12 
which show the distribution of pressure. But if we employ equa- 
tion (b) we must represent the result as dynamic isobaths inscribed 
on an isobaric surface and drawn—e. g., for unit differences of 5 
dynamic millimeters. Such a method of representation corresponds 
to that of a common topographical chart, but the contour lines on 
a dynamic chart instead of showing ordinary, linear heights, show 
levels of equal potential’ A dynamic topographical chart of a 
certain isobaric surface is the most approved method employed in 
modern dynamic oceanography to map ocean currents. 
APPLICATION OF DYNAMIC UNITS 
The number of unit equipotential sheets found in an isobaric 
sheet between two different station verticals represents a certain 
amount of potential energy existing between the two verticals. 
Fic. 4.—A vertical section through a sea basin and including the two stations A and B, with the 
respective points C and D separated by the distance L. C and D are at a depth of p decibars 
below the surface 
Figure 4 shows a section through a sea basin which includes two 
stations, A and B. The horizontal lines represent the intersections 
with some equipotential surfaces, and the oblique lines the inter- 
sections with some isobaric surfaces. The dynamic distance from the 
sea surface to the isobaric surface of p decibars is d, at station A, 
and d, at station B. According to equation (b) we have: 
dy = Pb Vp 
da =Pa Va 
But pa= Pp 
and therefore 
da—dp=p (Va—Vp) . . . . In terms of dynamic meters -- -- ------(c¢) 
d,—d,» represents the difference of potential energy, due to gravity, 
between the points D and C in Figure 4. This energy may be con- 
verted into work, d,—d,=k L, where kis a force and Lis the distance 
between the two points. Hence the force per unit mass due to gravity 
may be expressed 
k da—d»y _P(Va—Vd) 
L L 
