination of Salinity 



The salinity of sea water varies with the density of the water, and the simplest 

 method of determining the salinity, for use in hydrogi-aphic surveys, is to measure the 

 relative density, or specific gravity, with a hydrometer. Hydrometers and specific 

 gravity measurements are described in 475, and the method of recording the observed 

 data on Form 717, Record of Temperatures, Salinities, and Theoretical Velocities, is 

 described in 6341. The salinities, which are determined from the specific gravity 

 measurements by the use of tables or a graph, are also recorded on Form 717. 



The density of sea water depends not only on the amount of soluble matter con- 

 tained in a unit volume, but also on the temperature of the water. Sea water expands 

 and becomes hghter when heated and contracts and becomes heavier when cooled. 

 Therefore, the specific gravity by hydrometer must be corrected for any variation of 

 the water sample temperature from the standard of 15°C., for wliich the sahnity 

 tables are computed; consequently the temperature of the water at the time of the 

 specific gravity measurement is needed to determine the salinity (see 475). The 

 corrections for temperature and instructions for applying them will be found on pages 

 86 and 87 of Special Pubhcation No. 196 (1941 edition). Manual of Tide Observations. 

 Observed density, measured specific gravity, and apparent density or specific gravity 

 are all one and the same, and refer to the hydrometer reading uncorrected for tempera- 

 ture. With the corrected density, the corresponding salinity may be found from the 

 table on page 88 in Special Pubhcation No. 196. 



The salinity may also be found from a graph constructed from the values in the 

 tables for the required ranges of observed data. Figure 131 includes the range of 

 salinities from 28 to 40 °/oo. It may be seen from the figure that a change of 7 or 8 in 

 the fourth decimal place of specific gravity is required to produce a change of l°/oo in 

 the salinity. The hydrometer may generally be read accurately to 1 in the fourth 

 decimal place, so that the salinity may be determined with a theoretical accuracy of 

 almost 0.1 °/oo- 



