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 drawTi — 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. 



Fig. 4. — A vertical section tlirough 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 shotvs 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 sonie isobaric surfaces. The dynamic distance from the 

 sea surface to the isobaric surface of p decibars is d^. at station A, 

 and db at station B. According to equation (b) we have: 



<^b = Pb ^b 



But Pa = Pb 



and therefore 



da. — db = p{Va, — Vb) . . . . in terms of dynamic meters (c) 



(^a — dh 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, da,~d\) = Tc L, where Jc is a force and L is the distance 

 between the two points. Hence the force per unit mass due to gravity 

 may be expressed 



7. ^ ^a -db _ p{Vai- Vb) 



