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HISTORY OF ENGINEERING.

Book I.

of a counter pier. Vessels then could at all times enter, and shelter be obtained for any offrom 30 to 40 tons.

The width of this pier at the base is described to be 40 feet, and after fixing up leadingmarks from the first erected pier to the shore, stones were dropped in between them toform the foundation. These stones, which were rough masses of rock, were suspended bytackle in slings, and hooked or secured by a loop made of as many turns of rope yarnas would hold it, and by cutting the loop when the stone was in its proper position, itwas dropped. After the outlines were established, the internal stones were tumbled intothe area comprised between them.

The first stones dropped penetrated the sand, the second and third courses also dis-appeared in the same way, but after they had settled to a firm base, the rest of theconstruction was carried on by dropping the stones in a similar manner, taking care thatthe faces fell hack on each side, and that the wall diminished in thickness gradually; andwhen any settlement occurred, or the stones were displaced by a storm, they were imme-diately set right and the injury repaired.

The diameter of the circular part of the head was 2 feet more than the common breadthof the pier, and the foundation was 43 feet.

The masonry of the upper parts was built with stones laid flat on their beds withmortar ; the joints of the head of the pier radiated from a common centre, and every thirdcourse was securely cramped, and the centre stones retained by iron dogs. The cap or pierhead had every stone cut like a dovetail, and was so put together that its strength was suffi-cient to resist the force of the seas opposed to it.

Belfast is situated on the river Laggan, near where it discharges itself into an inlet of theIrish Channel. The tides flow for a short distance up this river, but ebbs entirely out atlow water, leaving a narrow and winding course through the sands for the river to flowout.

Spring tides rise in the roadstead 12 feet, and neaps 8, so that vessels which draw 10 feetwater cannot reach the quays at neap tides, but are obliged to lighten their cargoes atGarmoyle.

Bay of Dublin _On entering the nearest point of land, on the north and south are the

promontory of Ilowth, and the island of Dalky, which are distant from each other 6^ miles ;from the line uniting these points, to the end of the lighthouse of the south wall, thedistance is 3^ miles ; from the same line to Ringsend, 6^ miles.

Towards the south Howth presents a bold ascent, interspersed with rocks and adornedwith heaths of various colours; the mountains of Wicklow rise beyond in harmoniousconfusion, and the whole produces, on approaching this beautiful bay, great picturesqueattractions.

On the north and west are two dangerous sand banks, produced by the channel of theLifFey , and which extend to the lighthouse on the south wall. Hie channel between themis not very wide, and the entrance is difficult, the depth of water at the recess of spring-tides not being more than 5 feet. Near the northern extreme line of the banks called theSouth Bull, a pier has been constructed, which is much admired; it commences at the vil-lage of Ringsend, and continues as far as the pigeon-house, a distance of 7938 feet. It isformed by two stone walls, filled in with gravel, and completed about 1756.

The Pigeon House, before the harbour at Howth was constructed, was the place wherethe packets received and landed passengers; and there is here an artificial basin, 900 feet inlength, and 450 feet in breadth, which is nearly dry at low water.

Beyond the Pigeon House the pier is continued eastward 9816 feet, where it is terminatedwith a lighthouse ; this division of the pier was originally timber, but in 1796 two parallelwalls of hewn granite were built, without the aid of cement, and the intermediate spacefilled in with shingle and gravel; it is 32 feet broad at bottom, and tapers to 28 feet at top.

As it is merely a sea-wall, parapets have been dispensed with ; the lighthouse at itseastern extremity, built in 1768, is a truncated cone, three stories in height, with a stair-case on the outside, which leads to an octangular lantern; the whole is built of the moun-tain granite.

The pier secures the harbour from the sands of the South Bull, and with the quay wallsforms one continued barrier, from the lighthouse on the east, to Barrack Bridge, at the op-posite point of the compass, a distance of 6 miles.

The situation of Dublin Bay cannot, however, be considered favourable to the formationof a deep or commodious harbour; were it not for the discharge of the waters of theLifFey and Dodder, there w r ould be scarcely accommodation for the smallest class ofvessels.

The LifFey , being confined to a narrow course, the channel it cut through the sandswas of very small width, regular in its depth, but constantly altering its direction.

Captain John Perry, who was employed to stop the breach made in the Thames wall atDagenham, at the commencement of the last century, made some improvements in thisharbour by forming a pier of drift work, damming up the waters of the two rivers before