in Lambeth Reach." They are too extensive to be inserted here; but are truly interesting, and may be seen in Phil. Trans. Abridged, vol. vii. p. 133.

SECTION IV.-Effects of the Old London Bridge on the Tides, &c.

Such docu

In the first volume of Dr. Hutton's Tracts, 8vo. there are inserted some curious papers drawn up by Mr. Robertson, of Christ's Hospital, and others, on London Bridge, and on the probable consequences, in reference to the tides, of erecting a new bridge across the Thames, viz. Blackfriars. ments are not only interesting as matters of scientific history, but become valuable in process of time; as the comparison of facts with theoretic predictions is subservient to the correction of the theory itself. With a similar object in view, I here introduce abridged accounts of some valuable facts with regard to the motion and level of the tides in the Thames at London, collected in 1820 and 1821, when the project of a new bridge of five arches, instead of the old bridge, originally of 20 arches, and a very contracted water-way, came first under consideration.

Result of the levels of tides observed from 23d September to 25th October, 1820, between the entrance of the London Docks and Westminster Bridge. Also the transverse sections of the river Thames at London Bridge, Southwark Bridge, Blackfriars Bridge; Waterloo Bridge, and Westminster Bridge, collected from four drawings of the above surveys, made by Mr. Francis Giles, under the direction of Mr. James Montague, pursuant to an order of the Select Committee of Bridge House Lands, of the 8th September, 1820.

London Docks to London Bridge.

The high water of spring tides at the entrance of the London Docks, averaged a level of 1.5 inch higher and 10 minutes earlier time, than at the lower side of London Bridge. The low water of ditto at ditto averaged a level of 3 inches lower, and 9 minutes earlier time, than at ditto.

The high water of neap tides at ditto averaged a level of one inch, and 8 minutes earlier time, than at ditto.

The low water of ditto at ditto averaged a level of 2 inches lower, and 14 minutes earlier time, than at ditto.

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 inches 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 gradually 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 occasioned by the starlings were not taken away when this volume went to the press.