256 
DE. HAEOLD JEFFEEYS ON TIDAL FEICTION IN SHALLOW SflAS. 
6. Malacca Strait. 
This is a narrow triangular area, about 800 km. in length, separating Sumatra from 
the Malay Peninsula. The tide of the Bay of Bengal enters at the north-west end, 
and gradually increases in height as it advances along the strait towards Singapore. 
At the south end, however, the part of the tide that has not been reflected or 
dissipated on the way through the strait is overwhelmed by the diurnal tide of the 
South China Sea. Ample observations of the tides and tidal currents on both sides 
are available. The currents as far south as Cape Medang (nearly due west of 
Malacca) seem to reach maxima of \\ to 3 knots, the average amplitude at springs 
being practically 2 knots. The area of this region is 100,000 sq. km., and the 
dissipation is accordingly found to be about 9 x 10 17 ergs per second on an average. 
An alternative determination can be made by finding the rate of inflow of energy. 
At Kumpei, on the Sumatran side and near the north end of the strait, it is high 
water, full and change, at noon, and the amplitude at springs is 120 cm. The flood 
tide outside the bar, in water about 20 fathoms deep, sets south-east from three hours 
before high water till three hours after it, so that it reaches its maximum speed of 
l|- knots at high water. At Penang, near the opposite shore, it is high water at 
Oh. 21m. and the current reaches its maximum velocity of 2\ knots an hour before 
high water. As the strait is everywhere narrow in comparison to its length, and as 
these observations do not seem to have been taken on shoals, they are probably 
representative of that part of the strait. The amplitude of the tide at Penang is 
100 cm. We can therefore take the average height of the tide along the section from 
Kumpei to Penang to be 110 cm., and the average current when flowing strongest to 
be 2 knots, reaching its maximum half an hour before high water. The average 
depth is about 30 fathoms. A modification must be made in the previous procedure 
to allow for the fact that the current flows along the strait, which is not quite at 
right angles to the line of the section ; therefore, in finding the energy crossing the 
section, we must take for the length of the section, not the distance from Kumpei to 
Penang, but the projection of this on a line perpendicular to the strait, which is 
230 km. The flux of energy is hence found to be, on an average, 7 x 10 17 ergs per 
second. We also require the amount of this energy that emerges through the narrow 
part of the strait. Off Cape Medang, which marks the narrowest point of the strait 
away from the immediate neighbourhood of Singapore, the amplitude of the tide is 
120 cm., high water occurring at 6h. 30m. The tidal current flows at an average 
speed of about 2|- knots. There is no record of the tidal phenomena just opposite, 
but in Malacca Road it is high water at 7h. 30m., with an amplitude of 165 cm. ; the 
current there reaches its maximum of 2 knots an hour before high water. At the 
eastern end of South Sands, which lies near the Malay side, north-west of Cape 
Medang, it is high water about 6h. 0m., and the tidal stream has a maximum speed 
of l|- knots an hour before high water. The width of the channel at Cape Medang is 
