16

STEPHENSONS’ PATENT

and in summer is invisible only when the hygrometer is high ; its appearance is,indeed, a tolerably correct indication of the hygrometric state of the atmosphere.When the damper is lowered, the steam instantly becomes visible from the wantof hot air to absorb it, and it then issues from the chimney in dense whitevolumes.

Smoke-box Books. — A large door, 11 , (Plates XC. and XCII.,) is made in thefront plate of the smoke-box, for the purpose of affording access to the cylindersand the tubes ; there is a ledge fixed inside round the opening, against which thedoor is closely pressed by four finger nuts put upon screws fixed in the smoke-boxplate, and passing through projecting lugs upon the door. There is also a smalldoor, u, near the bottom of the smoke-box, for the purpose of clearing out thecinders and ashes that collect in it: both doors have to fit closely, that no air mayenter at them to impair the draught.

Feed Pumps. —The boiler is supplied with water by the two feed pumps K K,which are worked by the engine; their construction will be explained afterwards.One of them is sufficient to supply the necessary quantity of water for the boiler,and the other is thrown out of action; but two are furnished in the engine, inorder that if one should fail from any accident, the other may take its place withoutany delay being caused. On forcing the water into the boiler, it checks the generationof steam by its coldness, and the effect of the engine is also diminished in consequenceof the power required to work the pump ; for this reason the action of the feed-pumpis generally suspended when the engine is ascending an inclination, and requires itsgreatest power, and the supply of water is made up by working both pumps on asucceeding level or descent:—this is an additional advantage in having duplicatepumps. A small cock, called the pet cock, is fixed in the pipe leading to the boiler fromeach pump, and a long handle, v, is fixed to it, extending to within reach of the engine-man, standing behind the engine; these cocks are opened occasionally to ascertainwhether the pumps are working properly, when a stream of water should be forcedout at each stroke *.

* The pet cock was invented by Mr. George Stephenson , to obviate what was for some time anincurable defect in the feed pumps of locomotive engines,—the pumps could not be made to keep inaction, as they were fixed close to the boiler, and hot water entered from the leaking of the valves,causing them to be filled with steam instead of water at each stroke; thus preventing them fromforcing any water into the boiler, which could, therefore, be used only for a short time, whilst thewater in it would last, the boiler having then to be emptied of steam and refilled. The cause of thisstoppage of the action was at length discovered, and the pet cock applied, by opening which the steamin the pump was let out, and the action renewed; the name was given to it playfully, from its pettingor coaxing, as it were, the pump to do its duty. It is not now required for this purpose, as the pumpis separated from the boiler by a long tube with a valve at each end, and the hot water cannot get into it.