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. 
a =G) ID) Goes ocx way eam Ort Cleon ove Manse y se ee eee ete 2 (C) 
D=>p .... im 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. 
ea ey 
Decibars -_| 100 | 200 | 300 400 | 500 | 600 | 700 | 800 | 900 | 1,000 | 1, 200 | 1,400 | 1,600 | 1,800} 2,000 
Meters__-_| 99 | 198 | 298 | 397 | 496 | 595 | 693 | 792 | 891 990 | 1,187 | 1,385 | 1,582 | 1,779 | 1,975 
Dynamic 
meters___| 97 | 194 | 292 | 389 | 486 | 583 | 680 | 777 | 874 970 | 1,164 | 1,357 | 1,551 | 1,744 | 1,936 
It will be seen from the foregomg that under conditions as specified 
there is a difference 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 
grayity 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 with a thickness of 1 dynamic 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—26}—_3 
