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FAMILIAR LECTURES ON USEFUL SCIENCES.

ON PNEUMATICS.

DEFINITIONS.

evenly on the mouth of the glass, and the presPNEUMATICS is that branch of natural phi-the hand will be so great, that it will require sure of the atmosphere on the upper surface of

losophy, which treats of the weight, pressure, and spring of the air, and of the several effects connected with these properties.

Dilatation is an increase of volume without an

increase of matter: thus air is said to be dilated, when any portion of it is made to occupy more space than it did before.

When a quantity of air is reduced into a much smaller space than it filled before, it is said to be

[blocks in formation]

A valve is a kind of lid that opens one way, and closes the aperture more completely as the pressure upon it is greater; so that it either admits the entrance of a fluid, and prevents its return, or allows it to escape, and prevents its reentrance.

A tube is sealed hermetically, by melting the glass and consolidating it.

OF THE WEIGHT AND PRESSURE OF AIR.

some exertion to remove it from the glass. A quart of air weighs about 144 grains.

If the atmosphere were non-elastic, or of unithe base of the earth upwards, would not exceed form weight throughout, its whole height from 5 miles; but air is elastic, which gives it a density, or weight, proportioned to its compression, and causes the atmosphere to extend to an unlimited height. Some idea of the elasticity of air may be conceived by compressing a sponge, or piece of wool, which is no sooner set at lidistend in every direction till they have recoberty by the opening of the hand, than its parts vered their former bulk.

From various experiments it appears, that the spaces which air occupies when it is compressed by different weights, are reciprocally proportional to the weights themselves; for the more the air is compressed, the less space it takes up. Therefore as the pressure of the upper parts of the atmosphere upon the lower becomes less, according to the different heights, it must follow Atmospherical, or common air, is a thin, that the air in the higher part of the atmosphere transparent, and elastic fluid that surrounds the where the pressure is very inconsiderable, may earth to a considerable height, and revolves with be rarified to an almost unlimited extent. On it in its course round the sun. Air resembles the supposition that the atmosphere diminishes other fluids in its general properties, but it differs in weight exactly in proportion to the different from them in this, that it admits of being com-heights, it is calculated that at the height of pressed into any space, however small, and that it is incapable of being converted into a solid by cold. When the particles of air are acted upon by the voice, or any other moving power, they flow among, and over each other, in every direc-proportion. tion, and convey sound, &c. to distances proportional to the impulse they had received.

Air, like every other fluid, has weight, and presses in every direction. It compresses the animal body, and keeps the fibres from being forced out of their natural order; but as it presses equally on every part, we are insensible of its effects, except it be partially removed, as in the following experiment. Put a piece of burning paper into a wine-glass, the air contained in which will be displaced by the flame in a few seconds. Then place the fleshy part of the hand |

3 miles the atmosphere is about twice as rare as on the surface of the earth; that at seven miles it is four times as rare, at 14 miles 16 times as rare, at 21 miles 64 times as rare, and so on in

When wool, or any other elastic body is compressed, it resumes its former bulk when the pressure is removed; but air not only resumes its first bulk, but expands to any extent, diverg. ing in right lines, and in all directions as from a common centre. Hence soap-bubbles derive their spherical form, the air within them having an equal divergency from their centre.

The nature and properties of air have been clearly demonstrated by means of a machine called an air-pump. The construction of this pneumatical instrument is as follows:

H

L

V

P

B

S

with the machine, and shews in an accurate manner the changes which take place in the density, or weight of the air within the receiver.

The atmosphere is supposed to be about fifty miles high; but though this is merely conjectural, it is known with certainty that a column of air whose base is a square inch, and whose height that of the atmosphere, weighs fifteen pounds on the surface of the earth. Therefore estimating the surface of a man's body at 15 square feet, the pressure he continually supports is equal to 33,480 pounds, or upwards of 14 tons weight. The reason why we are insensible of this enormous pressure is, that there is within a quantity of air which counterbalances the pressure of the atmosphere upon our bodies. This re-action of internal against external air has been demonstrated by a variety of experiments, of which the following will suffice to establish the fact.

A and B are two brass barrels, or cylinders, within which are toothed rods, K and D, called pistons, which fall into a toothed wheel sunk in the block I V. P is the receiver, sometimes called the recipient. It stands on a brass plate that has a small hole in the middle, through which the air passes from the receiver into a closed channel made of brass, which communicates with the cylinders A and B. Near the bottom of each cylinder is a valve opening upwards; and above these valves are two others, which are moved up and down by the toothed rods. On turning the handle H, one of the pistons is raised and the other depressed, consequently a rarified space is formed between the upper and lower valve in one cylinder; and the air which is contained in the receiver rushes through the conducting pipe, and by its elasticity forces up the lower valve and enters the rarified part of the cylinder, when the valve closes, and prevents the return of the air into the receiver. When the motion is reversed, the other piston ascends, and that in the opposite cylinder is depressed; in its depression the elas-mispheres, which when put ticity of the air, contained between the two together form the sphere valves, forces open the uppermost valve, and A B, but may be separated at it escapes into the upper part of the cylinder; a touch. In the lower part then the valve closes again and prevents its re- D, there is a stop-cock which turn. The opposite piston performs the same communicates with a tube that operation, but the motions are alternate, so that screws into the plate of the air-pump, and by whilst one piston exhausts the air from the means of which the air may be withdrawn from receiver, the other is discharging it from the top the interior of the globe When this is effected, of the cylinder. Thus, by continued exhaustion, the stop-cock is shut to prevent the return of the the density of the air keeps decreasing in the air, and the counterbalancing force being removed receiver, till its elasticity is no longer able to from the interior of the globe, the pressure of the force up the lower valves, which terminates the atmosphere upon the surface will compress the effect of the machine. The air may be re-ad- two hemispheres so closely together, that they mitted into the recipient, by unscrewing a small cannot be separated without employing a consinut at S, and a barometer gauge Z, is connected derable force,

The machine, called the Magdeburg Hemisphere, consists of two hollow brass he

A

B

D

When the receiver is first placed upon the plate of the air pump, the pressure of the air contained within the receiver being equivalent to that which acts on the exterior part, it may, like our bodies, be moved with facility; but as the air is exhausted, the equilibrium is destroyed between the inner and outer surfaces, till the pressure of the atmosphere without fixes the receiver so firmly to the plate, that it requires a greater force than one man can exert to remove

it.

Place the hand upon the top of a small glass, called a hand glass, which is open at both ends, and stands on the plate of the air pump; then exhaust the air which it contains, and the fibres, or fleshy part of the hand, will distend, with a very painful sensation, which is occasioned by the want of atmospherical compression on that part of the hand which covers the mouth of the glass. In treating of the barometer, the pressure of air will be still farther illustrated.

ON HERALDRY.

IT has been justly observed "that in some cases, a book from its own nature cannot be rendered, without the utmost art, agreeable both to delicate tastes, and to correct judgments; and authors, in order to gain an extensive audience to their works, are sometimes seduced to seek after entertaining embellishments, more than is entirely consistent with that strict propriety which a just criticism demands in every literary production; but, after having bestowed infinite pains to prepare for the public useful compositions, instead of reputation, they do not rarely meet with mortification or disappointment." Such is the disadvantage of every treatise on Heraldry which is, or can be published.

A late author says, "of all the inventions of vanity, perhaps the most frivolous are armorial ensigns; but considered in a philosophical view, they afford to profound thinkers, who delight to examine the mechanism of the mind,' a curious example of that powerful principle termed association of ideas,' by which thought is continually led in its progress, and one image presents to the fancy another which has been discovered by experience, or which is supposed from habit to be connected with it. These badges, being intended to distinguish persons by whom martial achievements, or noble deeds had been performed, it is probable were originally personal, and were adorned, perhaps with designs or emblems ingeniously expressive of the actions which had procured them; but forming a part of the estates of the persons who had obtained them, and at their deaths passing to their heirs, they at last became relics, respected and precious, which, connecting their possessors with those on whom they had been bestowed, men would be proud to display; and they would be carefully preserved || on that account, as the fancies of those to whom they were shewn, as well as of those to whom they pertained, would be conveyed by them, with quickness and vivacity, to the persons who had

achieved them. Arms came thereby to be converted into marks of descent; and obscure hieroglyphics were gradually substituted for solid instruments adorned for shew, but made for use. Hence ranks and degrees were distinguished by fanciful conceits."

It will, however, be necessary to remark, what the learned and ingenious Mr. Boyer advanced concerning the usefulness of arms.

He says, "notwithstanding the great abuses that have crept into the use of arms, one cannot yet deny their usefulness, on considering, that as they are hereditary marks of honour and nobility, they are, or at least ought to be, a spur to excite those that bear them, to tread in the footsteps of their glorious ancestors who have acquired them by their virtue and noble achievements."

Secondly, that princes, when they rewarded those who signalized themselves in their service, did often annex great estates to the marks of honour wherewith they made them illustrious; and in order to perpetuate the grandeur of families, those honours and estates have generally been entailed on the eldest males of noble houses. Thus plain coats of arms do, of right, belong to the heads of families, and are thereby become, not only marks of honour, but also good titles, both for the enjoyment, and even for the recovery of certain estates, of which a house may have been dispossessed, either by intestine or foreign wars, or by other public calamities.

Again, it is to be observed, that arms being hereditary, the right that a man has to bear them is imprescriptible and almost inadmissible, since it is never lost, or forfeited, but by crimes which entirely degrade one from nobility, such as high treason in England, and what the French call La Majesté. In France, a family may, by divers accidents be reduced to the utmost indigence and poverty, and even in some countries, derogate from nobility, and become plebeian, by exercising mechanical arts: but nevertheless, it still

preserves its arms; and if, in process of time, it happens to rise again, they prove a good title towards its being restored to its ancient honour, and recovering its pristine lustre. Now, if by his personal merit and abilities, a man sees himself raised to an eminent post, either at court, in the army, or in the law, or, if by his labour, or industry, he raises a considerable fortune, it is natural for him to look back on his ancestors, and to cast about for a noble descent: and if by rummaging into old musty records, and ransacking or turning over the registers of honour, he is so happy as to find out a coat of arms, belonging to his family, what a pleasure and satisfaction it is to him, to be able to repay what he owes to his progenitors, and to add a fresh lustre to the glory he derives from them! In such a case the most illustrious families court his alliance; whereas when either by the smiles of blind fortune, or through the caprice of princes, sometimes blinder than fortune

itself, a man stars up on a sudden from obscurity into an eminem station; although he be en lowed with a superior genius, and extraordinary abilitics, he nevertheless can hardly escape the malicious slurs of envy and detraction, ever ready to reflect on his low extraction.

Lastly, arms and armoury serve to bring us acquainted with great and illustrious families; and therefore the study of Heraldry is absolutely necessary for all the princes of Europe, in order to know their alliances, pretensions, and interests; and for the same reason, is extremely useful to their ministers, for the management of impor ant affairs."

In these lights, therefore, we tru-t it will not be amiss to present our readers with a concise system of Heraldry.

[To be continued.]

H.

EATING.

CULINARY RESEARCHES.

DINNER is to the epicure the most interesting action of the day, the one in which he acquits himself with the greatest eagerness, pleasure, and appetite. Few therefore, excepting invalids, do not attach to this meal all the importance it deserves. A coquette would rather renounce the pleasure of being admired, a poct that of being praised, a Gascon believed on his word, an actor applauded, and a rich Midas flattered, than the seven-eighths of a great town would give up a good repast. We have often been surprised that no author has hitherto treated this subject with the importance it merits, and have not written a philosophical essay on dining. How many things may be said on this memorable deed which is renewed 865 times during the year?

If by some unforeseen event, or uncommon circumstance, the dinner be retarded only for half an hour, how the physiognomy of each guest lengthens, how the most animated conversation becomes languid, the visage darkens, the muscles are paralized; in short, how every eye is mechanically turned towards the dining rooms! Des the obstacle cease, does the butler announce that dinner is served, this little word produces the effect of a talisman; it contains a magic influ.nce which restores to each person his wonted serenity, liveliness, and wit. A good appetite is expressed in every eye, hilarity reigns in every heart, and the impatience with which each takes possession of his plate, is a manifest

and certain sign of the unanimity of wishes and the unity of sentiments; nature now assumes her rights, and even the flatterer allows his thoughts to be read in his countenance.

To shorten the ceremony usually attendant on sitting down, it would be a good plan to cause the name of each guest to be fixed to the plate destined for him. Every one seated, an universal silence prevails, which attests the strength and unanimity of sensations.

NEW AND EASY METHOD OF ACQUIRING AP

PETITE.

It is particularly necessary that merchants, and all men of business, should digest well; their fortune endows them with the means of keeping a good table, and to put in practice every advice they may receive; but their stomachs sometimes refuse their office. The mind must be perfectly free from care and inquietude for the inside to keep its digestive powers well in action, and it is very difficult with the foregoing professions to enjoy those advantages. We agree that diet may be called to the assistance of intemperance, but regimen, privation, and regrets, are melancholy resources to a glutton; and he then often envies the ostrich, towards whom Providence, in endowing her with the faculty of digesting iron, has shewn her a preference that more than one human stomach would wish to have been the object of.

If to repair sooner the strength he has abused, our glutton has recourse to rhubarb, treacle, dias

cordium, and all the tonical digestives which pharmacy offers, he will be but the more to be pitted, as he must soften the effect of the drugafter being cured of his complaint, and this cure is often more tedious and difficult than the other. Placed between diet and his apothecary, the glutton finds himself in quite a contrary situation from that of Buridan's ass. To get rid of it, he will again take the road of indigestion, and he again falls into debility for having depended too much on his strength.

Wisdom advises him to be temperate, to avoid excesses, and to consult his appetite rather than his sensuality; this is doubtless a very good

counsel, and readily hearkened to in sickness, but disdained in health. It is thus that the ma riner, timid and devout in the midst of a storm, braves new dangers as soon as the sky reassumes its serenity. When the winds are favourable he believes no more in hell, than a glutton does in medicine as long as he can digest.

But this is precisely the difficult point; and it ́ is to teach these gentlemen how, without any inconvenience, to give free scope to their appetites, that we allow this article a place in our sheets.

[To be continued.]

LETTERS ON BOTANY,

FROM A YOUNG LADY TO HER FRIEND.
[Continued from Vol. I. Page 538.]

LETTER XVIII.

MY DEAR EUGENIA.

Let us now endeavour to describe a noble beauty. Its name is Queen of the Meadows, or, common meadow-sweet, spirea almaria. The sweetest perfume is exhaled from its bosom. She alone does the honours of the meadows.

The stem rises to almost three feet. I believe it to be ligneous. It is straight, and has a profusion of branches around it. The palace of this Queen is a whole empire.

oak, which it resembles as well in the shape as in the manner it is notched. The under part is almost white, and so transparent that it has the appearance of being lined with Italian gauze.

The flowers are placed at the summit of each branch; the branch springs up to support them, and seems then to distend the leaves already shrunk into littleness.

The spirea forms an irregular corymb, loaded with an immense quantity of white flowers, whose crowded aggregation produces a very handsome bunch; she gives rather the idea of a flourishing

Its stem has five tints, irregularly dyed with a republic than that of a monarchy; and if the fiery red, or a pale green. spirea is queen of the meadows, she is (as Rome was) of the world.

||

Its leaves, more numerous, large, and open at the base, are carried on ligneous branches, and I have said that the flowers of the spirea form posed in the same way as those of the rose tree; a corymb, I do not know whether I am right; but the foliols have here no particular pe- the stem is separated into peduncles of unequal tioles, they are sessiles; and between the large height, which are also themselves divided; inleaves you see on the same branch, the appear-numerable flowers cover and bend them down. ance of numerous little leaves, which drawing It is not without some difficulty that I reck up the abundant juice that nourishes the plant,oned twenty stamina on each of these pretty cause its leaves and branches to thicken and grow stronger. These species of leaves are generally called compound leaves. Those of this plant are those which are called pinnated.

The end of the stem from which the leaf escapes, is almost entirely surrounded by another round leaf, resembling a frill, and notched like the others. The leaf that terminates the branch is not entirely separated from the two divisions, but on the contrary completes them. They are so very deep, that at the first glance one would imagine there were three leaves, and we are surprised in discovering them to be but one.

The leaf is of a dark green, like that of the

flowers. Their filaments are white, extremely delicate, straight, and each surmounted by a little yellow anther, about the size of the point of a pin. This forest of stamina, is not very perceptible at the first view, as you only distinguish on the large bunch of this plant a light transparent yellow, whose delicate flexibility adds to the elegant lightness of this charming flower.

The five little pistils with white heads, are more easily distinguished, whose ovaries are green; like Sultanas in a seraglio, they are guarded on all sides.

These Rowers 'n miniature have cach their

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