be about £2 each trumpet-consequently, the expense of a breakwater on this principle, equal to that at Plymouth, would be £160,000 -a very small sum compared with the expense of that magnificent work, which, however valuable as a protection to the harbour in severe gales of wind, has, in consequence of its unyielding materials, been the cause of the loss of several vessels; and as large open seaways are obliged to be left at each end of the breakwater, the swell and turbulence of the sea enters with great violence at times, rendering the passage in and out extremely dangerous. The breakwater, on Major Parlby's principle, may stretch completely across a channel from shore to shore, without offering much impediment to the seaway; and, certainly, no dangerous one. It is also capable of being removed at any time, without any considerable expense; a thing impossible to be done with stone breakwaters.

CAPTAIN NORTON'S LOTUS FLOATING BREAKWATER.

ON a lake or pond where the lotus grows, Captain Norton had observed, that when there were a strong breeze and waves on one side, on the other the water was comparatively smooth, resulting from the wind having no hold on the broad expanse of lotus leaves. He had also observed, after a storm at sea, the solid timbers of a wrecked vessel splintered in pieces by being driven against the shore, while a wicker basket escaped uninjured. These two results suggested the idea of constructing a floating breakwater of osiers, according to the singularly ingenious model in the Polytechnic Institution, the expense of which would be trifling compared with others.-Polytechnic Journ.

THE PLYMOUTH BREAKWATER LIGHTHOUSE.

THE last stone of the Lighthouse tower, at the western end of this stupendous sea-barrier, has been set. The tower is 122 feet in height from the level of the bottom of the sea, and fifty-six feet from the level surface of the breakwater. It is composed of thirty-one courses of large blocks of dressed granite, the first of which was laid on the 22d of February, 1841. The lighthouse is divided into five stories, in which are an oil-room, a store-room, a dwelling-room, a bed-room, and a watch-room. It has fourteen windows, seven of which are in the watch-room, the frames being constructed of bell-metal, as are also the outer doors. The lantern is the only thing now necessary to complete the lighthouse for service.

KEENE'S MARBLE CEMENT

Is described as a combination of sulphate of lime and alum. The gypsum undergoes the same preparation as for plaster of Paris, being deprived of its water of crystallization by baking. It is then steeped in a saturated solution of alum; and this compound, when recalcined and reduced to a powder, is in a fit state for use. The cement has been most extensively applied as a stucco; but the finer qualities (when coloured by the simple process of infusing mineral colours in the water with which the cement powder is finally mixed for working), being sus

ceptible of a high degree of polish, produce beautiful imitations of mosaic, and other inlaid marbles, scagliola, &c. The Cement is not adapted to hydraulic purposes, or for exposure to the weather, but has been used as a stucco in the internal decorations of Windsor and Buckingham Palaces. From its extreme hardness, it has been found serviceable when used for imbedding and setting the tiles of tesselated pavements, &c.; and it has been adopted for this purpose at the French Protestant church, the new fire-proof chambers in Shorter's-court, and the Reform Club-House. The extreme hardness of the cement is its principal recommendation, when applied as stucco and for mouldings.-Magazine of Science, No. 216.

CONCRETE.

MR. HAWKINS has made to the British Association a communication on the formation of Concrete, showing more particularly the importance of having the stones of the proper sizes, so that the smaller ones should, as nearly as possible, fill up the interstices of the larger. Where the sizes were properly adjusted, he found that one proportion of lime to twenty of shingle formed a stronger concrete than when larger proportions of lime were used. Some engineers are in the habit of using one of lime to six of shingle, and the proportions generally used, are as one to eight. A specimen of concrete made in the proportions he recommended, and with shingle of proper sizes, was found after a short time to be stronger than an old Roman wall. Mr. Macneill said he preferred artificial cement to lime, and he had found great advantage to result from allowing the mass of concrete to fall from a height, by which means the shingle became more compressed together. Mine dust mixed with the lime, he believed, made the most perfect concrete. Mr. Jessop and Mr. Taylor also approved of mine dust. The latter observed, that it was probably from the quantity of iron in mine dust, that its adhesive properties were derived.

THE NENE ESTUARY EMBANKMENT.

THIS extensive undertaking was designed for the purpose of enclosing from the sea a tract of most valuable land, amounting to about 4,000 acres this will, when enclosed, be principally the property of the Commissioners of the Nene Outfall, under whose auspices the works are being carried into effect, and in which they are assisted by the professional services of that eminent engineer, Sir John Rennie. The embankment is nearly three miles and a half in length, and for some distance averages twenty-eight feet in height, and at some parts of the line of works there is a depth at high tide of fourteen feet. About one mile and three quarters, or one-half the whole length, is already completed, and from this portion of the work, as a specimen, it is allowed by experienced persons that it will be one of the best examples of a sea-wall to be found in England. The land, it is estimated, will vary in value from £50 to £80 per acre, and, as a maiden soil, would be a fine site for a model farm of one of the agricultural

societies of England. The works are rapidly progressing under the superintendence of Mr. H. H. Fulton, resident engineer, and the contract was taken in August, 1842, by Mr. H. Sharp, for £60,000. The Nene Outfall Commission has already effected great improvement in the condition of part of the fens of Cambridgeshire and Lincolnshire, by procuring a natural drainage for the lands in lieu of the inefficient and expensive system of drainage by windmills and other mechanical means, at the same time improving the navigation of the river Nene from the sea to Wisbech, to such an extent that formerly Humber keels of seventy or eighty tons could with difficulty reach that port; whereas now, vessels of 400 or 500 tons can, without the assistance of a pilot, owing to the straightness of the channel, get up to Wisbech without the slightest difficulty. This investigation, as an artificial tidal channel, is said to be the finest of that description in the country. It was designed and executed under the direction of the late Mr. Thomas Telford and the present Sir John Rennie, and so important has been the result of these works that the trade of the port of Wisbech has been trebled during the last ten years.—Abridged from the Times.

FALL OF GREENWICH PIER.

THE failure of this pier took place on the 16th of May. It was not a matter of surprise to parties who understand the practical construction of such works. The immediate cause of the failure was dredging in front of the piles after the contractors had left the works, and the arrangement of the piles being faulty, as regards construction; the upper part is composed of brickwork in cement, eighteen feet high, and fourteen inches thick at top, capped with granite, one foot thick, backed with concrete, and standing upon a foundation of Yorkshire stone landings, laid on a small quantity of concrete, with a substratum of foul gravel. The landing in front rests on a row of cast iron piles, twenty-five feet long, and five feet apart, grooved to admit between them three cast iron plates, each six feet in height; these iron piles were fastened by four, or two pair of wrought iron land ties, two inches square, to wooden piles, eighteen feet long, and twelve inches square, driven in land at a distance of twenty-five feet and a half from the front, and five feet apart. The high water mark is about four feet from the top, and low water mark twenty-two feet below, or about seven feet below the stone landing. From inquiry, it is suspected the lower ties were not fixed. The superincumbent weight of the brickwork appears to have forced out the upper part of the iron piles to a considerable distance, and caused the brickwork above to slip down, and force out the iron plate.-Correspondent of the Civil Engineer and Architect's Journal.

THE GREAT CROTON AQUEDUCT.

THIS stupendous aqueduct of 32 miles, for conveying water to New York from the Croton River, is near completion. The work was commenced in 1835: it consists of:

First, an artificial reservoir, called the Croton River Lake, 45 miles from the Battery-the extreme part of the city: this lake is formed by a hydraulic stone-masonry dam, with two waste weirs or aprons, for the over fall of the water, one of 87 feet, and one of 180 feet, these being separated by a gate-house. The height of these waste weirs is 55 feet above the bed of the river, and 40 feet above the low water level.

The dam backs the water five miles, and makes a lake of an area of 400 acres, and of a capacity equal to 500 millions of gallons.

The water enters a gate-house, where the quantity is regulated, before it enters the aqueduct, which is a stone structure, lined and arched with brick.

The face of the interior of the aqueduct is at the bottom an inverted arch, width 6 ft. 9 in., height 8 ft. 51⁄2 in., area 53 square ft., about large enough for an omnibus and four to pass through. The line of the aqueduct being on a regular declivity of 13 inches to the mile down to the Harlein River, a distance of 33 miles, it has a line of tunnels of 6841 feet, being sixteen in number, sometimes through earth, and sometimes through solid rock; the deepest cut is 80 feet, and the least 25 feet. In Westchester only, the aqueduct crosses 25 streams of water, which are from 25 to 13 feet below the top of the aqueduct.

The grade line of aqueduct across the Harlem is 25 feet above tide water, and the top of the water now passes over Harlem River in one pipe of 36 inches, placed on the earthen dam made in the construction of the high bridge.

The bridge itself will be when finished one of the most stupendous works of the kind in the world. Its cost is estimated at one million of dollars, and its elevation is so great as not to impede the navigation of the stream. Some idea of this vast undertaking may be formed from the fact, that the excavation for one pier has been carried 34 feet below the surface of the water, and then a rock foundation not having been reached, 240 poles, from 30 to 40 feet long, were driven in for the purpose. Several piers having been already carried, by the aid of coffer-dams, from four to fifteen feet above high water mark. The river is 620 feet wide at water line, but the slope of the river banks adds an additional distance of 830 feet, making in all 1,480 feet.

The plan now in progress crosses the river with eight arches of 80 feet span, and on piers of 31 by 44 feet at the base, resting on the bed of the river, and seven arches on piers on the land from the edge of the water up the two banks of the river.

The spring of one of the arches is 95 feet above the lowest foundation put down; the top of the parapet will be 149 feet from the lowest foundation. It is intended that the water shall pass over this bridge in pipes, to have it secure against the possibility of danger.

The interesting works at Clendinning Valley, being a bridge over a valley of 1,900 feet in breadth, the greatest height of the aqueduct, is 50 feet from the bottom of the valley; beautiful archways are con

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structed for three streets, 34 feet for the carriage-way, and 10 on each side for side walks.

Next in interest is the reservoir at Eighty-sixth Street, which might well be called the detaining or clarifying reservoir. It has two divisions, together thirty-two acres-greatest depth of water 25 feet, containing one hundred and fifty millions of gallons. Two lines of thirtysix-inch pipes connect this with the reservoir at Fortieth Street, which has also two divisions, forming together an area of four acres-depth of water when filled thirty-six feet. From this point four and a half miles to the Battery. Whole length of line from the Battery to the artificial lake fifty miles. There are in this great work 55,000,000 of bricks, and 700,000 cubic yards of stone masonry.

The water in the aqueduct is regulated at the entrance gate, so as not to flow under any pressure; it has not been permitted to flow in the division near the city at a greater depth than two feet, but the works at the Croton dam required a few days back that more water should pass through the first division (the distance between Sing Sing and the Croton River), being eight miles, and it was found to pass seventy-five millions of New York gallons in twenty-four hours, and that its velocity was over two miles per hour.

The Croton Lake now retains, beyond the daily river supply, in reserve, five hundred millions of gallons; and a small expense would add other immense artificial lakes to hold back an additional supply; but the necessity of this is hardly conceivable. It is estimated that the London supply, from all the companies, is but twenty-four millions of gallons, and Paris four millions only.

NEW HYDRAULIC MACHINE.

MR. SCHWANFELDER, of Newington Causeway, has invented an apparatus for raising water, which, from its simplicity and extraordinary power, as compared with the common pump, will most probably, when generally known, be extensively patronized. It is merely a box, about nine inches square, and four thick, containing the principal motion, which is centrifugal: the handles for working it projecting on each side, and the ejection pipe placed in front; the pump barrel, which, in this case, is one inch and a quarter in diameter, proceeds from below to the water, within which is fixed a valve, which must, in every case, be within the usual atmospheric pressure-distance of the surface, as in the common pump. While the machine is in action, this valve remains open, and the water rushes in a continual stream; its use being to prevent the return of water on the pump ceasing work. This small model, with a discharge pipe not exceeding half an inch in diameter, will deliver about thirty-two gallons per minute, or eighty tons in twelve hours; and this with a force which a mere child might exert. On screwing on a jet, converting it into a force-pump, it threw a stream perpendicularly nearly forty feet, and horizontally a greater distance; and when water is required for the top of high buildings, its powers, as a lifting pump, are of the same powerful character. The box above mentioned will be, in the finished machine,