nution 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." (34.)

"I do not deny the fact, my dear: I only question your explanation of it. 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 the 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 was 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

2

explanation; and Tom inquired 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. (35.)

"I have before alluded to the relative compressibility of air and water, and the present appears a good oppotunity for proving the fact by an amusing experiment. See! here are the Bottle Imps,' vicar, which you may remember I promised to introduce to your respectful notice," said Mr. Seymour. "In this jar of water, carefully

closed, as you may perceive, by parchment, are two little enamelled figures, which shall be made to rise and fall, by alternately pressing upon and removing the hand from the cover: thus."

Why, the spirit of Simon Magus must surely possess thee!" exclaimed the vicar. The children, as may be readily imagined, were much astonished at so singular an effect, and expressed much anxiety to be informed by what mechanism it was produced. Their father accordingly proceeded with the following explanation.

"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."*

"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 an 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

*In the cod-fish the air-bladder is familiarly called the sound.

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."

"Very well: now examine what takes place. On propelling forward your second pellet, you condense the air which is enclosed between the two, until its elastic force becomes so great as to overcome the friction of the first pellet; thus released, the air expands with considerable force, and imparts a rapid motion to the pellet."

"I have frequently heard of the air-gun," said Louisa; "I suppose it depends upon a similar principle."

"It does; and it affords a very striking example of the surprising force which air is capable of exerting, when condensed to a considerable degree; for, by means of this instrument, bullets may be propelled with a force very nearly equal to that of gunpowder."

"It is a curious fact," observed the vicar, "that, although the air-pump is a modern invention, yet the air-gun, which is so nearly allied to it in the construction of its valves and condensing syringe, should have existed long antecedent to it; for it is recorded that an air-gun was made for Henry IV. by Marin, of Lisieux, in Normandy, as early as 1408; and another was preserved in the armoury at Schmetau, bearing the date of 1474."

"But the air-gun of the present day," said Mr. Seymour, "is very different from that which was formerly made, and which, like the pop-gun, discharged but one bullet, and that after a long and tedious process of condensation; while it is now made to discharge five or six without any visible variation of force, and will even act upon a dozen, but with decreasing effect."

"I feel very curious to learn something more about this air-gun," said Tom.

"There is a reservoir for the condensed air," replied Mr. Seymour, "which is secured by a nicely-constructed valve, and which is made to open by pulling the trigger of the gun, so that a portion only of the air is disengaged, which, rushing into the barrel, gives motion to the ball."

"But how is the condensed air introduced into the reservoir ?" asked Tɔm.