within it; and a flue contracted at the inlet and expanded as it
rises, so as to free, diffuse, and lighten the column of air, toward
the exit; _then_, initiate an induced current of air at the inlet, by
the injection of a jet of gas in the state of semi-explosive action
called flame; the pressure pushing upward from the crowded entrance
finds easier way and less resistance the farther it goes in the
expanding flue; the warmth and reduced gravity of the stream comes in
as an auxiliary in overcoming friction and any exceptional obstruction
in the state of the atmosphere; and now, as the ball is once set
rolling, with a little _aid_ instead of resistance from gravitation,
its initial impulse all the while sustained by the gas jet, and
friction reduced to a very small incident--there is nothing to prevent
the current rolling on with accelerated velocity (within the
limitations imposed by increasing friction) and rolling on forever. I
might, if I had time, add a curious consideration of the law of
_vortex motion_ in elastic fluids, demonstrated by Helmholtz, which
relieves the motion of such fluids from friction, as wheels facilitate
the movement of a solid; and which also sucks into the rolling vortex
the contiguous air, thus entraining it, as we have seen, so much more
effectively than could be done by a direct and rigid current, like a
jet of water, for instance. A wheel set in motion on an almost
frictionless bearing of metalline, runs without perceptible abatement
of velocity, until one begins to involuntarily question whether it
will ever stop. In the all but free winds that roll with minimized
friction in the higher atmosphere, there seems to be a self-moving
force; so persistent is simple momentum in a mass so infinitesimally
obstructed and so infinitely wheeled. An active current of air in a
ventilating flue is only less perfect in the same conditions; and so
it is quite conceivable, and not incredible, that such a current may
be gradually established and thenceforward permanently maintained by a