“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 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 sup pose 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 Tom.

"By means of a condensing syringe," replied his father; "but I will take an opportunity of exhibiting the instrument in operation." The reader will be pleased to recollect that the major agreed to pay a passing visit to the vicarage! it now becomes our duty to record what happened upon that memorable occasion; and we, perhaps, cannot better represent the nature of the discussion that took place than by relating the account, as it was given by the belligerent parties themselves, in conversation with Mr Seymour.

"Well, gentlemen," said Mr Seymour, "is it peace or war? I trust you have amicably adjusted all your differences."

"Upon my word," answered the vicar, "I have just reason to complain of the major's unjustifiable scepticism upon points that are perfectly unquestionable."

"You continue then to smart under the major's stinging criticisms, 'majore sub hoste. There is a Latin pun for your consolation," said Mr Seymour.

"The vicar alludes, I suppose," said the major, “to the doubt I expressed respecting the authenticity of his leathern money?"

"That is one of the many subjects upon which, I must say, you have betrayed a deficiency in historical knowledge. Seneca informs us that there was anciently stamped money of leather; and the same thing was put in practice by Frederick II. at the siege of Milan; to say nothing of an old tradition amongst ourselves, that, in the confused times of the barons' wars, the same expedient was practised in England."

"You strangely mistake me," replied the major; "I never questioned the truth of these historical statements; I know full well that numerous substances have, at different times, and in different countries, been adopted in exchange, as conventional representatives of property. I have already stated that cattle were employed as the earliest measure of value.* We find, for instance, in Homer, that the golden armour of Glaucus was valued at a hundred, and that of Diomedes at ten oxen.+ Among the Indians, cowries, or small shells, are used; and the Abyssinians employed salt, bricks, and beads for this very purpose. The ancient Britons, according to Cæsar, circulated iron rings as money. The Hollanders, we know, coined great quantities of pasteboard in the year 1754; and Numa Pompilius certainly made money both of wood and leather."

"And yet you doubt the authenticity of my leathern money, which I am fully persuaded was coined in 1360, by John, king of France, who, having agreed to pay our Edward the Third the sum of 3,000,000 golden crowns for his ransom, was so reduced as to be compelled to a coinage of leather for the discharge of his household expenses."

"I have only questioned the authenticity of that particular specimen which I saw in your cabinet," replied the major; "and so must any person who views it through a medium unclouded by prejudice. I will stake my whole library to a horn-book that our friend Mr Seymour will agree with me in pronouncing it a fragment of the heel of an old *Hence Pecunia from Pecus, and Opes | We derive the word 'Calculation,' from quasi Oves. that rudimental period, when pebbles (calculi) were used, as now, amongst savages, to facilitate the practice of counting.

The term CAPITAL, in its original signification, expressed the rude enumeration of the stock by the HEADS of the animals of which it was composed.

shoe; let him observe the perforation, and say, if he can, that it has not been produced by a nail or peg. But really, my dear Mr Twaddleton, you have forced me, much against my inclination, into this controversy."

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"Very good, Sir! very good! the heel of an old shoe, forsooth! But I thank you, Major Snapwell,” exclaimed the vicar with some warmth; "I thank you, Sir. Your assertion, while it evinces your own want of historical information, establishes, beyond doubt, the authenticity of my treasure, and the triumph of my opinion.” Assuredly," said Mr Seymour, with a wicked smile. I dare say there may be numerous holes in this leathern coin; for many have been the antiquaries who have, doubtless, pinned their faith upon it." Psha, psha!" cried the vicar: "for once, at least, Mr Seymour, let me entreat you to be serious; the subject, Sir, is important, and merits your respect. It is from that very hole that I am enabled to identify the coin; yes, major, from that very hole, which you affect to despise, I am enabled to derive its principal claim to antiquity. Are we not expressly informed that the leathern money of John of France had a little nail of silver driven into it?"

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Well, then," continued the major, "what say you to that tell-tale stitch, which I so unfortunately picked out with my penknife?"

"Admirable ingenuity! most refined sophistry! provoking perversion!" exclaimed the vicar. "It is really amusing to observe the address with which the prejudiced observer distorts every fact to his own advantage. Why, bless me, Sir, that stitch is strong enough to drag fifty such opponents out of the slough of unbelief."

"Do explain yourself," said Mr Seymour.

"Explain myself! the stitch speaks for itself, Sir. Were not these leathern coins strung together in different numbers to facilitate payments? For you will admit that it would have been extremely inconvenient to have coined single pieces of leather of different denominations. But stop, Sir, stop; look at this, look at it, major, with care and attention. That," said the vicar, as he drew a small coin out of his waistcoat pocket, with an air of imperturbable gravity and selfsatisfaction, "is a current halfpenny, in lead, of James II.; and if your eyes are not hoodwinked by prejudice, you may probably perceive a piece of copper in its centre, which, we are told, was thus introduced for the purpose of rendering the currency lawful."

The dinner was announced before the conclusion of the discussion; and as the reader will probably agree with us in thinking that a question of such grave historical importance ought not to be decided without due care and deliberation, we shall afford the disputants a reasonable time for reflection, and put an end to the chapter.

WE should have supposed, after the instructive accident related in the foregoing chapter, and the excellent advice it had elicited from the vicar, that Tom Plank would have desisted from his experimental vagaries, and that the doctor, at all events, would never have consented to become the subject of them; but there are those whom experience can never teach, nor failure ever discourage.

The very next morning after the catastrophe recorded in our preceding chapter, as the vicar was taking his early walk along Forestlane, gathering primroses and wild flowers, his placid and contemplative frame of mind was abruptly disturbed by the sudden vision of a dragon, vomiting flames of fire from its jaws, with a troop of dogs barking and yelling at its heels. As it drew nearer, the well-known person of the village doctor presented itself to his perplexed vision, mounted on its back, and shrouded in vapours, which the imagination of the vicar suggested could be no other than the pestilential effluvia from the nostrils of the unknown and frightful monster: but there