Nevertheless, so far as I can discover, these holes and flues, at best a little fire at the bottom of the latter, are the sole and all-sufficient expedients of science and architecture for ventilation to this day, in spite of their total failure in experience.  I can find nothing in standard treatises or examples from philosophers or architects, beyond a theoretical calculation on so much expansion of air from so many units of heat, and hence so much ascensional force inferred in the ventilating flue—­a result which never comes to pass, yet none the less continues to be cheerfully relied on.  Unfortunately for the facts, they contradict the philosophy, and are only to be ignored with silent contempt.  A French Academician’s report on the ventilation of a large public building, lately reprinted by the Smithsonian Institution, states with absolute assurance and exactness the cubic feet of air changed per minute, with the precise volume and velocity of its ascension, by burning a peck of coal at the bottom of the trunk flue.  No mention is made of the anemometer or any other gauge of the result asserted, and we are left to the suspicion that it is merely a matter of theoretical inference, as usual; for every one who has had any acquaintance with practical tests in these matters knows that no such movement of air ever takes place under such conditions, unless by exceptional favor of the weather.

I have seen a tall steam boiler chimney induce through a four inch pipe a suction strong enough to exhaust the air from a large room as fast as perfect ventilation would require.  But this, it is well known, requires four hundred or five hundred degrees of heat in the chimney.  I never saw an ordinary domestic fire of coals produce any noticeable ventilating suction, without the use of a blower, urging the combustion to fury, and I presume nobody else ever did.

But, while nobody ever saw an active suction of air produced by the mere heat of a still or unexcited fire—­unless the quantity of heat were on a very large scale—­everybody has seen a roaring current sucked through the narrowed throat of a chimney or a stove by a blazing handful of shavings, paper, or straw.  It is very remarkable, when you come to think of it, that the burning of an insignificant piece of paper, with less heat in it, perhaps, than a pea of anthracite, will cause a rush of air that a bushel of anthracite cannot in the least degree imitate.  It is not only a curious but a most important fact.  In short, it is the cardinal fact on which ventilation practically turns.  But what is the nature of it?  There are three factors in the phenomenon.  In the first place, the mechanical peculiarity of flame, or gas in the moment of combustion, as compared with a gas like air merely heated, is an almost explosive velocity of ascent. The physical peculiarity from which this results is the intensity of its heat—­commonly stated at 2,000 degrees, as to our common illuminating gas—­acting instantaneously throughout its mass, just as in gunpowder.  The gas goes up the flue in its own flash, like the ignited charge in the barrel of a gun:  the burning coals can only send, and by a leisurely messenger, namely, the moderately heated gases, and contiguous air, that rise only by the gravitation or pressure of the surrounding atmosphere.