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car, although driven by a steam engine, really derives its power from heat, but, instead of the combustible or fuel being burned and converted into pressure in the cylinder of the engine, as in the internal combustion engine of the petrol car, it is burned under a boiler. The expansive or elastic force of the steam pressure generated by the heat of the fire in its turn drives the engine, which gives the car its motion. The heat energy of the fuel is released by combustion; this heat is used to generate steam in the boiler, and the energy of the steam is transformed into motion after being admitted into the engine. Thus the three main essentials, of the propelling apparatus of a steam car are the fire or burner, the steam boiler or generator, and the engine.

Fuel.—Coal or coke is not used for pleasure cars, as either is too cumbersome and dirty, and the fire requires constant attention, liquid fuel in the form of petroleum (paraffin), or petroleum spirit (petrol or motor spirit), being universally adopted. Although petrol will ignite instantly if a match be applied to it, and paraffin will not, both must be vaporised or transformed into a gas by heat before they can be economically and cleanly used as heating agents. Not only so, but when vaporised, they must be burned mixed with air on the Bunsen principle.

Examples.—An ordinary domestic gaslight is produced by burning gas in air, but the atmospheric or Bunsen burner burns a mixture of gas and air in air.