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.

The automobile builders have a problem that troubl