. As a means
of road traction the steam-engine was for half a century almost
entirely discomfited and routed by horse-power, partly owing to this
mechanical defect and partly, as we have seen, through legislative
partisanship.

The explosive type of engine was next called into requisition to do
battle against the living competitor of the engineer's handiwork.
Petroleum and alcohol, when volatilised and mixed with air in due
proportion, form explosive mixtures which are much more nearly
instantaneous in their action than an elastic vapour like steam held
under pressure in a boiler, and liberated to perform its work by
comparatively slow expansion. The petroleum engine, as applied to the
automobile, does its work in a series of jerks which provide for the
unequal degrees of power required to cope with the unevenness of a
road.

As against this, however, there are certain grave defects, due mainly
to the use of highly inflammable oils vapourised at high temperatures;
and these have impressed a large proportion of engineers with a belief
that, in the long run, either electricity or steam will win the day.
Storage batteries are well adapted for meeting the exigencies of the
road, just as they are for those of tramway traffic, because, as soon
as an extra strain is to be met, there is always the resource of
coupling up fresh batteries held in reserve--a process which amounts
to the same as yoking new horses to the vehicle in order to take it up
a hill. In practice, however, it is found that the jerky vibratory
motion of the gasoline automobile provides for this in a way almost as
convenient, although not so pleasant.

The chance of the steam-engine being largely adopted for automobile
work and for road traffic generally depends principally on the
prospects of inventing a form of cylinder--or its equivalent--which
will enable the driver to couple up fresh effective working parts of
his machinery at will, just as may be done with storage batteries. A
new form of steam cylinder designed to provide for this need will
outwardly resemble a long pipe--one being fixed on each lower side of
the vehicle--but inwardly it will be divided into compartments each of
which will have its own separate piston. Practically there will thus
be a series of cylinders having one piston-rod running through them
all, but each having its own piston.

Normally, this machine will run with an admission of steam to only one
or two of the cylinders; bu