THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
TIDES.
939
The tides in the harbour of Hongkong are semi-diurnal but considerable difference often exists between the heights of two consecutive tides, the higher being called the "superior" and the lower the "inferior" tide.
The difference in height of superior and inferior tides depends mainly upon the moon's declination either north or south of the equator.
At the times of the moon crossing the equator the tides are equal.
The maximum range of tide may be taken as eight feet and the highest water under ordinary circumstances as 8 feet above Ordnance Datum and the lowest 1 foot below that Datum.
Tidal Observations.-With a view to ascertaining the general effect of the tidal currents at the points proposed for the sewer out-falls, ie., Belcher's Point to the west of the city and North Point to the east, I made in conjunction with Mr. JAS. SAMPSON of the Public Works Department several float experiments on the 27th and 28th of February last during the North East Monsoon, and again on the 21st and 22nd of August during the South West Monsoon.
The floats used consisted of wooden buoys conical in shape, five feet in height and two feet in diameter from which were suspended wrought iron vanes 4 feet by 1 foot 6 inches of T shape construction and so designed as always to keep the larger surface of the vane square to the current. The total weight of a float was 236 lbs. and the depth of the vane below the surface of the water was about 7 feet.
Surface floats constructed of wood were also set adrift and watched during the day, but their course followed so closely on that of the sunken vanes that further observation of them was abandoned soon after sunset as unnecessary. A boat with adequate crew was detailed to watch the drifting of each float and to follow it day and night, and having a steam-launch at our disposal we were able to visit each float periodically and to note its change of position.
On the plan submitted herewith the relative positions of the several floats at the various times of our visits are shewn, the approximate course between each observation being sketched in. I will now briefly describe the courses of the floats.
On the morning of February the 27th (during the North East Monsoon) at 10.15 A.M. float No. North Point on the ebb tide, the course followed by this float is indicated in the plan by a red line.
was set adrift off
At 10.30 A.M. float No. 2 was started about three quarters of a mile east of North Point; its course is indicated by a blue line.
Both these floats got into an eddy in Quarry Bay. No. 2 grounded in shallow water and on being moved out to the position of the surface float it closely followed the course of No. 1.
At 11 A.M. float No. 3 was set adrift off North Point and nearly made the Lyemun Pass; its course being indicated by the green line.
At 12.30 p.m. No. 4 was started off North Point; its course is indicated by the brown line.
At 1 P.M. No. 5 was started about a mile and a half east of North Point; its course is indicated by the yellow line.
The tide turning shortly after 1 P.M. a strong current set in through the Lyemun Pass carrying the floats westwards.
At 2.55 P.M. float No. 6 was started off North Point on the flood tide; its course is indicated by a red line.
This float passing to the west of Green Island found its way out to sea on the afternoon of the following day.
At 5.15 P.M. float No. 7 was started off North Point, after travelling a short distance westwards this float returned on the ebb as far as the Lyemun Pass but missing the mid channel current returned on the flood. Its course is indicated by a blue line.
At 6.45 P.M. No. 8 was set adrift off North Point and after travelling westwards for four hours it returned on the ebb and passed through the Lyemun Pass at 2.30 A.M. Its course is indicated by a brown line. This completed the float experiments at North Point.
On the following day, February the 28th, a similar series of experiments was effected at Belcher's Point. At 8.45 A.M. No. 1 was started on the ebb tide and taking a westerly course, as indicated by the red line, got into shallow water to the north-west of Green Island. On being set free again this float continued its westerly course round Green Island and turning southwards made for the open sea.
At 11 A.M. float No. 2 was started and passing direct through the Sulphur Channel gained the sea; its course is indicated by a blue line.
At 1.45 P.M. No. 3 was started and also passed through the Sulphur Channel, but meeting the flood tide was carried in a north westerly direction outside of Green Island; its course is indicated by a brown line.
At 3.50, 5.10 and 6 P.M. respectively, Nos. 4, 5 and 6 were set adrift. These as indicated by the blue, red and brown Inies made little progress.