ler, the steam is directed by the pipes, the
piston is moved by the steam pressure, and the fly-wheel rotates
because of proper mechanism between it and the piston. No one who has
given attention to the successive steps in the process is so puzzled as
to feel the need of inventing a particular force, or a new kind of
matter, or any agency, at any stage of the process, different from the
simple mechanical ones represented by a push or a pull. Even if he
cannot see clearly how heat can produce a push, he does not venture to
assume a genii to do the work, but for the time is content with saying
that if he starts with motion in the furnace and stops with the motion
of the fly-wheel, any assumption of any other factor than some form of
motion between the two would be gratuitous. He can truthfully say that
he understands the _nature_ of that which goes on between the furnace
and the wheel; that it is some sort of motion, the particular kind of
which he might make out at his leisure.

Suppose once more that, across the road from an engine-house, there was
another building, where all sorts of machines--lathes, planers, drills,
etc.--were running, but that the source of the power for all this was
out of sight, and that one could see no connection between this and the
engine on the other side of the street. Would one need to suppose there
was anything mysterious between the two--a force, a fluid, an immaterial
something? This question is put on the supposition that one should not
be aware of the shaft that might be between the two buildings, and that
it was not obvious on simple inspection how the machines got their
motions from the engine. No one would be puzzled because he did not know
just what the intervening mechanism might be. If the boiler were in the
one building, and the engine in the other with the machines, he could
see nothing moving between them, even if the steam-pipes were of glass.
If matter of any kind were moving, he could not see it there. He would
say there _must_ be something moving, or pressure could not be
transferred from one place to the other.

Substitute for the furnace and boiler a galvanic battery or a dynamo;
for the machines of the shop, one or more motors with suitable wire
connections. When the dynamo goes the motors go; when the dynamo stops
the motors stop; nothing can be seen to be turning or moving in any way
between them. Is there any necessity for assuming a mysterious agency,
or a force of