“If the atmosphere exerts so enormous a pressure, and has so much weight,” observed Louisa, “it is strange that it should not fall down on the earth.”
Mr. Seymour replied, “that the air was a peculiar fluid, which, from its elastic properties, was distinguished by the term of an elastic fluid, the particles of which were too far distant from each other to exert any cohesive attraction amongst themselves.”
“But I suppose,” said Tom, “that it gravitates, or is attracted by the earth; what then can be the reason, as Louisa says, that it does not fall, like any other body, to the ground?”
“And so it actually does,” replied Mr. Seymour. “The lower stratum of the atmosphere rests upon the ground, but the strata above it do not fall, because they are supported by the particles beneath them, in the same manner as the water at the surface of a basin is supported by that at the bottom: the only difference in these two cases arises from the one being an elastic, and the other an inelastic fluid; so that the air after compression resumes its original dimensions; and since the atmosphere, by the action of gravity, is always in a state of compression, so is it always endeavouring to expand itself.”
“If, then, the force of gravity were diminished,” observed Louisa, “the air would become much lighter, and I suppose that is the true reason of its being so much less dense in the upper regions.”
“Scarcely,” replied her father. “Have you forgotten the explanation[[36]] which I lately gave you, of the diminution in the weight of bodies at a distance from the earth’s surface?”
“I recollect it perfectly,” exclaimed Tom; “and it explained to us the reason that a marble fell from the top of a house, and from the ball of St. Paul’s with the same velocity.”
“And yet I am quite sure,” said Louisa, “that I have lately read an account of the air being so extremely light upon the top of a high mountain as to affect the breath and occasion great uneasiness.”
“I do not deny the fact, my dear; I only question your explanation of its cause. Can it not, think you, be accounted for upon some other principle than that of the diminished force of gravity?”
Louisa was unable to suggest any other probable reason.