STATXONARY ENGINES.

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opened and closed by these appliances is limited only by consider-ations of safety to the gates, the time taken in actual practicebeing about two minutes. In nearly all the cases in which hydraulicpressure has been applied for the moving of dock gates, it is alsoused for opening and closing the levelling shuttles, and in manycases also for working the capstans. The former purpose is effectedby the direct application of a cylinder and piston fixed above theshuttle: and the latter is accomplished by throwing the capstan Cinto gear with the shafting A.

WATER-PRESSURE ENGINES FOR DOCK GATES, &c.

These engines consist of a combination of three oscillating cylin-ders, working cranks inclined 120° to one another. The cylinders A(Fig. 210) are fitted with plungers B, instead of pistons, and are there-fore only single-acting. The slide valves V are worked by the oscil-lation of the cylinders, communicated through the levers L. Whenthe back end of the cylinder is depressed, the slide valve is lowered,and allows the water to enter from the pressure pipe P through thepipe C to the cylinder, where it acts upon the plunger in the outstroke; and in the return stroke the back end of the cylinder is raised,the cylinder port C is closed to the pressure pipe P, and open tothe exhaust E. A small relief valve is fitted to the cylinder pipe C,opening against the pressure, which prevents any shock when thecommunication with the exhaust is closed at the end of the returnstroke. These engines have occasionally been made with pistons,so as to be double-acting; but for the great pressures employedwhere accumulators are used, the single-acting arrangement withplunger is preferred. It will be observed that with the arrange-ment of cranks dividing the path into three equal parts, there is noliability of the engine stopping on the “dead centre,” as it is termed,the one crank assisting the other over the extreme points.

In working swing bridges by means of water pressure, a centralpress is generally applied to lift the entire bridge clear of itsSupports, and it is then turned by an application similar to thatused for swinging a crane. An example of a swing bridge on thisprinciple is seen at Wisbech , having an opening of 85 feet, arrangedfor a double roadway in one leaf, weighing about 450 tons. Thepower is derived from an accumulator charged by a hand forcepump, and notwithstanding its great length and weight the bridge