Chap. VIII.
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weighs upwards of 20 tons, and has two sets of sluices, working on brass facings, in irongrooves, and so constructed that one set is raised whilst the other is lowered; this iseffected by having the sluice rod attached to a rack that turns a spur-wheel working inanother rack attached to the other sluice rod; thus a capacious opening is obtained withoutweakening the gates.
The bridges over are on the balance or lifting principle, and are moved by means of fourcrabs, two on each side; the handle is applied to a 6-inch pinion, which drives a spur-wheel 4 feet in diameter, on the axis of which is a 12-inch pinion, working in a toothedsegment, the radius being 5 feet 9 inches ; this is attached to the outer rib of the bridge.Each bridge weighs nearly 100 tons, and it can be opened or shut by three men in less thana minute.
Cleansing the Docks of the mud deposited by the waters of the Humber, is a matter ofconsiderable moment, as it is said the quantity annually deposited in the Humber Dockwas 36,000 tons. The dredging machine employed is placed on a flat-bottomed vessel,80 feet in length and 20 feet in breadth, drawing 5 feet water. It is worked by a steam-engine of six-horse power; its stroke is 2 feet, forty times a minute ; by means of a bellcrank, motion is given to four cog-wheels ; on the axis of the upper one is a squaretumbler, with a corresponding one at the lower end of the bucket-frame. There aretwenty-nine iron buckets, revolving on an endless chain, which deposit the mud they bringup, by means of a spout, into lighters. This endless chain turns on an axis at the upperend, and the lower end passes through an opening in the middle of the boats, and is raisedor lowered by a crab, and tackling fixed over it; by this means, the buckets are placed atthe proper level for dredging. An engine man and three assistants are required to managethis machine, and two others to attend the lighters. Sixty tons per hour is sometimesraised, but usually twelve boats, each of 500 tons, are employed, and the ordinary workperformed is about 45 tons per hour. The mud boats are flat-bottomed, and draw 4 feetwater ; they are in length 48 feet in width 17 feet 6 inches, and carry about 40 tons.
The tide basin is scoured by two cast-iron mains, 4 feet in diameter next the lock, anddiminishing to 2 feet 6 inches. At the outer end, branching out of these on each side, areten 18-inch pipes, which discharge through the basin wall, about 5 feet above the levelof the sills; other mains are connected with the docks, but they only scour where the wateris discharged, and leave the bed in furrows, to be cleared away by other means.
In consequence of the waves of the Ilumbcr acting with violence against the outer gatesof the dock, it was found necessary to build two piers to protect them. The main pilesare 14 inches square, the outer waling the same scantling, and the inner wales 12 inches by6; the cap sill 12 inches by 10; joists 7 inches by 4; ties 12 inches by 6; sheet piling6 inches in thickness, and the planking 3 inches in thickness.
Spurn Point Lighthouses were commenced near the Humber mouth by John Smeaton , inthe year 1771; they were built of brick, and his instructions were to make the largest90 feet in height from the mean surface of the ground to the centre of the light, andthe smallest 50 feet high ; both were to be provided with inclosed lanterns for fire-lights.
The original lighthouse was a strong brick building of an octangular form, 60 feet high :the light was hoisted on a swape, a provincial term for a lever, fixed upon a centre whicheould be turned in any direction by the hand. A naked coal fire was employed, which
TUNS OP SPURN POINT LIGHTHOUSE.
burnt with unequal force; during the storm of 1703, the draught was so great that itmelted down the iron bars on which it was laid like lead. In the year 1777, the two newlighthouses were completed; in design they did not difter, but the lower one con-