“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. “The fact, then,” said her father, “is simply this; since the air is elastic, or capable of yielding to pressure, so, of course, the lower parts must be more dense, or in a greater state of compression than those which are above them. In a pile of fleeces of wool, are not the lower fleeces pressed together by the weight of the superior ones, and do they not lie light and loose, in proportion as they approach the uppermost fleece, which receives no external pressure, and is confined merely by the force of its own gravity?” “Clearly,” said Louisa. “Well then, we will suppose, for example, that the whole column of the atmosphere were divided into a hundred parts, and that each of these parts weighed an ounce; would not the earth, and all things on its surface, be, in such a case, pressed upon with the whole hundred ounces?” “No one can deny that,” said Tom. “The lowest stratum of air,” continued Mr. Seymour, “would be pressed upon by the ninety-nine ounces above it; the next by ninety-eight; and so on, until we arrived at the ninety-ninth stratum from the bottom, which would, of course, be subjected to no more than one ounce of pressure, or to the weight of the last and highest stratum.” The children were perfectly satisfied with this simple explanation; and Tom enquired whether, for the same reason, the water at the bottom of the sea must not be very dense, and unlike that we are accustomed to observe on the surface: his father, however, corrected this notion, by stating that water, not being, like air, elastic and compressible, would not suffer any material diminution in volume, although pressed even by the enormous weight of the superincumbent ocean.(30)