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. 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. 4. Observations on the Rise of the Tides at Woolwich, Deptford, Billinsgate, Old Swan Stairs, and Westminster Bridge, May 25th and 26th 1823. Full Moon May 23d. From these observations we learn, that the relative time of the flood at these places, from a mean of the two days, is, And that the low water at Woolwich precedes that at 22 20 22′ Deptford precedes that at Billinsgate by 0 Old Swan Stairs Westminster Bridge And the mean rise of the tides at these stations is, 5. Mean of Six Weeks' Observations on the velocity of the Ebb and Flood, Neap and Spring Tides, at Woolwich. |