BREAKWATER

Estimate of the Probable Expense of a Cut-Stone Pier and Two Lighthouses to be built on the top of the Great Breakwater.

L.119,900 0 0

Breakwater, 1,528,639 0 0

Total estimate of completing the works.....L.1,171,100 0 0

The estimate for the works of Plymouth Breakwater now stands thus (1854):—

It was not until the opinions of the best engineers, men of science, and naval officers eminent in their profession, had been collected, compared, and seriously considered, that Mr Yorke determined to carry into execution this great undertaking; one of the chief supporters of which was Sir John Barrow, then secretary of the Admiralty. The principal objection started against it was, that it might cause the anchorage in the Sound to be destroyed in the course of time by the deposition of mud and silt along the whole eddy within it. But there does not appear to be any

Break-water. solid ground for this objection. The water brought by the tides from the sea is at all times perfectly clear and transparent, and that which proceeds from Hamoaze, and is supplied by the Tamar and the Tavy, is almost wholly free from any alluvial matter, these rivers holding their course through a fine granite soil. The fact is sufficiently proved by the circumstance of no deposit being made in the recesses of Hamoaze along the dockyard wall leading into the docks, nor in the numerous eddies that are caused by the projecting jetties and salient angles of that wall. Another objection started against the undertaking was, that by the diminished quantity of water thrown by the tide into Hamoaze and Catwater, the Sound would gradually fill up, and these harbours be destroyed. But no perceptible alteration has as yet taken place in the height of the water in Hamoaze, or in the strength or set of the tides.

Commencement of the work. A rock of limestone, or rather gray marble, situated at Oreston, on the eastern shore of Catwater, consisting of a surface of twenty-five acres, was purchased from the Duke of Bedford for the sum of £10,000. Quays for shipping the stone were erected in front of it; iron railways leading from the quarries to the quays were laid down; ships were hired by contract to carry off the stone, and others built at the dockyard. Mr Whidby was appointed superintendent of the work. The quarries were opened on the 7th August 1812; the first stone was deposited on the 12th of the same month; and, on the 31st March 1813, the breakwater made its first appearance above the surface of the Sound at low water of the spring-tide. The system of quarrying the stone was conducted with admirable skill, and stones of the proper size were obtained with less waste of small rubble than might have been expected. In working these quarries an extraordinary phenomenon was discovered in the very body of the great mass of this old marble rock. At the depth of sixty-five feet from the summit of the rock, and twenty-five from the margin of the sea, a cavity, or rather a nodule of clay, was discovered, of twenty-five feet long and twelve square, or thereabouts, in the midst of which were found several bones of the rhinoceros, in a more perfect state, and containing less animal matter in them than any fossil bones that have yet been dug out of rock or earth.

Machinery employed. The vessels employed for conveying the large blocks of stone were of a peculiar construction, adapted to carry with ease masses of marble weighing from three to five tons each. These great blocks of marble were placed on trucks at the quarries, and run down thence on iron railways to the quays, against which the vessels lay with their sterns. The two stern ports were made sufficiently large to receive the trucks with the stones upon them. Each truck was passed separately through the port-hole on an inclined plane, and run to the fore-part of the vessel in the hold on an iron railway. The two sides of the hold of the vessel were calculated each to contain eight of these loaded trucks, which, at five tons on each truck, gave eighty tons of stone for one cargo. The stones thus placed on the trucks remained till the vessel arrived at the point in the line of the breakwater where they were to be deposited. By means of a crane on the deck of the vessel, the two trucks nearest to the two stern ports were then drawn up the inclined plane, and run upon a frame on movable hinges, called the typing-frame; by the falling of this frame in the manner of a trap-door the stone or stones were discharged from the trucks on the slope of the breakwater; but the typing-frame remained, by means of a catch, in the position in which it was left at the moment of discharging the stones, until the empty truck was pulled up by the crane to the after-part of the deck, whence it was run forward to make room for the second pair of loaded trucks in the hold. The catch being now disengaged, the typing-frame returned to its former position, ready to receive the next pair of loaded trucks, and so on

till the whole sixteen were discharged; and the light trucks ran upon the deck of the vessel, ready to be run out at the quay, and thence to the quarries, to take in fresh loads of stone. In this manner a cargo of eighty tons was discharged in the space of forty or fifty minutes. The vessels were placed in the proper places for depositing the stones by means of buoys, and the exact line of the breakwater was preserved by observing lights or staves placed at a distance on the shore.

The following description, referring to Plate CXXXIX., will convey an accurate idea of these excellent vessels for the purpose for which they were constructed.

Fig. 1 shows the stern of the vessel in the act of depositing the stones. The runner R being hooked to the fore-part of the truck, raises it up, and by that means tips the stone overboard. When the stone is in the act of being drawn up out of the hold on the inclined plane B (fig. 3), the runner is hooked to the fore-part of the truck, and lashed down to the after-end over the stone, which prevents the latter from sliding off the truck in its progress up the inclined plane. The empty trucks are for the most part lodged on the fore-part of the deck, and some placed on edge against the side of the vessel.

Fig. 2 shows the stern of the vessel when loaded, with the ports up or closed.

Fig. 3 is a longitudinal or sheer-section of the vessel when loaded, with the trucks on one side of the hold and deck, showing the number which the vessel usually stows on each side. The stones being frequently longer than the trucks, the number carried in the hold must be proportioned accordingly. In bad weather it is unsafe to send many trucks on deck; and, in general, not more than four are sent into the Sound in that way at one time; the amount of the cargoes, therefore, vary, according to circumstances, from forty to sixty-five tons; the largest stone hitherto deposited being about eight tons.

The after-part of the deck under the tiller is divided into two parts lengthways, and made to move up and down; the fore-parts are secured to a beam by hinges. This movable deck, when raised as at X, allows the stones to come out of the hold, and when down, as at Y, serves to convey the empty truck from the port to the deck, in order to make room for another stone.

D is a common windlass for heaving the trucks out of the hold up the inclined plane B.

C, the hinges of the typing-frame.

Ten vessels of this construction, for carrying large masses of stone, built in the King's Yards, and forty-three hired by contract, averaging about fifty tons each, were employed in conveying stones from the quarries. The contractors' vessels were not of the same construction as those in the immediate employ of government; they carried stones of less weight, which were hoisted out of the hold by a chain and windlass, and thrown overboard. A load of fifty tons was discharged from one of these vessels in about three hours. By all these vessels the quantity of stone deposited in 1812 was 16,045 tons; in 1813, 71,198 tons; in 1814, 239,480 tons; in 1815, 264,207 tons; and in 1816, up to 12th August, 206,033 tons, at which time the total quantity of stone sunk amounted to 896,963 tons; and at the conclusion of the year to upwards of 1,000,000 tons.

Of this quantity the proportions of the different sizes of the blocks deposited were nearly as follow:

The original contract price for quarrying the stone was 2s. 9d. per ton, and the original contract price for conveying it to the breakwater 2s. 10d. per ton; but the former was