And yet it is not the small flame itself that roars in the chimney but the rush of air induced by it.  The semi-explosion of flame is but for an instant, though constantly renewed, and its explosive impulse cannot carry its light products of combustion very far through stationary and resistant air.  It is the induction of air carried with it by such semi-explosive impulse (under proper mechanical conditions) that is strange to our observation and understanding, and is the second factor in the phenomenon we are accounting for and preparing to utilize.

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