Our rocket cars were only perfected in the usual slow course of invention, and could neither have been constructed nor propelled a hundred years ago, for neither was the metal of which they are constructed produced, nor had the method of propulsion or even the propulsive power been developed. Inventors had to wait till science had given us in abundance a metal less than a quarter the weight of iron, but as strong and durable, and this was not until some fifty years ago when a process was discovered for producing cheaply the beautiful metal calcium. But calcium would have been little use alone. Aluminium, which is now so plentiful, had to be alloyed with it, and aluminium was not used to any great extent till the beginning of this century, when an electric process of reducing it quickly from its ore--common clay--was discovered. The metal known as calcium bronze, which is now so common, is an alloy of calcium, 0.75; aluminium, 0.20; and 0.05 of other metals and metalloids in varying proportions according to different patents. This alloy has all the useful properties of the finest steel with about one-fourth its weight, and is besides perfectly non-oxydisable and never tarnishes. Without the production of a metal with all these combined qualities, we might still in our journeys, be dawdling along at sixty miles an hour in a cumbrous railroad car behind a snorting, screaming locomotive. 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,