Feet. Inches.

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 FLOOD TIDE.

The head at low water above

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 tides is 15 minutes earlier below than above bridge.

neap

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

0

224

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.

The tabulated observations, from which the above abstract has been made, were taken by Mr Pullman, superintendant master at Woolwich Dockyard, and may be confided in for their accuracy. They were taken at about mid-tide with a ship's log, and with every possible care and attention.

The following experimental results, as to the velocity at ebb-tide above bridge, were furnished by Mr Jessop, civil engineer. The velocity was ascertained by throwing into the stream turnips and potatoes every 10 minutes for 45 minutes during low water, in the middle, and on each side, of the river. The greatest velocity thus deduced, was 21 feet in 5.17 seconds or about 25 miles per hour, and the mean of each series gave as follows:

Near the London shore, 21 feet in 8.4 seconds.

6.-Line of High and Low Water.

The mean low water-line has a fall of 12 inches between Westminster and London Bridge, and from London Bridge to the London Docks at spring-tides, a fall of 3 inches; at neap-tides, 2 inches. Mean 24 inches.

The high water-line has a fall, in the contrary direction, from London Docks to London Bridge, of 12 inch at spring-tides, but it is a dead level at high-water neap-tides, as it is also between the London Docks and Blackwall. And from London Bridge to Richmond the high water-mark is, according to the survey of Mr Giles, the city surveyor, one dead level; and this gentleman informs me, that he has found the same circumstance to obtain in several tide rivers; although the Severn and some others observe very different laws.

7. Experiments to ascertain the Velocity of the general body of Waters of the Thames.

As the velocity, found as above, was obviously that of the surface of the water, I thought it desirable to ascertain whether