Even if the pedrail is not made to carry weapons of destruction, we can, after our experiences with horseflesh in the Boer War, understand how important it may become for commissariat purposes. The feats which it has already performed mark it as just the locomotive to tackle the rough country in which baggage trains often find themselves.
To conclude with a more peaceful use for it. When fresh country is opened up, years must often pass before a proper high road can be made, yet there is great need of an organised system of transport. Whither ordinary traction-engines, or carts, even horses, could scarcely penetrate, the pedrail tractor, thanks to its big, flat feet, which give it, as someone has remarked, the appearance of "a cross between a traction-engine and an elephant," will be able to push its way at the forefront of advancing civilisation.
At home we shall have good reason to welcome the pedrail if it frees us from those terrible corrugated tracks so dreaded by the cyclist, and to bless it if it actually beats our roads down into a greater smoothness than they now can boast.
OIL ENGINES--ENGINES WORKED WITH PRODUCER GAS--BLAST FURNACE GAS ENGINES
If carbon and oxygen be made to combine chemically, the process is accompanied by the phenomenon called _heat_. If heat be applied to a liquid or gas in a confined space it causes a violent separation of its molecules, and power is developed.
In the case of a steam-engine the fuel is coal (carbon in a more or less pure form), the fluid, water. By burning the fuel under a boiler, a gas is formed which, if confined, rapidly increases the pressure on the walls of the confining vessel. If allowed to pass into a cylinder, the molecules of steam, struggling to get as far as possible from one another, will do useful work on a piston connected by rods to a revolving crank.
We here see the combustion of fuel external to the cylinder, i.e. under the boiler, and the fuel and fluid kept apart out of actual contact. In the gas or oil-vapour engine the fuel is brought into contact with the fluid which does the work, mixed with it, and burnt _inside_ the cylinder. Therefore these engines are termed _internal combustion_ engines.
Supposing that a little gunpowder were placed in a cylinder, of which the piston had been pushed almost as far in as it would go, and that the powder were fired by electricity. The charcoal would unite with the oxygen contained in the saltpetre and form a large volume of gas. This gas, being heated by the ignition, would instantaneously expand and drive out the piston violently.