The mobility of air seems almost preternatural, when the proper conditions for setting a current in motion are supplied. But without a current established, it is surprising in turn to find how obstinately and elusively immovable it can be. It is like tossing a feather; or trying to drive a swarm of flies; dodging and evading every impulse applied. 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. 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 small motor flame barely more than enough to overbalance the minimized friction. This is not a supposed or theoretically inferred fact, like the facts of ventilation sometimes alleged by theorists. On the contrary, the theory I have offered is merely an attempt to explain facts that I have witnessed and that anyone can verify with the anemometer. But the _theory_ by no means covers the art and mystery of ventilation; for ventilation is truly an _art_ as well as a mystery. The art lies in a consummate experience of the sizes, proportions, and forms of flues, their inlets, expansions, and exits, with many other incidental adaptations necessary, in order to insure under _all_ circumstances the regular exhaustion of any specific volume of air required, per minute. And this art has by one man been achieved. It would be a double injustice if I should neglect from any motive to inform my audience to whom I am indebted for what I know about ventilation practically, and even for the knowledge that there is any such fact as a practicable ventilation of houses; one who is no theorist, but who has felt his way experimentally with his own hands, for a lifetime, to a practical mastery of the art to which I have attempted to fit a theory; every one present who is well informed on this subject must have anticipated already in mind the name of Henry A. Gouge.