DR. HAROLD JEFFREYS ON TIDAL FRICTION IN SHALLOW SEAS. 
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later at Port Arthur. On the opposite side of the Strait of Pe-Chili it is slightly earlier 
than at Port Arthur ; then on the way round the Shan-tung peninsula and out of 
the sea again the tide again becomes steadily later. It therefore looks as if the tide 
enters up the coast of Korea, gradually passes up the sea, losing energy all the way, 
and a reflected wave from the Pe-Chili strait emerges down the Chinese coast. The 
tides and the currents on the Korean side are noticeably stronger than those on the 
Chinese side, and it does not seem likely that this is due wholly, or even largely, to 
the shoals on the former coast, for at islands in deep water, such as the Mackau, 
Myangoru, and Bate groups, velocities of 3 to 5 knots are recorded, while in 
shallower water the velocities are not usually greater than these, though local strong 
streams exist. Apparently the effects of friction are great enough to counterbalance 
those of the diminution in depth. They are also seen in another respect. Among 
the islands of the Korean archipelago the tidal stream sets west from four hours 
before till two hours after high water, whereas in most places elsewhere there is little 
or no current at high water. Thus work is continually being done on the sea, and 
the energy entering is dissipated in it. An effect of the approximate agreement in 
phase between the tide and the velocity may be utilized to give an estimate of the 
currents and tide height in the entrance to the sea, far from the nearest land, and 
hence of the amount of energy entering. In any motion in a channel where, on 
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