Oldalképek
PDF
ePub

my taking the liberty of fending you a male and a female fly, alfo one of the animals in its caterpillar, and one which is in its chryfalis ftate, for your infpection, hoping that the public may be come acquainted with the means of preventing in future fo great a calamity.

Left the flies may become diffigured in travelling, it may be prudent to fay, that their wings are four; that their antennæ are clubbed, and about one-third of the length of their body, each being compofed of nine joints, namely, two next the head, above which two there is a joint fome. what longer than the reft, and above this fix more joints, fimilar to the two below; that near the point of the tail of the female there is a black fpeck, outwardly fringed with hair; but which, opening longitudinally, appears to be the end of a cafe, containing a delicate point or fting (about one-twentieth of an inch in length) which on a curfory view appears to be a fimple lanceolated inftrument, with a ftrong line paffing down the middle, and ferrated at its edges; but, on a clofer infpection, and by agitating it ftrongly with the point of a needle, it feparates into three one edged inftruments, hanger-like as to their general form, with a fpiral line fpiral line

of

wrinkle winding from the point to the bafe, making ten or twelve revolutions, which line, paffing over their edges, gives them fome appearance of being ferrated.

By the help of thefe inftruments, I apprehend, the female depofits her eggs in the edge of the turnipleaf (or fometimes, perhaps, in

the nerves or ribs on the under furface of the leaf); thus far I can fay, and I think with a confiderable degree of certainty, that having put fome fresh turnip-leaves into a glafs containing feveral of the male and female flies, I perceived (by the means of a fimple magnifier) that one of the females, after examining attentively the edge of the leaf, and finding a part which appeared to me to have been bitten, unfheathed her inftruments, infinuated them into the edge of the leaf, and having forced them afunder fo as to open a pipe or channel between them, placed her pubes (the fitua tion of which from repeated and almoft inceffant copulations I had been able to afcertain precifely, and to the lower part of which these inftruments feem to be fixed) to the orifice, and having remained a few feconds in that pof ture, deliberately drew out the inftruments (which the transparency of the leaf held against a ftrong light afforded me an opportunity of feeing very plainly) and proceeded to fearch for another convenient place for her purpose.

The caterpillar has twenty feet (fix of its legs being of confi derable length, the other fourteen very fhort) and in its first stage is of a jetty black, fmooth as to a privation of hair, but covered with innumerable wrinkles. Hav ing acquired its full fize, it fixes its hinder parts firmly to the leaf of a turnip, or any other fubftance, and breaking its outer coat or flough near the head, crawls out, leaving the fkin fixed to the leaf, &c. The under coat, which it now appears in, is of a blueish or lead colour, and the

caterpillar

caterpillar is evidently diminished in its fize. In every refpect it is the fame animal as before, and continues to feed on the turnips for fome days longer: it then entirely leaves off eating, and be comes covered with a dewy moifture, which feems to exfude from it in great abundance, and appearing to be of a glutinous nature, retains any loofe or pliant fubftance which happens to come in contact with it, and by this means alone feems to form its chryfalis coat. One I find laid up in the fold of a withered turnip leaf (that which I have the honour of inclofing you) was, among fix others, formed by putting common garden mould to them while they were in the exfudatory ftate above defcribed.

From the generic characters of the fly I conclude it to be a Tenthredo of Hill; but whether that voluminous author be fufficiently accurate; or whether, from being an almost entire ftranger to natural hiftory, I may, or may not, fufficiently understand my book, I muft beg leave to fubmit to your fuperior knowledge of the fubject.

I am endeavouring to extend my obfervations on thefe infects, and am making fome experiments concerning them, the refult of which I fhould be extremely happy in being permitted to communicate to you; and it may be proper to add here, that I fhould not have taken the liberty of troubling you prematurely with this letter, had I not luckily met with an opportunity of procuring fome live flies (which are now become very fcarce; and I flatter myfelf they VOL. XXVI,

[blocks in formation]

WE

opportunity of this recent publication, to lay before our readers a brief, hiftorical account of the very interefting difcovery which has of late attracted the notice of the whole philofophical world; and which our fanguine neighbours did not fcruple, at the very firft, to dignify with the name of Aerial Navigation.

Although the author of this book be known to have warmly efpoufed the party of Montgolfier, in oppofition to that of Charles (for there are parties even con cerning balloons), yet his reputation, as a man of learning and veracity, is fufficiently established, and the facts he here alledges are in general, as we have had opportunities to afcertain by collateral evidence, flated with fufficient accuracy to juftify us in taking him for our guide in this narrative.

The Preface contains a fhort

furvey of what projects have formerly been fuggefted for the purpofe of floating heavy bodies in the atmosphere; the principal of which are thofe of Lana, a Jefuit of Brefcia, and of Galien, a Dominican of Avignon; both which however were, upon well eftablifhed principles, found by theory

F

[ocr errors]

to be impoffible in the execution*. Due honour is paid to Mr. Cavallo of London, who, in 1782, feemingly with a view to this difcovery, tried to fill bags of paper and bladders with inflammable air; but failed in his attempts, by the unexpected permeability of paper to inflammable air, and the 100 great proportional weight of the common fized bladders. Had he then thought of employing gummed filk, or gold-beater's fkin, he probably would have plucked the very laurels that now adorn the brows of Montgolfier and Charles.

I. The honour of the difcovery is certainly due to the brothers Stephen and Jofeph Montgolfier, proprietors of a confiderable paper manufacture at Annonay, a town in the Vivarais, about thirty-fix miles fouth of Lyons: and their invention is the more to be admired, as it is not the effect of the late discovery of a permanent elaftic fluid lighter than the common air, but of properties of matter long known, and in the hands of the many acute philofophers of this and of the laft century. They conceived that the effect they looked for might be obtained by confining vapours lighter than common air, in an inverted bag, or covering, fufficiently compact to prevent their evaporation, and fo light, that when inflated, its own weight, added to that of the inclofed vapour, might fall fome

what short of the weight of the air which its bulk difplaces.

On these principles, they prepared matters for an experiment, They formed a bag, or balloon, of linen cloth, lined with paper, nearly fpherical, and measuring about 35 feet in diameter t; its folid contents were about 22,000 cubic feet, a space nearly equal to that occupied by 1980lb. of common air, of a mean temperature, on the level of the fea. The vapour, which, by conjecture, was about half as light as common air, weighed 990lb. The balloon, to. gether with a wooden frame fufpended to the bottom, which was to ferve as ballaft, weighed 490lb. whence it appears that the whole must have been about 500 lb. lighter than an equal bulk of common air. This difference of fpecific gravity, by which these bodies are made to rife, we shall henceforth, without warranting the propriety of the expreffion, call their power of afcenfion.

The 5th of June, 1783, was fixed on for the difplay of this fingular experiment. The ftates of Vivarais, who were then af fembled at Annonay, were invited to the exhibition. The flaccid bag was fufpended on a pole 35 feet high; ftraw and chopped wool were burnt under the opening at the bottom; the vapour, or rather smoke, foon inflated the bag, fo as to diftend it in all its parts; and, on a fudden, this immenfe

The impoffibility of Lana's project was demonstrated by Hook; see his Philofophical Collections, No. I. p. 28. And fince by Leibnitz. Galien's never needed any confutation.

All the measures here given are French. The French foot is to the English at 144 to 135; a French toife is fix French feet, or, fix and three, eighths English feet.

mafs

mafs afcended in the air with fuch velocity, that in lefs than ten minutes it appeared to be about 1000 toifes above the heads of the fpectators. A breeze carried it about 1200 toifes from the fpot whence it departed; and then the vapour, either efcaping through fome loop-holes that had been accidentally left in the conftruction, or being condenfed by the coldnefs of the circumambient air, the globe defcended gradually on vineyard, with fo little preffure, that none of the ftakes were broken, and scarce any of the branches of the vines bent.

a

II. The rumour of this fuccefsful experiment foon reached the me. tropolis, and rouzed the emulation of the Parifian philofophers. Without waiting for particular inftructions from the inventors, they reflected on a method of their own; and refolved, inftead of vapour, to ufe inflammable air; the fpecific weight of which, when pure, they knew to be that of common air nearly as ten to one *.

The procefs of producing this air being very expenfive, the author of the book now before us, fet on foot a fubfcription; and having foon raifed a fufficient fum, M. Charles, profeffor of experimental philofophy, and M. Robert, a methematical inftrument maker, were fet to work: and they conftructed a globe of luteftring (taffetas), glazed over with elaftic gum diffolved in fome kind of fpirit or effential oil. After many difficulties and difap

pointments, which will ever attend firft effays, they fucceeded, in two days, to fill this globe with inflammable air, produced from 1000lb. of iron-filings and 498 lb. of vitriolic acid, diluted in four times its quantity of water. This globe measured 12 feet two inches in diameter, its folid contents were 943 feet fix, lines cubic, and its power of afcenfion was found equal to 35 lb.

The 27th of Auguft 1783, have ing been fixed on for the exhibition of this experiment, the balloon was conveyed, in the preceding night, floating in the air, from a court near the Place des Victoires, where it had been conftructed, to the Champ de Mars. Our author indulges his lively imagination in a lofty defcription of this nocturnal proceffion, which, he fays, moved along in the dead of night, attended by a party of guards, with lighted torches, and feemed fo awful, that the hackney coachmen who happened to be in its way, defcended from their feats, and devoutly proftrated themselves before the fupernatural being that advanced in fuch folemn ftate.

At

The concourfe of people, on foot and in carriages, were fo immenfe in the Champ de Mars, that a large body of troops were drawn out to prevent difturbances. five o'clock in the afternoon, a fignal having been given by the firing of a mortar, the cords that confined the globe were cut, and it rofe, in less than two minutes, to a height of near 500 toifes. It

* In justice to our country, we must here at least commemorate the name of Cavendish; to whom, it is acknowledged on all hands, the discovery of the fpecific gravity of inflammable air, as well as of many other of its properties, is folely due. See Phil. Tranf, Vol. lvi. p. 150.

F 2

there

there entered a cloud, but foon appeared again, afcending to a much greater height; and at last it was loft among other clouds.

Our author juftly cenfures the conduct of this experiment; obferving, that too much inflammable air, and that even fome common air had been introduced into the globe, which being clofed on all fides, left no room for the expanfion of this of this elaftic fluid when it fhould arrive to a more rarefied medium. We find, in fact, that it must have burft in confequence of this expanfion; fince, after having floated about three quarters of an hour, it fell in a field near Goneffe, a village about five leagues (15 miles) N. N. W. of the Champ de Mars. It must be allowed, that the mere evaporation of the air could not well have been the cause of its defcending fo foon. Many periodical papers have already entertained the public with ludicrous accounts of the aftonishment of the peasants who found it, and of the rough treatment it received at their hands.

III. It may easily be imagined, that thefe brilliant fucceffes animated the zeal of all the curious in the metropolis; and that many effays were made to repeat the fame experiments upon a smaller fcale. Our author, accordingly, in a third chapter, mentions a number of thefe fecondary atwhich tempts; upon we fhall dwell no longer than only to obferve, that they fucceeded with globes made of gold-beater's fkin, and only 12 inches in diameter, which being thought the leaft that could be made to afcend, confi

dering that the proportionate

weight of the materials increase as the bulk is diminished, were called minimums.

IV. M. Montgolfier junior, having arrived at Paris a few days before the experiment at the Champ de Mars, was defired by the Royal Academy of Sciences to repeat the experiment of Annonay. He accordingly conftructed, in a garden, in the Fauxbourg St. Germain, a balloon of an elliptical form, 70 feet high, and 40 feet in diameter. It was lined, both infide and outfide, with paper. Its power of afcenfion was found, upon calculation, to be about 1250 lb. It was filled in ten minutes by the burning of 50lb. of ftraw and 10lb. of chopped wool.

It was loaded with a weight of 500lb. and afcended, fastened to ropes, on the 12th of September, in the prefence of the deputies of the Royal Academy. But it proving a very rainy day, the whole apparatus was fo effentially damaged, that it was not thought proper to fet it loose.

V. We come now. to the experiment made on the 10th of September, in the prefence of the king and queen, the court, and all the Parifians who could procure a conveyance Verfailles. This balloon was 57 feet high and 41 in diameter. Its power

to

of afcen

fion, allowing for a wicker cage, containing a fheep, a cock, and a duck, which was fufpended to it, was equal to 696 lb. As only four days had been allowed for the making this machine, it could not, therefore, be lined with pa per. M. M. had predicted, that it would remain in the air about 20 minutes; and, with a moderate wind, might float to a dif

tance

1

« ElőzőTovább »