gliding, noiseless impulse. Yet it is one that would push aside massy
tons of dead weight, that would almost unimpeded crush a hole through
the enclosing wall, that whirls upon the rails the drivers of a
locomotive weighing sixty tons as though there were no weight above
them, no bite upon the rails. There is an enormous concentration of
force somewhere; of a force which perhaps no man can fairly estimate;
and it is under the thin shell we call a boiler. Were it not elastic it
could not be so imprisoned, and when it rebels, when this thin shell is
torn like paper, there is a havoc by which we may at last inadequately
measure the power of steam.

We have in modern times applied the word "engine" almost exclusively to
the machine which is moved by the pressure of steam. Yet we might go
further, since one of the first examples of a pressure engine, older
than the steam machine by nearly four hundred years, is the gun. Reduced
to its principle this is an engine whose operation depends upon the
expansion of gas in a cylinder, the piston being a projectile. The same
principle applies in all the machines we know as "engines." An
air-engine works through the expansion of air in a cylinder by heat. A
gas-engine, now of common use, by the expansion, which is explosion,
caused by burning a mixture of coal-gas and air, and the steam-engine,
the universal power generator of modern life, works by the expansion of
the vapor of water as it is generated by heat. Steam may be considered a
species of _gradual_ explosion applied to the uses of industry. It
often becomes a real one, complying with all the conditions, and as
destructive as dynamite.

It cannot be certainly known how long men have experimented with the
expansive force of steam. The first feeble attempt to purloin the power
of the geyser was probably by Hero, of Alexandria, about a hundred and