Glaisher and Mr. Coxwell went up five miles and a half, Mr.
Glaisher's veins began to swell, and his head grew dizzy, and he
fainted. The air was too thin for him to breathe enough in at a
time, and it did not press heavily enough on the drums of his
ears and the veins of his body. He would have died if Mr.
Coxwell had not quickly let off some of the gas in the balloon,
so that it sank down into denser air. (*100 cubic inches near the
earth weighed 31 grains, while the same quantity taken at four
and a half miles up in the air weighed only 12 grains, or two-
fifths of the weight.)
And now comes another very interesting question. If the air gets
less and less dense as it is farther from the earth, where does
it stop altogether? We cannot go up to find out, because we
should die long before we reached the limit; and for a long time
we had to guess about how high the atmosphere probably was, and
it was generally supposed not to be more than fifty miles. But
lately, some curious bodies, which we should have never suspected
would be useful to us in this way, have let us into the secret of
the height of the atmosphere.
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