as the engine of that most reliable automobile the railway locomotive. Most steam cars have two cylinders, but for the sake of simplicity we will describe a single-cylinder engine, as the principle of working is identical. Fig. 16 shows the cylinder a, in which a piston b is free to move up and down. The steam is admitted alternately at the top and the bottom

Fig. 16 of the cylinder by means to be described later. To the piston b a piston-rod c is fixed, and it issues through a hole in the bottom of the cylinder. Both the piston b and the hole through which the piston-rod issues from the cylinder are rendered steam-tight by means to be presently described. The piston-rod c is attached by a hinged joint d to the connecting rod e, which at the other end encircles the crank-pin f of the crank g, which moves, as shown by the arrow and dotted line, in a circle of which h is the centre. We will assume that steam is admitted to the top of the cylinder a. It blows the piston b downwards, which in its turn depresses the piston-rod c, and, as this is connected to crank c by the connecting rod e, the rotation of the crank is started. When b gets to the bottom of the cylinder, the steam supply on the top of it is stopped, and steam is admitted underneath the piston, so that it is blown upward from the underside, and the crank is pulled up. As b ascends it expels the steam from the cylinder which had driven it on its downward stroke. This action is kept up as long as the engine is at work, and by the interposition of the connecting rod and crank, the reciprocating or