On the top of each hydraulic ram is a heavy sheave or pulley, 6 feet in diameter, over which pass eight steel cables, 2 inches in diameter, making in all 288 cables. One end of each cable is anchored in the bed-plates supporting the hydraulic cylinders, while the other end is secured to the side girders of the platform. Each of the cables has been tested with a load of 80 tons, so that the total test load for the ropes has been 21,000 tons.

In lifting a ship the load is never evenly distributed on the platform. There is, in fact, often more than one ship on the platform at once. Some rams, therefore, may have a full load and others much less. Under these conditions, to keep the platform a true plane, irrespective of the irregular distribution of the load, Mr. Dickie designed a special valve gear to make the action of the dock perfectly automatic. Down each side of the dock a shaft is carried, operated by a special engine in the power house. At each hydraulic ram this shaft carries a worm, gearing with a worm-wheel on a vertical screw extending the full height reached by the stroke of the ram. This screw works in a nut on the end of a lever, the other end of which is attached to the ram. Between the two points of support a rod, working the valves—also carried by the ram—engages with the lever. If at a given moment the screw-end is raised, say, six inches, the lever opens the valve. As the ram rises, the lever, having its other end similarly lifted by the rise, gradually assumes a horizontal position, and the valve closes.

To lift the dock the engine working the valve shaft is started, and with it the operating screws. These, through the levers, open the inlet valves. The rams now begin to move up: if any one has a light load it will move up ahead of the other, but in doing so it lifts the other end of the lever and closes the valve. In fact, the screws are continually opening the valves, while the motion of the rams is continually closing them, so that no ram can move ahead of its screw, and the speed of the screw determines the rate of movement of the lifting platform.

To lower the dock, the engine operating the valve shaft is reversed, and the screws and levers then control the outlet valves as they controlled the inlet valves in raising. When the platform has reached the limit of its movement, a line of locks on top of the foundation girders, thirty-six on each side, are pushed under the platform by an hydraulic cylinder, and the platform is lowered on to them, where it rests until the work is done on the ship; then the platform is again lifted, the locks are drawn back, and the platform with its load is lowered until the ship floats out. All the operations are automatic.

Since the dock was opened well over a thousand ships have been lifted in it without any accident whatever; the total register tonnage approaching 2,000,000. The great favour in which the dock is held by shipowners and captains is partly due to the fact already mentioned, that the ship is lifted above the level of tide water, where the air can circulate freely under the bottom, thus quickly taking up all the moisture, and where the workmen can carry on operations with greater comfort.

When extensive repairs have to be undertaken on iron or steel vessels, the fact that this dock forms part of an extensive shipbuilding plant, and is located right in the yard, enables such repairs to be executed with despatch and economy. Several large steamships have had the under-water portions of their hulls practically rebuilt in this dock. The steamship Columbia, of the Oregon Line, had practically a new bottom, including the whole of the keel, completed in twenty-six days. This is possible, because every facility is alongside the dock and the bottom of the vessel is on a level with the yard.

This being the only hydraulic dock controlled automatically (in 1897), it has attracted a large amount of attention from engineering experts in this class of work. English, French, German, and Russian engineers have visited the Union Iron Works to study its working, and their reports have done much to bring the facilities offered to shipping for repairs by the Union Iron Works to the notice of shipowners all the world over.

FOOTNOTES:

[21.] The Romance of Modern Engineering, pp. 383 foll.

[22.] For explanation of the "accumulator," see the chapter on Hydraulic Tools ([p. 81]).