drives the crank direct through the connecting rod k, exactly the same as in a petrol engine. l is the flywheel, and m a tap for letting dirty lubricating oil out of the crank chamber. Each of the four cylinders works in a precisely similar way to the one described. In some engines the valves are parallel with the cylinders instead of slightly inclined as shown in fig. 26.
Piston-rings.—These scarcely require description, as they are the same in principle as those used in petrol motors, and by turning to the chapter dealing with these the reader will be
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Fig. 26.—The Serpollet Engine
able to find out how the piston, which must be a free sliding fit in the cylinder, is also pressure-tight.
Stuffing Boxes.—As the piston-rod issues from the bottom of the cylinder it is necessary that this should also be a free sliding fit, and at the same time steam-tight. These ends are attained by having a circular cavity, into which packing is inserted and held firmly by a screw ring or gland, and a locknut. Two forms of stuffing-box are shown in figs. 27 and 28. The packing usually consists of some substance in which asbestos and graphite are mainly used.
Condensers.— In damp weather, when using a full supply of
