246 
DR. HAROLD JEFFREYS ON TIDAL FRICTION IN SHALLOW SEAS. 
rather wide straits, Carimata Strait and Caspar Strait. In both of these the tide is 
mainly diurnal; in the latter, in fact, it is entirely so. At Pontianak, near the north¬ 
westerly point of Borneo, the tide is still diurnal; thus the part of the China 
Sea south of Pontianak and Singapore probably contributes little to the secular 
acceleration of the moon. 
Apparently the main tidal stream from the China Sea strikes the Malay Peninsula 
somewhere near Cape Patani and spreads out from there ; for on the coast north of 
this point the flood stream sets to the north, while south of it it sets to the south. 
The tide in this region is definitely semi-diurnal, though the heights of the two daily 
high waters may be unequal. The depths in the western part of the sea and in the 
Gulf of Siam are mostly about 30 fathoms, but there are many shoals around the 
coast where the depth is onty a few fathoms, and it is therefore necessary to be verj^ 
critical of the sites of observations of currents. The best results seem to be given at 
small islands with rapidly shelving sides, for the currents there are modified little by 
the form of the bottom and can be regarded as fairly typical of the general currents 
in the neighbourhood. 
At the Anamba Islands (see map, fig. l) the semi-diurnal tide appears to be 
usually much less than the diurnal one. The £ China Sea Pilot,’ vol. 3, states that for 
a few days in each month, when the moon is near the Equator, there are two high 
tides in the day. It is easily seen that for two tides to occur in the day the 
amplitude of the semi-diurnal term must be at least a quarter of that of the diurnal 
term. It would not, however, vary much with the moon’s declination, whereas that 
of the diurnal term vanishes when the moon is on the Equator ; and the above fact 
shows that the semi-diurnal tide only attains this fraction of the diurnal tide when 
the latter is at its least. The true semi-diurnal tide at the Anamba Islands must 
therefore be insignificant. The same is evidently true of the currents, for the tidal 
streams take a day to run backwards and forwards. 
In the Gulf of Siam also the tides are mainly diurnal. The oscillation in this is a 
forced one due to that in the South China Sea, and as the latter is diurnal so is that 
in the Gulf of Siam. Actually the only place where the semi-diurnal tide is 
considerable is Bangkok Harbour, at the head of the gulf. This tide seems to 
increase in relative importance towards Bangkok, for at Kamput on the eastern side 
and places in about the same latitude on the western side the tides are said to be 
very irregular, indicating the presence of some complicating influence. On the whole, 
therefore, it seems that the dissipation in the Gulf of Siam will not be underestimated 
if we assume that the semi-diurnal current reaches a maximum of one knot, this being 
one-third of the diurnal tidal current observed off Cape Patani, at all places north of 
the parallel of 11° N. The area of this region is about 70,000 sq. km., giving a 
maximum dissipation of 2x 10 17 ergs per second. 
An estimate may be made of the dissipation of the energy of the diurnal tide in 
the same regions. At Pontianak there is a diurnal current of two knots when 
