11 



pressure of 1 decibar. We should note the coincidence that 1 meter 

 below the surface the gravity potential is very nearly 1 dynamic 

 meter less, and the pressure 1 decibar more. 



p^q D .... in terms of decibar units (a) 



D=v p . . . . in terms of dynamic meter units (b) 



In order to show the close coincidence existing between dynamic 

 units and pressure units of this system for increasing depth, we may 

 regard the various values for the three arguments, viz, common 

 meters, dynamic meters, and decibars, as they exist in a sea of 0° C. 

 temperature, and 35 per mille salinity. 



Decibars .. 



Meters 



Dynamic 

 meters... 



2,000 

 1,975 



1,936 



It will be seen from the foregoing that under conditions as specified 

 there is a dift"erence of about 1 per cent between a depth expressed 

 in pressure decibars and that expressed in common meters. This 

 difference becomes even smaller under natural conditions prevailing 

 on the earth, and thus being so insignificant, when contemplating 

 the horizontal extension of ordinary sea areas, permits us, with the 

 same number, to express a depth either in common meters or in 

 decibars. The difference between dynamic meters and common 

 meters averages about 2 per cent, and between dynamic meters and 

 decibars about 3 per cent, and these are of a magnitude that can not 

 be disregarded. ^ 



The two foregoing equations (a) and (b), in the case of equili- 

 brium, expresses as simply as possible the relation existing between 

 gravity potential, pressure, and specific volume. Thus it follows 

 that we may by (a) find the pressure in decibars at a given dynamic 

 depth, or by (b) the dynamic depth of a certain given pressure. 



We have already described the equipotential gravity surfaces and 

 the potential sheets \vith a thickness of 1 d3aiamic meter. Now 

 the surfaces of equal pressure are given, called isobaric surfaces, 

 which are separated by isobaric sheets 1 decibar thick. It is seldom 

 that we have under natural conditions a motionless water mass, 

 and so then it will usually be found that isobaric and level surfaces 

 intersect. In other words, an isobaric surface contains varying 

 potentials of gravity, and a level surface, in like manner, contains 

 many baric variations. The intersections of these two surfaces may 

 be considered as lines of the one inscribed on the plane of the other, 

 accordingly as we employ equation (a) or (b) . If the lines of inter- 

 section are considered inscribed on the level surfaces, they are 

 isobars, and the chart is similar to the ordinary meteorological charts 

 71321— 26t 3 



