burners giving off their heat below. The burners quickly bring the coils to a red heat, and a small stream of water is pumped into a (fig. 10), and almost instantly converted into steam. It passes right through the coil, issues at b into the next coil above, and so on to the engine. The upper coil superheats the steam—that is to say, it makes it very much hotter than it would be in either of the types of boilers we have previously described—as after being converted into high-pressure steam in the lower parts of the coils, it is still subjected to great heat in the upper lengths of the coil before it passes to the engine. The expansive force of the steam is considerably increased by this superheating, and not only so, but it is very different from the steam from a fire-tube or water-tube boiler, being much drier, as well as hotter, c (fig. 9) indicates the air-inlet to burner box, and the arrows show the direction of the air currents. d is the chimney.
Pumps.—When steam is up, and the burner in full operation, the water in the boiler is quickly evaporated, and the renewal of the supply is performed by pump. The pump is usually driven off the cross-head, a part of the engine which has a constant up-and-down motion. On most of the smaller cars the water supply is carried in a horseshoe-shaped tank at the back, which half encircles the boiler. In the Serpollet the water-tank takes the place of the motor bonnet of a petrol car.
The pump, fig. 11, has a tightly fitting plunger, which is pulled up and pushed down by the engine. As it makes its upward movement it sucks water in on the right, and the flap valve, which only opens in an inward direction, freely admits the water. As the plunger is pushed down it compresses the water, which at once closes the valve on the right and opens the one on the left, forcing the water to the boiler. The valve on the left is opened by the pressure of water from the pump and closed by the pressure of steam on the other side from the boiler as soon as the down-stroke of the pump ceases. Fig. 12 shows the form of pump more commonly employed on steam
