15 



reciprocal of the specific gravity and is chosen in preference to the 

 density because its value leads to the simplest method of dynamic 

 calculations. It is, in such cases, combined directly with other parts 

 of the term pressure, and furnishes a result in terms of gravity 

 potential. (See equation (b), p. 11.) 



In the depths of the sea observations are not made directly of spe- 

 cific volume, but it is obtained only after first finding the temperature, 

 salinity, or density at a given temperature, and then correcting for 

 the particular depth below the surface at which the observation was 

 made. The temperature is, of course, a direct instrumental obser- 

 vation. The salinity is calculated ordinarily by determining the 

 chlorine content of the sample and substituting in Knudsen's formula : 



s = 0.30 + 1.805 CI 



The two foregoing characters of sea water have been tabulated by 

 Knudsen with regard to corresponding values of density, within the 

 range of that normally met in the oceans. 



It is vitally necessary in the course of djTiamic computations, 

 moreover, to know the specific volume in situ — that is, the actual 

 specific volume as it existed at the particular depth at which it was 

 found. Thus, after the specific volume has been determined from the 

 temperature and salinity, it must be corrected for a tliird variable, 

 viz, compression. It is easy to appreciate that a mass, even such as 

 water, becomes more and more compressed the deeper down we pene- 

 trate beneath the surface. Naturally, the more compressed a body 

 becomes, the denser it grows — i. e., its specific volume becomes in- 

 creasingly less. The compressibility of sea water is not entirely de- 

 pendent upon the depth below the surface, but it is also influenced by 

 the temperature (and to a much slighter degree by the salinity) pre- 

 vailing in the water itself. Generally speaking, the warmer and 

 saltier is a water mass, the less it can be compressed. Investigations 

 have been made regarding the compressibility of sea water at various 

 depths under different combinations of temperature and salinity by 

 Ekman. (cf. Die Zusammendriieckbarkeit des Meerwassers, etc. 

 Pub. de Constance, Copenhagen, 1908. 



In order to construct tables for specific volume in situ, it is neces- 

 sary to combine the two previous tables — namely, those of KJiudsen 

 for temperature and salinity with those of Ekman for compressi- 

 bility. It is impossible, however, to arrange one convenient and 

 accurate table for specific volume in situ, because of the multitudi- 

 nous combinations arising between the three variables, viz, tempera- 

 ture, salinity, and compression, as they commonly range in the sea. 

 Direct tablulation, according to V. Bjerknes, would require something 

 like 256,000 pages of 500 numbers each, if intervals of 0.1 degree tem- 

 perature, 0.01 per mille salinity, and 10 decibars pressure were em- 



