“The density of the air,” replied his father, “would, undoubtedly, materially increase as we descended. It has been calculated that, at the distance of thirty miles below the surface, the air would have the same density as water; and, at the depth of forty-two miles, that of quicksilver; while, at the centre, it would be more solid than any substance of which we have any idea, for its density would be thousands of millions of times greater than that of mercury.” Mr. Seymour then informed his young pupils, that after the lesson they had just received, they would never again be puzzled by the motions of the barometer, which had so often excited their wonder. “As the quicksilver is contained in a closed tube, I do not exactly understand how the air can act upon it; and if the tube were not closed, it would of course run out from its weight,” observed Louisa. “You are altogether in error,” said her father. “In the first place,” he continued, “I will show you that the bulb at the lower extremity of the tube is open, in order that the quicksilver may freely communicate with the atmosphere, upon which, indeed, its action entirely depends; while the upper space is a perfect vacuum, so as to obviate any counteracting pressure. As to the quicksilver running out, have you so soon forgotten that the air presses upon every body on the surface of the earth, in the proportion of about fifteen pounds upon every square inch? Now it is from this circumstance that the column of quicksilver is sustained in the tube, the ascent and descent of which thus indicates the varying pressure of the atmosphere; so that, when the barometer falls, we know the air presses less heavily upon the earth, and the contrary when it rises.” “That I understand: but what can cause the pressure of the air to vary at different times?” asked Tom. “Cannot you imagine the atmosphere to be an airy ocean, and to be therefore thrown into enormous waves, so that we may sometimes have a longer column of air above us than at other times; this is one explanation, there may be other causes not so intelligible,” answered Mr. Seymour. “But enough of this for the present. Now, before we quit the subject of the air’s elasticity, let us consider the philosophy of the _pop-gun_; an amusement with which, I have no doubt, you are well acquainted.” “Indeed I am, papa; but do you allude to the quill, or to the wooden pop-gun?” “The principle in both is the same; tell me, therefore, the origin and nature of the force which enables you to shoot your pellet to so considerable a distance.” “It depends upon the action of the air,” replied Tom. “Undoubtedly: but your answer is too general; I wished you to state, in precise terms, the changes which the air undergoes upon this occasion. You first ram in your pellet to the further end of the tube, do you not?” “To be sure; and then I drive in a second pellet, and on forcing this forward, the first flies out with prodigious force.”