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 equipotertial 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 Ferrelian 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 
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