44 



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 generahties 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. 



