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 Cl 
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 dynamic 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 third 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 
Kkman. (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 Knudsen 
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- 
