Most steam automobile boilers are of the water-tube variety—­that is, water to be turned into steam is carried through the flames in pipes, instead of the heat in pipes through the water, as in the ordinary flue boilers.  Compactness, quick-heating, and strength are the characteristics of motor-car boilers.  Some of the boilers are less than twenty inches high and of the same diameter, and yet are capable of generating seven and one-half horse-power at a high steam pressure (150 to 200 pounds).  In these boilers the heat is made to play directly on a great many tubes, and a full head of steam is generated in a few minutes.  As the steam pressure increases, a regulator that shuts off the supply of gasoline is operated automatically, and so the pressure is maintained.

[Illustration:  THE “LIGHTHOUSE” OF THE RAIL The switchman’s house (on the left), commanding a view of the railroad yard, from which the switches of the complicated system are worked and the semaphore signals operated.]

The water from which the steam is made is also fed automatically into the boiler, when the engine is in motion, by a pump worked by the engine piston.  A hand-pump is also supplied by which the driver can keep the proper amount when the machine is still or in case of a breakdown.  A water-gauge in plain sight keeps the driver informed at all times as to the amount of water in the boiler.  From the boiler the steam goes through the throttle-valve—­the handle of which is by the driver’s side—­direct to the engine, and there expands, pushes the piston up and down, and by means of a crank on the axle does its work.

The engines of modern automobiles are marvels of compactness—­so compact, indeed, that a seven-horse-power engine occupies much less space than an ordinary barrel.  The steam, after being used, is admitted to a coil of pipes cooled by the breeze caused by the motion of the vehicle, and so condensed into water and returned to the tank.  The engine is started, stopped, slowed, and sped by the cutting off or admission of the steam through the throttle-valve.  It is reversed by means of the same mechanism used on locomotives—­the link-motion and reversing-lever, by which the direction of the steam is reversed and the engine made to run the other way.

After doing its work the steam is made to circulate round the cylinder (or cylinders, if there are more than one), keeping it extra hot—­“superheated”; and thereafter it is made to perform a like duty to the boiler-feed water, before it is allowed to escape.

All steam-propelled automobiles, from the light steam runabout to the clumsy steam roller, are worked practically as described.  Some machines are worked by compound engines, which simply use the power of expansion still left in the steam in a second larger cylinder after it has worked the first, in which case every ounce of power is extracted from the vapour.