ページの画像
PDF

FAMILIAR LECTURES ON USEFUL SCIENCES.

ON PNEUMATICS.

DEFINITIONS.

levenly on the mouth of the glass, and the pres

sure of the at:nosphere on the upper surface of I'NEUMATICs is that branch of natural phi-||

the hand will be so great, that it will require losoply, which treats of the weight, pressure,

some exertion to remove it from the glass. A and spring of the air, and of the several effects

quart of air weighs about 144 grains. connected with these properties.

If the atmosphere were non-elastic, or of uniDilatalion is an increase of volume without an

form weight throughout, its whole height from increase of matter: thus air is said to be dilated,

the base of the earth upwards, would not exceed when any portion of it is made to occupy more

| 54 miles; but air is elastic, which gives it a denspace than it did be.ore.

sity, or weight, proportioned to its compression, When a quantity of air is reduced into a much

and causes the atmosphere to extend to an unsmaller space than it filled before, it is said to be

limited height. Some idea of the elasticity of condensed,

air may be conceived by compressing a sponge, A racuuin is a space from which all the air has

or piece of wool, which is no sooner set at libeen taken away.

berty by the opening of the hand, than its parts A ralre is a kind of lid that opens one way,

distend in every direction till they have recoand closes the aperture more completely as the

vered their former bulk. pressure upon it is greater; so that it either ad

From various experiments it appears, that the mits the entrance of a fluid, and prevents its re

spaces which air occupies when it is compressed tur, or allows it to escape, and prevents its re

by different weights, are reciprocally proportional entrance.

to the weights themselves; for the more the air A tube is sealed hermetically, by melting the

is compressed, the less space it takes up. Thereglass and consclidating it.

fore as the pressure of the upper parts of the OF THE WEIGHT AND PRESSURE OF AIR.

atmosphere upon the lower becomes less, ac

cording 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 srace, however small, and that || 35 miles the atmosphere is about twice as rare it is incapable of being converted into a solid by ll as on the surface of the earth; that at seven miles cold. When the particles of air are acted upon it is four times as rare, at 14 miles 16 times as by the voice, or any other moving power, they || rare, at 21 miles 64 times as rare, and so on in flow among, and over each other, in every direc- l proportion. tion, and convey sound, &c. to distances propor || When wool, or any other elastic body is comtional to the impulse they had received.

pressed, it resumes its former bulk when the Air, like every other fluid, has weight, and pressure is removed; but air not only resun e; presses in every direction. It compresses the its first bulk, but expands to any extent, diverg. animal bodly, and keeps the fibres from being ing in right lines, and in all directions as from forced out of their natural order; but as it presses a common centre. Hence soap-bubbles derive equally on every part, we are insensible of its their spherical form, the air within them having effects, except it be partially removed, as in the an equal divergcacy from their centre. following experiment. Put a piece of burning 1 The nature and properties of air have been paper into a wine-glass, the air contained in clearly demonstrated by means of a machine which will be displaced by the fame in a few called an air-pump. The construction of this seconds. Then place the fiely part of the hand pneumatical instrument is as follows:

A and B are two brass barrels, or cylinders, ll with the machine, and shews in an accurate within which are toothed rods, K and D, called manner the changes which take place in the pistons, which fall into a toothed wheel sunk l density, or weight of the air within the receiver. in the block I V. P is the receiver, sometimes The atmosphere is supposed to be about filty called the recipient. It stands on a brass plate miles ligh; but though this is merely conjecthat has a small hole in the middle, through tural, it is known with certainty that a column which the air passes from the receiver into a ll of air whose base is a square inch, and whos closed channel made of brass, which communi-height that of the atmosphere, weighs fiteen cates with the cylinders A and B. Near the pounds on the surface of the earth. Therefore bottom of each cylinder is a valve opening up estimating the surface of a man's body at 15 wards; and above these valves are two others, square feet, the pressure he continually supports which are moved up and down by the toothed is equal to 33,480 pounds, or upwards of 14 tons rods. On turning the handle H, one of the Il weight. The reason why we are insensible of pistons is raised and the other depressed, conse

I the other depressed, conse- || this enormous pressure is, that there is within a quently a rarified space is formed between the quantity of air which counterbalances the pres. upper and lower valve in one cylinder; and the sure of the atmosphere upon our bodies. This air which is contained in the receiver rushes ll re-action of internal against external air has been through the conducting pipe, and by its elas- demonstrated by a variety of experiments, of ticity forces up the lower valve and enters the which the following will suffice to establish Tarified part of the cylinder, when the valve | the fact. closes, and prevents the return of the air into the machine, called the the receiver. When the motion is reversed, the Magdeburg Hemisphere, conother piston ascends, and that in the opposite sists of two hollow brass hecylinder 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 communicateswith 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 ll 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,

[ocr errors]

When the receiver is first placed upon the Place the hand upon the top of a small glass, plate of the air pump, the pressure of the air called a hand glass, which is open at both ends, contained within the receiver being equivalent to and stands on the plate of the air pump; then that which acts on the exterior part, it may, like exhaust the air which it contains, and the fibres, our bodies, be moved with facility; but as the or fleshy part of the hand, will distend, with a air is exhausted, the equilibrium is destroyed very painful sensation, which is occasioned by between the inner and outer surfaces, till the the want of atmospherical compression on that pressure of the atmosphere without fixes the part of the hand which covers the mouth of the receiver so firmly to the plate, that it requires a glass. In treating of the barometer, the presgreater force than one man can exert to remove sure of air will be still farther illustrated.

ON HERALDRY.

It has been justly observed “ that in some || achieved them. Arms came thereby to be concises, a book from its own nature cannot be ren. 1 verted into marks of descent; and obscure hierodered, without the utmost art, agreeable both to || glyphics were gradually substituted for solid delicate tastes, and to correct judgments; and instruments adorned for shew, but inade for use. authors, in order to gain an extensive audience to

IIence ranks and degrees were distinguished by their works, are sometimes seduced to seek after fanciful conceits.” entertaining embellishments, more than is entirely It will, however, be necessary to remark, what consistent with that strict propriety which a just the learned and ingenious Mr. Boyer advanced criticism demands in every literary production ; || concerning the usefulneis of arms. but, after having bestowed infinite pains to pre He says, “notwithstanding the great abuses pare for the public useful compositions, instead that have crept into the use of arms, one cannot of reputation, they do not rarely meet with morti- ll yet deny their usefulness, on considering, that fication or disappointment.” Such is the dis- \| as they are hereditary marks of honour and nobis advantage of every treatise on Heraldry which is, || lity, they are, or at least ought to be, a spur to or can be published.

excite those that bear them, to tread in the foot. A late author says, “ of all the inventions of steps of their glorious ancestors who have acquired vanity, perhaps the most frivolous are armorial en them by their virtue and noble achievements.” signs; but considered in a philosophical view, they Secondly, that princes, when they rewarded afford to profound thinkers, who delight to ex those who signalized themselves in their serice, amine the mechanism of the mind,' a curious did often annex great estates to the marks of example of that powerful principle termed “as- | honour wherewith they made them illustrious ; sxiation of ideas,' by which thought is conti- û and in order to perpetuate the grandeur of faminually led in its progress, and one image presents lies, those honours and estates have generally been to the fancy ano:her which has been discovered entailed on the eldest males of noble houses, by experience, or which is supposed from habit to Thus plain coats of arms do, of right, belong to be connected with it. These badges, being in the heads of families, and are thereby become, tended to distinguish persons by whom martial a not only marks of honour, but also good titles, chievements, or noble deeds had been performed, both for the enjoyment, and even for the recovery it is probable were originally personal, and were of certain estates, of which a house may have adorned, perhaps with designs or emblems ingeni. been dispossessed, either 'by intestine or foreign ously expressive of the actions which had pro wars, or by other public calamities. cured them; but forming a part of the estates || Again, it is to be observed, that arıns being of the persons who had obtained them, and at || hereditary, the right that a man has to bear them their deaths passing to their heirs, they at last || is imprescriptible and almost inadmissible, since became relics, respected and precious, which, it is never lost, or forfeited, but by crimes which connecting their possessors with those on whom entirely degrade one from nubility, such as higı they had been bestowed, men would be proud to 1 treason in England, and what the French call La display; and they would be carefully preserved Majesté. In France, a family may, by divers on that account, as the fancies of those to whom accidents be reduced to the utmost indigence and they were shewn, as well as of those to whom poverty, and even in some countries, derogate they pertained, would be conveyed by them, with | from nobility, and become plebeian, by exercisquickness and vivacity, to the persons who had ling mechanical arts : but neveriheless, it still presei ves its arms; and is, in process of time, it itself, a man star. up on a sedlen from ob-curiiy happens to rise again, they prove a god title into an eninen station ; although he be en lowed towards its being restored to its ancicut licnour, with a superiorganius, and extraordinary abilities, and recovering its pricine lusore. Now, if by his he nevet!ieless can hardly escape the malicious personal merit and abilities, a man sees himself, slurs of envy and detraction, ever ready to reflect raised to an eininent post, either at count, in the on his low extraction. army, or in the law, or, if by his labour, or indus- Lastly, arms and armcury serve to bring us try, he raises a considerable fortune, it is natural acquainted with great and illustrious families; for him to look back on his ancestors, and to castill and therefore the study of Heralury is absolutely about for a noble descent: and if by rummaging necessary for all the princes of Europe, in order to into old musty records, and ransacking or tuning, know their alliances, pretensions, and interess; over the registers of honour, he is so happy as to and for the same reason, is extremely useful to find out a coat of arms, belonging to his family, their eministers, for the management of imporant what a pleasure and satisfaction it is to him, tol affairs." be able to repay what he owes to his progenitors, ll ln these lights, therefore, we tru-t it will not be and to add a freslı lusire to the glory he derives amiss to present our readers with a concisesystem from them! In such a case the most illustrious of Heraldry. families court his alliance; whereas when either

H. by the smiles of blind fortune, or through the

[To be continucd.] caprice of princes, sometimes blinder than fortune |

CULINARY RESEARCHES.

PETITE.

EATING.

}) and certain sign of the unanimity of wishes and Dinner is to the epicure the most interest

the unity of sentiments; nature now assumes ing action of the day, the one in which he

her rights, and even the flatterer allows his acquits himself with the greatest eagerness, plea

thoughts to be read in his countenance. sure, and appetite. Few therefore, excepting

To shorten the ceremony usually attendant on invalids, do not attach to this meal all the in- sitting down, it would be a good plan to cause portance it deserves. A coquette would rather the name of each guest to be fixed to the plate renounce the pleasure of being admired, a poct

destined for him. Every one seated, an universal that of being praised, a Gascon believed on his

silence prevails, which aitests the strength and word, an actor applauded, and a rich Midas unanimity of sensations. flattered, than the seven-eighths of a great town would give up a good repast. We have often

NEW AND EASY METHOD OF ACQUIRING APbeen surprised that no author has hitherto treated this subject with the importance it merits, and l It is particulirly necessary that merchants, and have not written a philosophical essay on dining. all men of business, should digest well; their How many things may be sail on this memorable || fortune endows them with the means of keeping deed which is renewed 965 times during the || a good table, and to put in practice every advice year?

they may receive; but their stornachs someIf by some unforeseen cvent, or uncommon limes refuse their office. The mind must be circunstance, the dinner be retarded only for perfectly free from care and inquietude for the half an hour, how the physiognomy of each guest inside to keep its digestive powers well in action, lengthsns, how the most animated conversation and it is very difficult with the foregoing probecomes languid, the visage darken, the muscles fessions to enjoy those advantages. We agree are pralized ; in short, how every eye is me- hat diet may be called to the assistance of inchanically turned towards the dining rooms! | temperance, but regimen, privation, and regrets, Dies the obstacle cease, does the buller announce are mclancholy resources to a glution; and he that dinner is served, this little word produces then often envies the ostrich, towards whom the effect of a talisman; it contains a magic in- Providence, in endowing her with the faculty of f. u. nice which restore3 10 cach person his digesting iron, has shewn her a preference that wonted serenity, liveliness, and wit. A good more than one human stomach would wish to appetite is expressed in every eye, hilarity reigns have been the object of. in every heart, and the impitience with which If to repair sooner the strength he has abused, cach takes possession of his plate, is a manifest llour glution has recourse to rhubarb, treacle, diasa Cardiurn, and all the tonical digestives which counsel, and readily hearkened to in sickness, pharmacy offers, he will be but the more to be but disdained in health. It is thus that the mapitied, as he must soften the effect of the drug riner, timid and devout in the midst of a storm, after being cured of his complaint, and this cereli braves new dangers as soon as the sky reassuines is often more ielious and dishcult than the other. ll its serenity. When the winds are favourable he

Placed between diet and his apothecary, the believes no more in hell than a glutton does in gluton finds himself in quite a contrary situation medicine as long as he can digest. from that of Buridan's ass. To get rid of it, he But this is preciscly the difficult point; and it will again take the road of indigestion, and he is to teach these gentlemen how, without any again falls into debility for having depended 100 || inconvenience, to give free scope to their apmuch on his strength.

Il petites, that we allow this article a place in our Wisdom advises him to be temperate, to avoil || sheets. exceses, and to consult his appetite rather than ||

[Tu be continued.] his sensuality; this is doubtless a very good ||

LETTERS ON BOTANY,
FROM A YOUNG LADY TO HER FRIEND,

[Continued from Vul. I. Page 538.]

LETTER XVIII.

oak, which it resembles as well in the shape as

in the manner it is notched. The under part is MY DEAR EUGENIA.

almost white, and so transparent that it has the Let us now endeavour to describe a noble appearance of being lined with lialian gauze. beauty. Its name is Queen of the Meadows, or, || The flowers are placed at the summit of each cominon meadow-sweet, spiraa almaria. The branch; the branch springs up to support thiem, swectest perfume is exhaled from its bosom. She and seems then to distend the leaves already alone does the honours of the meadows. shrunk into littleness.

The stem rises to almost three feet. I believe The spiræa forms an irregular corymb, loaded it to be ligne015. It is straight, and has a pro- with an imniense quantity of white Howers, whose fusion of branches around it. The palace of crowded aggregation produces a very handsome this Queen is a whole empire.

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.

spiræa is queen of the meadows, she is (as Roine Its leaves, more numerous, large, and open at was) of the world. the base, are carried on ligneous branches, and I have said that the lowers of the spiræa form posed in the saine way as those of the rose tree; 1 2 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 alsa themselves divided; inleives you see on the same branch, the appear-nuinerable flowers cover and bend them down. ance of numerous litile leares, which drawing | It is not without some difficulty that I reckup the abundant juice that nouishes the plant, oned twenty stamina on each of these pretty cause its leaves and branches to thicken and flowers. Their filaments are white, extienely grow stronger. These species of leaves are ge- ll delicate, straight, and each surmounted by a little nerally called compound leaves. Those of this I yellow anther, about the size of the point of plant are those which are called pinnated. a pin. This furcst of stamina, is not very

Tne end of the stem from which the leaf perceptible at the first view, as you only disescapes, is almost entirely surrounded by another tinguish on the large bunch of this plant a light runt leaf, resembling a frill, and notched like transparent yellow, whose delicate flexibility the others. The leaf that terininates the branch adds to the elegant lightness of this charming is not entirely separated from the two divisions, | power. but on the contrary completes them. They are! The five little pistils with white heads, are so very deep, that at the first glance one woultnicre casily distinguined, whose ovaries are imagine there were three leaves, and we are sur l green; like Sultanas in a seraglio, they are prised in discovering then to be but one, ll guarded on a'l siles,

The leaf is of a dark green, like that of thell These lowers in miniature have cach heir

« 前へ次へ »