A. The same quantity of air is expanded to three or four times its original volume; and is made so much lighter than common air, that even when all the paper, wire, and cotton are added, it is still lighter bulk for bulk.
Q. What is meant by being lighter “bulk for bulk?”
A. If the balloon be 3 square feet in size, it is lighter (when inflated) than 3 square feet of common air, and therefore floats through it; as a cork (at the bottom of a tub of water) would rise to the surface.
Q. Why does smoke rush up a chimney?
A. The heat of the fire expands the air in the chimney; and (being thus made lighter than the air around), it rises up the chimney, and carries the smoke in its current.
Q. Why has a long chimney a greater draught than a short one?
A. Because air rises faster and faster the higher it ascends in a chimney flue; the same as a stone falls faster and faster the nearer it approaches to the ground.
Q. Why will a long chimney smoke, unless the fire be pretty fierce?
A. If the fire be not pretty fierce, its heat will not be sufficient to rarefy all the air in the chimney; and then the chimney will smoke.
Q. Why will the chimney smoke, if the fire be not big enough to heat all the air in the chimney flue?