“I have here,” said he, “a figure exactly similar to those in the bottle, which we will now examine. You will observe, that in its centre there is a cavity terminating in a small orifice in the lower part; this cavity may be made to contain any quantity of air, so as to give the required buoyancy to the figure: now mark!--I press my hand upon the parchment cover, and the figure, you perceive, descends; I now remove the pressure, and see, it immediately reascends. The water in the bottle, as I have told you, is incompressible; when, therefore, I press upon the surface, it rises into the interior of the figure, and, consequently, by compressing the air into a less space, renders it less buoyant; but no sooner is the hand removed, than the enclosed air resumes its former volume, and expels the intruding water; in consequence of which the figure regains its former lightness, and reascends. Do you understand me?” asked Mr. Seymour.
“Perfectly,” said Tom, “and many thanks for the explanation;” and in this opinion did the whole party concur.
“Well, then,” continued Mr. Seymour, “you will now understand the use of the air-bladder in fish, for it is constructed upon a precisely similar principle. When the fish desires to descend, it presses upon the bladder by means of its muscles, and thus condenses the included air into a smaller volume.”[[37]]
“I now also perceive why the water at the bottom of the sea cannot be much more dense than that on the surface; but, if we could dig a pit to the centre of the earth, the air, in that case, would be highly dense, because, unlike water, it is compressible,” said Tom.
“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.”