244 
DR. HAROLD JEFFREYS ON TIDAL FRICTION IN SHALLOW SEAS. 
3. The North Sea. 
A satisfactory estimate of the dissipation in the North Sea is practically impossible. 
Velocities up to over 3 knots are recorded here and there, but all the observations are 
in the coastal region, which is very much complicated by shoals. The maximum in 
the outermost part of the Moray Firth is about l'l knot, and this is probably fairly 
typical of the whole of the North Sea. Taking the area to be 5x 10 5 sq. km. and 
adopting the above value of the velocity, we see that the maximum dissipation is of 
the order of l'8x I0 18 ergs per second. 
4. Other European Waters. 
In the Mediterranean there is probably little or no dissipation of tidal energy, for 
the Atlantic tidal wave can only enter through the very narrow Straits of Gibraltar, 
and partly for this reason and partly on account of the great length and considerable 
depth of the sea there is very little tidal movement in it. The same argument applies 
to the Baltic, for the entrance through the Kattegat is largely blocked up by the 
Danish islands, so that little water can enter to produce a tide. The Bay of Biscay 
is mostly too deep to have any important current, while the White Sea is too small 
and landlocked to give as much dissipation as the Irish Sea. 
The average dissipation in a period is 4/37r of the maximum. If we find the 
maximum for the Irish Sea on this basis, we obtain for the total dissipation in 
European waters when the spring tide currents are flowing strongest about 6'Ox 10 18 
ergs per second ; the average at spring tides is 2'4 x 10 18 ergs per second. 
Asiatic Seas. 
It has already been pointed out that the tidal currents in mid-ocean are insufficient 
to give any important dissipation.* Accordingly we need consider only those places 
where the currents are very much magnified by great decreases in depth. On 
referring to a physical map of Asia it is at once seen that the places around the coast 
where the depth is less than 100 fathoms are the Straits of Malacca, the South China 
Sea (with the Java Sea), the Gulfs of Siam and Tongking, the Yellow Sea, the 
Persian Gulf, and parts of the Seas of Japan and Okhotsk and the Bering Sea. 
These regions will be dealt with separately. The Persian Gulf may be omitted at 
once, as its narrow entrance prevents the tide from being great. 
1. The South China Sea. 
This sea is in the form of a letter T. The middle stroke points north-east and lies 
between Annam and Southern China on the one side, and Borneo and the Philippines 
* ‘ Philosophical Magazine,’ May, 1920, vol. 39, pp. 578-586. 
