PRINCIPLES OF HYDROSTATICS. 



45 



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meter and decibar, or occasionally also to other decimal parts or multiples of the 

 dynamic meter and the corresponding decimal parts or multiples of the bar. 



As a first simple example we can consider pure imcompressible water of unit 

 specific volume. Here there is full coincidence between isobaric andequipotential 

 unit-sheets. The standard isobaric sheets of i decibar (section 5) have the thick- 

 ness of 1 dynamic meter, exactly as the standard equipotential sheets (section 4). 

 Disregarding the atmospheric pressure on the sea's surface, and considering only 

 what we have called the sea-pressure (section 26), 

 we get this simple rule for finding the depth where 

 the pressure has a given value. The number ex- 

 pressing a given sea-pressure in decibars expresses 

 at the same time the depth of this sea-pressure in 

 dynamic meters. This rule, being exact for the 

 case of pure incompressible water, remains a use- 

 ful approximate rule also for the case of common 

 sea-water. 



As a second example we may consider sea- 

 water of 35 / 00 salinity at o C. In table 8 h of 

 Hydrographic Tables we have registered the spe- 

 cific volume of this water for the differences of 

 pressure of 1 bar (10 decibars). The unit of 

 gravity potential corresponding to this difference 

 of pressure is the dynamic decameter. Forming 

 the arithmetic mean of two and two successive 

 numbers in table 8 h, we get the mean specific vol- 

 ume of the water in isobaric sheets of 1 bar, i. e., 

 the thickness of these sheets expressed in dynamic 

 decameters. Adding these thicknesses from the 

 surface downward, we get the depths of all iso- 

 baric surfaces for the interval of pressure of 1 bar. 

 The dynamic depths found in this way are given 

 in table 7 H of Hydrographic Tables, the units 

 being again turned into dynamic meters and deci- 

 bars. The equilibrium relation connecting dynamic 

 depth and pressure according to this table is illus- 

 trated by the first vertical of fig. 1. The divi- 

 sions to the right of this vertical give the pressures 

 in decibars, and the divisions to the left the corre- 

 sponding depths in dynamic meters. The second 

 vertical of the figure gives in corresponding man- 

 ner the equilibrium relation between pressure and 

 specific volume, i. e., the relation contained numerically in table 8 h. 



If we had constructed complete tables of the specific volume of atmospheric air, 

 we should have been able to determine the heights of given pressures in the atmos- 



3 



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-1OO0O 1OOO0 



ton 



ioooo->- 10000- 



Fig. 1. State of equilibrium of sea- 

 water of 35 / 00 salinity and o C. 



