ancestors. Only by means of contrasts, startling in their details, can
we arrive at an adequate estimate, even as a moral farce, of the power
of steam as embodied in the modern engine in a thousand forms.

* * * * *

Perhaps it might be well to attempt to convey, for the benefit of the
youngest reader, an idea of the actual working of the machine we call a
steam-engine. There are hundreds of forms, and yet they are all alike
in essentials. To know the principle of one is to know that of all.
There is probably not an engine in the world in effective common
use--the odd and unusual rotary and other forms never having been
practical engines--that is not constructed upon the plan of the cylinder
and piston. These two parts make the engine. If they are understood only
differences in construction and detail remain.

Imagine a short tube into which you have inserted a pellet, or wad of
any kind, so that it fits tolerably, yet moves easily back and forth in
the bore of the tube. If this pellet or wad is at one end of the tube
you may, by inserting that end in your mouth and putting air-pressure
upon it, make it slide to the other end. You do not touch it with
anything; you may push it back and forth with your breath as many times
as you wish, not by blowing against it, so to speak, but by producing an
actual air-pressure upon it which is confined by the sides of the tube
and cannot go elsewhere. The only pressure necessary is enough to move
the pellet.

Now, if you push this little pellet one way by the air-pressure from
your mouth, and then, instead of reversing the tube in the mouth and
pushing it back again in the same way, reverse the process and suck the