spoken, is what we have hitherto practiced, as results abundantly
prove. Fallacies, both vulgar and scientific, obstruct our way. A
fundamental fallacy respects the very nature of the work, which is
supposed to be _to get in fresh air_. In point of fact, this care is
both unnecessary and comparatively useless. Take care of the bad air,
and the fresh air will take care of itself. Only make room for it, and
you cannot keep it out. On the other hand, unless you first make room
for it, you cannot keep it _in_; pump it in and blow it in as you may,
you only blow it _through_, as the Jordan flows comparatively
uncontaminated through the Dead Sea. This is a law of fluids that must
be kept in view. The pure air is quite as ready to get out as to get
in; while the air loaded with poisonous vapors is as sluggish as a
gorged serpent, and will not budge but on compulsion. Such compulsion
the grand system of wind _suction_, actuated by the sun, supplies on
the scale of the universe; and this we must imitate and adapt for our
more limited purposes.

It would seem as if we need not pause to notice so shallow though
common a notion as that which usually comes in right here, namely,
that confined air will move off somehow of itself, if you give it
liberty; being supposed to be much like a cat in a bag, wanting only a
hole to make its escape. Air is ponderable matter--as much so as
lead--and equally requires force of some kind to set it or keep it in
motion. But applied philosophy itself relies on a fallacious, or, at
best, inadequate source of motive power for ventilation. It gravely
prescribes ventilating flues and even holes, and promises us that the
warmed air within the house will rise through these flues and holes,
carrying its impurities away with it, from the pressure of the cooler
and denser air without. But we very well know that the best of flues
and chimneys will draw only by favor of lively fires or clear weather.