Q. What effect has heat upon the air?
A. Heat rarefies the air, and causes it to expand.
Q. How do you know that heat causes the air to expand?
A. If a bladder half full of air (tied tight round the neck), were laid before a fire, the heat of the fire would expand the air so much, that the bladder would soon be entirely inflated; (in this case, the air in the bladder is expanded to twice its original bulk, by the heat of the fire).
Q. What effect is produced upon air by rarefaction?
A. It causes the air to ascend through colder strata, as a cork (put at the bottom of a basin of water) would ascend through the water.
Q. How do you know that rarefied air ascends?
A. When a boy sets fire to the cotton of his balloon, the flame heats the air inside the balloon; and the air becomes so light, that it ascends, and carries the balloon with it.
Q. What effect is produced upon air by cold?
A. Air is condensed by cold, or squeezed into a smaller compass; in consequence of which, it becomes heavier, and descends towards the ground.