The process, as it actually is, may be clearly exhibited by a very simple means. Let anyone take a tube, say an inch in diameter - a roll of paper will do as well as anything - and, applying it closely to his mouth, try the whole force of his lungs through it upon any light object. The amount of effect will be found surprisingly small; and unless the tube is a short one, it will be so far absorbed by friction and atmospheric resistance as to be almost imperceptible. Then let him hold the same tube near to the mouth, but not in contact, and repeat the experiment. With the best adjustment, the effect may be described as tenfold or fifty-fold, or almost any fold - the effect of the simple blowing being merely nominal as compared with the induced current added by blowing into the tube instead of in it. The blast enters the free and open orifice with all the contiguous air which its surface friction and the vacuum of its movement can involve in its rolling vortex. While the entrance is thus crowded with pressure, the exit is free; and the result at the exit is a blast of well sustained velocity and magnified volume; ready itself to repeat the miracle on a still larger scale if provided with the apparatus for doing so.
To test this, now place a second and larger tube in such position as to prolong the first in a straight line, but with a slight interval between the meeting ends; so that the blast, as magnified in volume in entering the first tube, may enter in like manner the second tube and be magnified again. With correct adjustments this experiment will prove more surprising than the first. Put on a third and still larger tube in the same way, and still larger surprise will meet a still larger volume and force of blast, like a stiff breeze set in motion by the puny effort of a single expiration. Of course, the prime impulse must bear a certain proportion to the result; and the inductive or tractional friction of the initial blast, of flame or breath, will be used up at length unless re-enforced. In ventilating practice, there is such re-enforcement, from an excess of gravity in the cooler atmosphere outside the flue in which the flame is operating with its heat as well as its ascensional traction; so that there has been found no limit to the extensions and fresh inductions that may be added to the first or trunk flue, with increase rather than diminution of power at every point.
But the terms on which such extensions must be made have been referred to in our illustration, and must be accurately ascertained and observed. They constitute what is, in effect, the third factor in the phenomenon of a roaring draught, and also, therefore, ineffective ventilation. That is, the entering or induced current of air must always find its channel of progress and exit certain correct degrees larger than the opening by which it entered. Every one knows that a stove or chimney wide open admits of but little suction in connection with even the blaze of paper or shavings.
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.
Read by Wm. C. Conant before the Polytechnic Association of the American Institute, New York, May 10, 1883.