They are from twenty to thirty feet in diameter.
With what are balloons filled?
Balloons are filled with a gas lighter than air.
What gas is often used for filling balloons?
The coal gas, such as is used to light our cities at night.
What lighter kind of gas is sometimes used?
Hydrogen gas is sometimes used.
Is hydrogen gas much lighter than the air?
It is so much lighter than the air, that fifteen cubic feet of hydrogen gas weigh no more than one cubic foot of air.
How much less will a balloon thirty feet in diameter weigh, when filled with hydrogen gas, than when filled with air?