About a foot below one of the side windows was a circular opening,
eight inches in diameter, and fitted with a brass rim adapted in its
inner edge to the windings of a screw. In this rim was screwed the
large tube of the condenser, the body of the machine being, of course,
within the chamber of gum-elastic. Through this tube a quantity of the
rare atmosphere circumjacent being drawn by means of a vacuum
created in the body of the machine, was thence discharged, in a
state of condensation, to mingle with the thin air already in the
chamber. This operation being repeated several times, at length filled
the chamber with atmosphere proper for all the purposes of
respiration. But in so confined a space it would, in a short time,
necessarily become foul, and unfit for use from frequent contact
with the lungs. It was then ejected by a small valve at the bottom
of the car- the dense air readily sinking into the thinner atmosphere
below. To avoid the inconvenience of making a total vacuum at any
moment within the chamber, this purification was never accomplished
all at once, but in a gradual manner- the valve being opened only for
a few seconds, then closed again, until one or two strokes from the
pump of the condenser had supplied the place of the atmosphere
ejected.
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