and the fire continues to burn, the water in the coil will be forced back into the air-chamber, and the production of steam will cease.
When the water is exhausted, the engine simply stops until the supply is renewed. At the right of the pump a relief-valve is placed to limit the pressure. It is not a safety-valve, as, even if it were absent, an explosion could not occur, as the mechanism of the pump is not strong enough to produce a bursting pressure. The construction of the generator is more fully shown in Fig. 7. It consists of a cylindrical shell in which is coiled about thirty-five feet of seamless copper tubing. In the base of this is a gas-burner, L, and around it is a second shell, leaving an annular space for the entrance of air. The air is also admitted through openings at the bottom of the shell. The burning 
Fig. 8. gases pass up through the center of the coils and down between the two, and out by the draught-pipe. The exhaust steam is discharged through the regenerator, where it partly heats the water in the coil, into this draught-pipe. On first lighting the gas, the cap A’ is removed and the flame allowed to burn up into the air, but as soon as the formation of steam begins the cap is replaced, and the exhaust steam then creates sufficient draught to carry the burning gases through the coils into the draught-pipe. The generator has but one fiftieth the cubical capacity of that required by an ordinary boiler to run the same engine. The engine is of the simple oscillating type, and the whole apparatus is attached to a bed-plate forming the top of a vase-shaped receptacle. Fig. 8 shows the appearance of the complete motor. In another form the vase is supported by a bracket attached to the wall instead of by the tripod. The vase is used to hold the water from which the pump supplies the reservoir. Four or five quarts of water once in as many hours is generally sufficient in the ordinary use of the machine.
The engine in this size is far from being economical, requiring twelve cubic feet of gas an hour, while it only furnishes a thirty-third of a horse-power. Efficiency, however, is not very important with such a small motor. This amount of fuel, with
