This gas, being specifically much heavier than common
air, descends into the bottom of such unoccupied fortresses, and
remains stagnant like water in old reservoirs. The current of pure
air continually passes over, without being able to carry off the mass
of stagnant air below; and the only way to render such places
habitable is to make large openings in the walls on all sides, from
the top to the bottom, so that the foul air may be driven out by the
current of pure atmospheric air, which will then be continually
rushing in. When these fortresses are thickly peopled, the continual
motion within tends, I think, to mix up this gas with the air above;
while the numerous fires lighted within, by rarefying that below,
tend to draw down a regular supply of the atmospheric air from above
for the benefit of the inhabitants. When natives enter upon the
occupation of an old fortress of this kind, that has remained long
unoccupied, they always make a solemn religions ceremony of it; and,
having fed the priests, the troops, and a crowd of followers, all
rush in at once with beat of drums, and as much noise as they can
make.
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