This I had anticipated. Gravity on
the surface of Mars is less than half what it is on Earth; the total
mass of the planet is as two to fifteen. It was consequently to be
expected that the extent of the Martial atmosphere, and its density
even at the sea-level, would be far less than on the heavier planet.
Rigging the air-pump securely round the aperture, exhausting its
chamber, and permitting the Martial air to fill it, I was glad to find
a pressure equal to that which prevails at a height of 16,000 feet on
Earth. Chemical tests showed the presence of oxygen in somewhat
greater proportion than in the purest air of terrestrial mountains. It
would sustain life, therefore, and without serious injury, if the
change from a dense to a light atmosphere were not too suddenly made.
I determined then gradually to diminish the density of the internal
atmosphere to something not very much greater than that outside. For
this purpose I unrigged the air-pump apparatus, and almost, but not
quite, closed the valve, leaving an aperture about the twentieth part
of an inch in diameter.
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