But, given a flue, to define and conduct a stream; an upright
flue, to take advantage of the slighter gravity of the warmed air
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.
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