Our swiftly darting cars were not at first constructed on such
perfect principles as now. Invention seems to follow certain laws,
and has to take its time. A new discovery in physics has to be
supplemented by one in chemistry, and one in chemistry by another
in physics, and so on through a whole century, perhaps, before any
great invention is perfected. Thus it happens that, though the
principle of the rocket has been known for an age, it is only
comparatively recently that it has been applied to the propulsion
of cars. An invention, too, always presents itself to an inventor
at first in the most complicated form, and frequently many years
are passed in attempts at simplification. What a wide interval is
there between the steam locomotive with all its complex mechanism,
and the magnificently simple rocket car! A century of ceaseless
invention is comprehended between the two! Before the simplicity
of our cars was arrived at, inventors had to give up boilers,
fire-boxes, valves, steam-pipes, cylinders, pistons, wheels,
cranks, levers, and a host of minor parts. Wheels died hard.
Electric locomotives using them were brought out and were
considered to do the very fastest thing possible in locomotion,
and such was in fact the case while wheels were used, for wheels
could not have borne a faster pace without flying to pieces from
centrifugal force. But when an inventor devised a machine on
runners to move on lubricated rails, a great step was gained,
though the invention was not a success, and when, after this,
liquid carbonic acid, or carbonic acid ice expanding again to a gas
was employed as a motive power, another advance was made. Then the
greatest lift of all was given. The solidification of oxygen and
hydrogen by an easy process was discovered and mankind presented
with a new motive power. In due time a way was found to make the