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 troubles locomotive builders
very little--that is, compensating the difference between the speeds of
the two driving-wheels when turning corners. Just as the inside man of a
military company takes short steps when turning and the outside man
takes long ones, so the inside wheel of a vehicle turns slowly while the
outside wheel revolves quickly when rounding a corner. As most
automobiles are propelled by power applied to the rear axle, to which
the wheels are fixed, it is manifest that unless some device were made
to correct the fault one wheel would have to slide while the other
revolved. This difficulty has been overcome by cutting the axle in two
and placing between the ends a series of gears which permit the two
wheels to revolve at different speeds and also apply the power to both
alike. This device is called a compensating gear, and is worked out in
various ways by the different builders.

The locomotive builder accomplishes the same thing by making his wheels