London Bridge.

High water of the highest spring tides occurs at three or four o'clock. High water of the lowest neap tides occurs at eight or nine o'clock.

Spring tides flow four or five hours, and ebb seven to eight and a half hours.-Neap tides flow five to five and a half hours, and ebb six and a half to eight hours.

The high water of spring tides produced an average fall through London Bridge of 8 inches, but the greatest fall at high water was 1 foot 1 inch.-October 24th.

The low water of ditto, through ditto, of 4 feet 4 inches, but the greatest fall at low water was 5 feet 7 inches.-September 27th.

The high water of neap tides through ditto of 5 inches. The low water of ditto, through ditto, 2 feet 1 inch, but the least fall at low water was 1 foot 1 inch.-October 16th. 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 increased to about 8 inches at high water.-The first flow of these tides, nevertheless, began above bridge about 20 minutes after low water time below bridge, although the water was then about 2 feet 6 inches higher above than below bridge; the time of low water below bridge averaged 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 of 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 time 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 nchies at high water.-The first flow of this tide, nevertheless, began above bridge, about 1 hour after low water time 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 gra dually sunk to its greatest fall at low water.-The time of high water of 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.

London Bridge to Westminster Bridge.

The high water line from the upper side of London Bridge to Westminster Bridge is generally level, unless influenced by winds and land floods.

The time of high water is about 10 minutes earlier at London than Westminster Bridge.

Areas of the Transverse Sections in the River Thames at

London Bridge.

London, 12th March, 1821.-(Published in a letter addressed to G. H. Sumner, Esq. M. P. by a scientific architect.)

Gradation of the Ebbing and Flowing of the tide at London Bridge, taken above and below, on the 29th of July, 1821; being the day of the new moon.

The object of this statement was to show, that the old bridge tended to retain the water above bridge and assist the navigation up the river

Difference between the Levels of High and Low Water Spring Tides, between Rotherhithe and Battersea in the year 1820

From Battersea Bridge to London Bridge, 5 miles; from London Bridge to Old Horse Ferry, 14 miles. From London Bridge to the Nore, 44 miles.*

SECTION V.-Watermills.

1. The impulse of a current of water, and sometimes its weight and impulse jointly, are applied to give motion to mills for grinding corn and for various other purposes. Sometimes the impulse is applied obliquely to floatboards in a manner which may be comprehended at once by reference to a smokejack. In that, the smoke ascends, strikes the vanes obliquely, and communicates a rotatory motion. Imagine the whole mechanism to be inverted, and water to fall upon the vanes, rotation would evidently be produced; and that with greater or less energy in proportion to the quantity of water and the height from which it falls.

Water-wheels of this kind give motion to mills in Germany, and some other parts of the continent of Europe. I have, also, seen mills of the same construction in Balta, the northernmost Shetland Isle. But wherever they are to be found, they indicate a very imperfect acquaintance with practical mechanics; as they occasion a considerable loss of power.

2. Water frequently gives motion to mills, by means of what is technically denominated an undershot wheel. This has a number of planes disposed round its circumference, nearly in the direction of its radii, these floatboards (as they are called) dipping into the stream, are carried round by it; as shown in the accompanying diagram. The axle of the wheel, of

* The preceding results will always be valuable, as they supply striking evidence of the effect of a broken dam, such as many of our old bridges present. I regret that the contrast occasioned by the large arches of the new bridge cannot yet be presented for though the old bridge is removed, the entire obstructions occasione by the starlings were not taken away when this volume went to the press

course, by the intervention of proper wheels and pinions, turns the machinery intended to be moved. Where the stream is large and unconfined, the pressure on each floatboard is that corresponding to the head due to the relative velocities (or difference between the velocities of stream and floatboard): this

pressure is, therefore, a maximum when the wheel is at rest; .but the work performed is then nothing. On the other hand, the pressure is nothing when the velocity of the wheels equals that of the stream. Consequently, there is a certain intermediate velocity, which causes the work performed to be a maxi

mum.

The weight equal to the pressure is a ( ¤ — √ h)2, q being the quantity of water passing in a second, н the height due to v the velocity of the water, and h that due to u the velocity of the floatboard. Considering this as a mass attached to the wheel, its moving force is obtained by multiplying it into u and as Hh varies as vu, this moving force varies as (v u) u which is a max. when u = v. In this case, then, the rim of the wheel moves with of the velocity of the stream; and the effect which it produces is

so that the undershot wheel, according to the usual theory, performs work of the moving force.

=

Friction, and the resistance of fluids, modify these results; but Smeaton and others have found that the maximum work is always obtained when u is between v and

v.

3. Where the floats are not totally immersed, the water is heaped upon them; and in this case the pressure is that due to

2 H.

4. When the floatboards move in a circular sweep close fitted to them, or, in general, when the stream cannot escape without acquiring the same velocity as the wheel, the circumstances on which the investigation turns become analogous to what happens in the collision of non-elastic bodies. The stream has the velocity v before the stroke which is reduced to u, and the quantity of motion corresponding to the difference, or to vu, is transferred to the wheel; this turns with the velocity u; and therefore the effect of the wheel is as VU U3 ; which is a maximum when y = 2U;

VU

u, or,

V

being then of the moving power.