40 
topographical chart or ocean current map. This particular illustra- 
tion delineates the dynamic form of the 750-decibar surface as it was 
found to lie by the International Ice Patrol, May 5-7, 1922, south of 
the Grand Banks. 
It was pointed out on page 13 that the greatest variations in the 
physical state of oceans and consequently the obliquity of the iso- 
baric surfaces take place in the upper levels; constant conditions and 
a level position of isobaric surfaces, on the other hand, are most 
nearly approached in the greater depths. Therefore, in order to 
compare dynamic measurements with the same level plane, and 
ultimately to record the obliquity of the sea surface at several points 
of observation, it is necessary to measure upwards from a relatively 
deep-seated, level, isobaric plane, instead of downwards from the 
usually tilted surface of the sea. It is obvious, then, that the best 
representation of the real movements taking place on a sea surface 
is contained in a dynamic topographical chart of a relatively deep 
isobaric surface. Since the 750-decibar surface was the maximum 
depth to which the Ice Patrol’s investigations extended, this 
has been taken as representing the surface of minimum motion. The 
record of its form, as shown by Figure 19, approximates very closely 
to a surface current chart of the southern Grand Banks region. 
It is customary on dynamic topographical charts to record the dy- 
namic contours of an isobaric surface for multiples of dynamic milli- 
meters difference in depth, but sometimes, such as the present example, 
due to the great inclination of the baric lene only every 5 dynamic 
centimeters is graphically possible. It will be noted that the 750- 
decibar surface was found at a depth of 729.0848 dynamic meters at 
station 206 in the northern edge of the Gulf Stream (see column 12, 
p. 28), but at station 205, 32 miles nearer the continental slope, it was 
recorded at a depth of only 728.7215 dynamic meters. [If it is as- 
sumed in conformity with previous remarks, that the 750-decibar 
surface most nearly approached a position level, then there remains 
but one alternative, namely, that the sea surface was approximately 
36 centimeters higher at station 206 than at 205. This obliquity of 
the sea surface, and to a less degree in the other depths of observa- 
tion, south of the Grand Banks, May 5-7, 1922, is shown in greatly 
exaggerated (but proportionate) profile, Figure 5, page 14. In 
order to give the reader an idea of the slight inclination necessary 
in the sea surface to drive a current, we call attention to the differ- 
ence in actual dynamic height recorded, viz, about 1 centimeter per 
mile, which, according to computations, page 31, represents a gradient 
that impelled the surface layers at a velocity of 1.3 knots per hour. 
The direction in which the water is flowing also may be determined 
by comparing the dynamic heights of any two points of observation. 
If it is assumed that the current is more rapid in the upper levels 
than in the depths we shall be facing, in the direction of flow in the 
