HHAVIER THAN AIR 237

shouts and screams of the spectators. At first it was all
very fine; they enjoyed their sail and crouched in the
bottom of the balloon-car, chattering to each other at a
great rate, for they had no idea of their danger. The bal-
loon was kept up in the air by its great, big, varnished
silk bag, being full of hydrogen gas, which is about sixteen

”

times lighter than air—” Then Henri spoke:

“Did it float in the air something as soap-bubbles do ?
because they ’re so much lighter than anything else.”

“Yes,” said Monsieur, wondering whether it were worth
while to explain that the soap-bubble is just a bag made
of a very thin sheet of water and filled with warm breath.
“Anything that is lighter than air, if set free, will rise. But
the air grows thinner and lighter the higher one goes up
above the earth, and when a balloon gets into very thin
air, the gas within the bag, finding that it is not pressed upon
so much by the heavy air outside, begins to swell and try to
get out; and the higher it goes, the more the gas pushes,
until, at last, it bursts the bag—then what happens?”

“The monkeys get frightened,” said Marie, gazing earn-
estly at the picture. ,

Henri was older and wiser; so he answered that “most
likely the balloon would all shrivel and tumble down if
the gas came out of it, just as a toy balloon would if some
one should prick it.”

“Very good,” assented Monsieur. “ Now, in the top of
all balloons there is a valve or little door for letting out the
gas when it begins to swell, and a man seated in the bal-
loon-car has only to pull a certain string when he wishes
to open the valve. But our monkeys knew nothing of