COMPOUND LOCOMOTIVES. The compound loco motive is one in which the steam is admitted to one cylinder called the high-pressure cylinder, where it partially expands in doing its work, and whence it exhausts into the steam-chest of a larger cylinder. called the low-pressure cylinder, where it completes its expansion and finishes its work. The compound principle has been applied to locomotives in three ways: (1) Two-cylin der compounds, with a cylinder on each side: the steam passes from the high-pressure cylinder on one side through the smoke-box and across the engine to the low-pressure cylinder on the oppo site side; (2) three-cylinder compounds, with a high-pressure cylinder on each side, the steam from which exhausts into a low-nressure cylinder beneath the smoke-box; (3) four-cylinder com pounds, with a high-pressure and a low-pressure cylinder on each side of the engine, each pair working independently of the other. In the United States four makes of two-cylinder loco motives are in use; they are named the Rich mond, Schenectady, Pittsburg. and Brooks, from the names of the works at which they originated. These four makes differ from each other princi pally in the construction and operation of their intercepting valves. This valve is an ingenious and rather complicated mechanism. func tion it is to admit live steam from the boiler directly into the low-pressure cylinder, and then to cut off that flow of steam when the exhaust of the high-pressure cylinder has raised the pres sure in the receiver leading to the low-pressure cylinder sufficiently to enable it to deliver steam to the latter. The necessity of being able to
deliver live steam into the low-pressure cylinder arises when the crank of the high pressure stops on the dead centre. Intercepting valves are some times designed to cut off the live steam auto matically, and sometimes this operation is per formed from the cab by suitable lever arrange ments.
Three-cylinder locomotives have only been used experimentally 'a the States. but they have met witn considerable favor in Europe. Four-cylinder compound locomotives are of two forms; in one, the high-pressure cylinder is set above the low-pressure cylinder, as shown in Fig,. 15. and in the other the two cylinders on each side are set end to end, or tandem. The form of four-cylinder compound most used in the United States is that having each pair of cylin ders arranged as shown in Fig. 15. This draw ing shows the piston-valve between the two cylin ders, and by means of the arrows the course of the steam from its entrance to the high pressure cylinder to its exhaust from the low pressure cylinder. The general use of compound locomotives is of comparatively recent date. the first locomotive of this type having been built In 1575. The great advantage claimed for the com pound locomotive is economy of fuel. which is variously stated to he from 10 per cent. to 30 per cent. over single engines.