ad 
is revealed by reference to certain current charts (dynamic topog- 
raphy) of which the Ice Patrol has record. As an example, we might 
call attention to Figure 19, page 39, wherein the position of the Gulf 
Stream relative to the 4,000-meter contour, clearly indicates that the 
current flooded in up the grade occasioned by the southeasterly 
extension of the Grand Banks (about 53° west longitude), but meet- 
ing increased resistance in the constantly shallowing depths it was 
deflected to the right, offshore. The current, proceeding along in this 
new direction where the depths increase, tended to swing to the left, 
inshore; and so in this fashion its course may be traced as it flowed 
along the continental slope in a serpentine path. 
Inshore, over continental shelves, it has been found that the 
coastal waters are in a slow primary circulation which Huntsman 
believes due to the pumping action of the tides combined with the 
effect of terrestial rotation. It has been expressed in a general state- 
ment, viz, bottom configuration in the Northern Hemisphere tends to 
deflect currents to the right (cum sole), around islands and shoals, 
and to the left (contra solem), around basins and deeps. 
TIDES 
One of the external forces provoking currents in the sea was 
ascribed to the tides (see p. 1). Such currents rotate in their move- 
ment either clockwise or counterclockwise one complete cycle, when 
unaffected by other influences, in a period of 12.4 hours. The theory 
of semidiurnal tides is based upon a series of waves which are known 
sometimes to be propagated great distances, but the discussion of the 
form of such waves, and the theory of orbital motion given to the water 
particles, is too lengthy to be included in this paper. Those inter- 
ested in a more detailed exposition are referred to Darwin (cf. Tides 
and their Kindred Phenomena in the Solar System.) We mayremark, 
however, a few generalities regarding the various tidal phenomena as 
they affect certain oceanographical problems. Tidal currents in the 
deep ocean basins are of comparatively subordinate importance, but 
near continental slopes and over shelves they may attain great magni- 
tude. Even when such slopes and shelves extend far out into an ocean 
basin, tidal effects remain quite prominent. The sheet of water lying 
over the Grand Banks, for example, averaging a thickness of about 
35 fathoms (65 meters), receives a regular tidal clockwise rotation 
which attains velocities ranging from 0.2 to 0.5 knots per hour. Well- 
marked rippling on the surface during periods of calm sea, moreover, 
has been observed along the eastern edge of the Grand Banks, which 
it is easy to believe were caused by the semidiurnal tidal wave meet- 
ing the rise as presented by the eastern face of the bank. Icebergs 
around the Grand Banks have often been carried inshore and grounded 
temporarily during calm weather when no other cause seemed so 
plausible as that of a favorable tidal set at the time and place. 
