38 



previously discussed on'page 28. It possesses great practical advan- 

 tages in that it presents two pertinent, desirable pieces of information, 

 viz, (1) the direction of movement, and (2) the relative rate of flow 

 of the current, over any given area. Let us suppose that the tem- 

 perature and salinity data, surface to 750 decibars, have been collected 

 from a sufficient number of stations in the region south of the Grand 

 Banks. This, as a matter of fact, corresponds to an actual oceano- 

 graphical investigation carried out by the International Ice Patrol 

 in these waters during the spring of 1922. Dynamic treatment of 

 these data leads through the accepted methods of calculation as shown 

 on page 28. Column 12 on that page contains the dynamic depths of 

 the successive surfaces of observation, and also the material for the 

 construction of a dynamic topographical chart, of which Figure 19, 

 page 39, is an example. 



An isobaric surface, the dynamic topography of which is the sub- 

 ject of interest, may be visualized as spread out beneath the surface 

 of the sea, an undulating floor, the depth of which we plumb with the 

 same reality as the more tangible floor of the ocean is sounded out by 

 the hydrographer. As a first step toward the mapping of currents, 

 let us investigate any one of the standard isobaric planes of observa- 

 tion adopted by the International Ice Patrol, viz, 50, 125, 250, 450, 

 and 750 decibars, by plotting its dynamic soundings on a map at those 

 positions in latitude and longitude where the respective stations have 

 been located. This procedure, it is plainly seen, is identical to that 

 in which depths to the bottom are fixed on any ordinary navigational 

 chart. If, as a next step, equipotential (level) planes are passed at 

 frequent heights through the selected isobaric surface which is under 

 investigation, a number of lines of intersection are formed, which for 

 convenience may be called dynamic isobaths. If now we recall the 

 fact that when the accelerating force of friction is disregarded the 

 movement of water particles on an isobaric surface tends along such a 

 surface, as well as along the same equipotential surface (see p. 34, 

 fig. 16), it is not difficult to appreciate the significance of dynamic 

 isobaths. The small sketch in the lower left-hand corner of Figure 

 19, page 39, shows a series of dynamic isobaths and the direction of 

 the two forces which are always present wherever there prevails trans- 

 latory movements of water particles in a steady current. Friction, 

 for all practical purposes, may be disregarded, (See p. 43). (1) AE 

 illustrates the resultant of the forces which impel and maintain gradi- 

 ent flow; (2) AC represents the Ferrehan force acting in a plane 90*^ 

 to the right of the current; and (3) AB is the path of the actual estab- 

 lished movement following along the dynamic isobaths. When these 

 latter are recorded on an ordinary geographical map, as a series of 

 dynamic contours, it permits the reader, at a glance, to picture the 

 course followed by a water particle throughout the region which is 

 under survey. Figure 19 is shown as an example of a dynamic 



