25 



It is assumed that the specific volumes in situ have been calculated 

 from the tables, as based upon the temperature and salinity records 

 from the observed depths, viz, 0, 50, 125, 250, 450, and 750 meters. 

 The values of p are, for all practical purposes, equal to the depth in 

 meters — that is, at a depth of 750 meters the pressure is 750 decibars 

 (see p. 11). In order to compute as accurately as possible the value 

 of D (the dynamic depth) for the vertical AC, it is necessary to 

 consider the change at frequent depths which occurs in the value of v 

 (the specific volume) . In order to comprehend the method of mathe- 

 matical computation customarily followed, e. g., p. 28, it will be help- 

 ful to regard Figure 9. 



dp 



M 



N 



Fig. 9.— a graphic means of illustrating the mathematical integration 

 customarily employed in computing the dynamic depth to a given 

 observed pressure beneath the sea surface 



The area MNPO (assumed equal numerically to Da) is formed part- 

 ly by the ciu've NP, which represents varying values of v, and the 

 side MO, which indicates the scale of pressure. The value of Da, 

 therefore, represented by the area MNPO is equal in value to the 

 sum of aU the smaller areas <Zi, d^, d^, c?4, and d^. The area di (and 

 the values of all the other small rectangles in similar manner) may, 

 with sufficient accuracy, be put equal to 



i.-(^-^)iv. 



or 



which equals the value of the gravity potential, relatively to the sea 

 surface, expressed as a depth in dynamic meters at which the pressure 



