« ПредишнаНапред »
Some particulars relative to the Tides in the upper part of the
River Thames, and of the obstructions caused by the present London Bridge. By P. Barlow, F.R.S., Mem. Imp. Acad.
Petrop., &c. (Communicated by the Author). London Bridge, which has for so many years bestrided the Thames, is now doomed within a very short time to be remo ved, and a considerable change will be, there can be no doubt, effected in the state of the River from the present site of the Bridge upwards. It may not therefore be uninteresting to record some particulars relative to the present state of the tides, and of the river, in order hereafter the better to compare the change which the removal of the bridge may occasion. When this question was before Parliament, I was summoned to attend the Committee to state my opinion relative to some points connected with these probable changes, and it was on that occasion that I collected together the several facts given in the following pages, and which, if they should not be found to furnish any present important information, may hereafter be referred to as matters of interest by the curious inquirer. Some doubts having been suggested as to the damage which might be sustained by the wharfs, &c. above bridge, by the rising of the river above its present level at high water, and the exposure of the sewers at low water, the data here given are such as are connected more particularly with these questions; they consist,
1. Of the sectional area of water-way at the different bridges, at various states of the tide.
2. Of the hourly rise and fall of the tide, and the difference of level at different times of the tide, immediately above and below London Bridge.
3. The rise and fall, and interval between the time of high and low water, at Woolwich, and at several other places on the river, ascertained by observations made on the same days.
4. Experiments and observations made on the velocity of the tide at ebb and flood at Woolwich, and other places on the river.
5. The difference of level between the high and low water, at several places on the river, and other miscellaneous particulars.
1. Sectional areas of water-way at London Bridge, and at
Southwark, Blackfriars, Waterloo, and Westminster Bridges, at different periods and states of the tide.
At an extraordinary high
tide, 2 feet above the usual average spring tide high water-mark, the water-way through
the different bridges is, 8130 15,260 At the Trinity, high wa
ter mark or datum, 7360 13,940 At an average spring tide,
high water below Lon.
water above Bridge, 5293 11,135 Spring and neap tide, low
water above Bridge, 1488 5,012 Neap tide low water below Bridge,
1030 Spring tide low water below Bridge,
The linear water-way at London Bridge between the Piers
above the Starlings,
From this table it appears, that, at low water spring-tide, the sectional area of the water-way at London Bridge is not more than about two-fifths of that at Waterloo Bridge, which has the least water-way at low water of the other four bridges; this contraction acts as a dam, and causes the water to accumulate so much above bridge, that the sectional area below bridge is very little more than one-third of that above bridge.
And at high water spring-tide, the water-way at London Bridge is, at a medium, about half that at Southwark, which has the least section of the other four bridges at high water, and this again causes an accumulation below bridge, but by no means so great as in the former case.
The effect of this blockage on the hourly rise and fall of the water at the bridge is shewn in the following table :
2. TABLE of the Ebbing and Flowing of the tide at London
Bridge, taken above and below on the 29th of July 1821.
By means of this table we readily find the head of water above and below bridge at each successive hour of the tide,
viz. by subtracting from the depth of water on one side, the depth on the other. Thus it appears that, on the day in question,
The above deductions are from the observations of a particular day, and are not quite the mean results even for a day, because the high water above and below bridge does not happen exactly at the same time. From a mean, however, of several days, it appears, that the average fall
3. Some other particulars relative to the periods of Rise and
Fall, and of High and Low Water, above and below Bridge, may be stated as follows :
1. The flood of spring-tides, of October 21st and 23d, produced slack water through the bridge in about 40 minutes after low water below bridge; from which time a-head gradually increased below bridge to 1 foot 10 inches at half flood, and then regularly decreased to about 8 inches at high water.
The first flow of these tides, nevertheless, began above bridge about 20 minutes after low water below bridge, although the water was then about 2 feet 6 inches higher above than below bridge; the time of low water below bridge averages 10 minutes earlier than above bridge.
The ebb of these tides produced slack water at the bridge about 30 minutes after high water, and then gradually sunk to their greatest fall at low water.
The time of high water, October 21st and 23d, was the same below as above bridge ; but the average time of high water spring tides is 9 minutes earlier below than above bridge.
The flood of neap-tide, October 30th, produced slack water through the bridge, in about two hours after low water below bridge, when there was some land-flood in the river; from which time a head gradually increased below bridge to 1 foot 3 inches at two-thirds flood, and then regularly decreased to 4 inches at high water.
The first flow of this tide, nevertheless, began above bridge about 1 hour after low water below bridge, although the water was then 1 foot higher above than below bridge ; but the average time of low water below bridge is 32 minutes earlier than above bridge.
The ebb of this tide produced slack water at the bridge about 15 minutes after high water above bridge, and then gradually sunk to its greatest fall at low water.
The time of high water, October 30th, was 15 minutes earlier below than above bridge; and the average time of high water neap tides is 15 minutes earlier below than above bridge.