Pagina-afbeeldingen
PDF
ePub

a

be bigger and denser than in others, texture than could arise from mere exwhich might not only occasion several halations, whether formed into a smaller explosions as the fire went sphere, and then burning; or disposed along, but also those greater ones, into a kind of train, and consumed by which were like the blowing up of a running fire." large magazines."

After adding many other arguments Here, though the Doctor has at- against the two hypothefes of Dr. Haltempted to thew how these vapours ley, and also to prove that these bodies may rise above the sensible limits of are not terrestrial exhalations, he conour atmosphere, yet it must be observed cludes his remarks in the following that he has changed his opinion greatly words: “ If it is then probable that with respect to the manner in which these balls of fire come from regions they operate, and the principles on far beyond the reach of our vapours; which their motion depends. I shall if they approach often so near to our not hazard a conjecture concerning the earth, and so seldom or never touch merits or demerits of either opinion; it; if they are moved with so much some objections may undoubtedly be celerity, as in that respect to have the made to both, arising from discoveries character of celestial bodies; if they made by later philosophers; and the are seen flying in all directions, and Doctor was himself aware of others, consequently have a motion of their which he has endeavoured to obviate. own, independent of that of our globe;

The late truly worthy, and very in- if they part with great quantities of genious Sir John PRINGLE, P. R.S. elastic fluid, a phlogistic matter, and after giving all the accounts he could probably an acid, surely we are not to meet with of that remarkable meteor consider them as indifferent to us, much which was seen on the 26th of No less as fortuitous masses, or trains of vember 1758, subjoins fome remarks terrestrial exhalations in the athereal on meteors of this kind, in which he regions; but rather as bodies of a nobrings many objections against both bler origin, possibly revolving about those hypotheses, and drops hints that some centre, formed and regulated by imply a persuasion of those bodies the Creator for wise and beneficent being of a much more durable kind purposes, even with regard to our atthan is yet generally supposed. For, molphere; which, during their comafter speaking of the valt height that bustion, they may supply with some meteor was at, the exceeding rareness subtile and falutary matter, or remove of the atmosphere at that height, the from it such parts as begin to be favery small report which gun-powder, perduous, or noxious to the inhabi. puliis fulminans, &c. make when fired tants of the earth.” in an exhausted receiver, and the pro- Some may perhaps be as little fatisdigious great one which that meteor fied with the opinion of this learned made when it burit over the city of and good man as with those of Dr. Glasgow, or thereabout, he adds," I Halley : I, for my part, an equally

“ would infer from hence, that the se- disinclined to comment on either; but paration of the elastic matter must have having laid before my readers the been perforined with a velocity exceed- whole itock of our knowledge on this ing all imagination; as the intensity of subject, both in fact and theory, refound depends so much on the relitt- cominend to every one of them to add ance of the air, and as this elastic mat- to it whatever may be in their power, ter could fly off with so much celerity, by observing and recording, in some as to find so great an opposition from publication or other, such circumso thin a inedium.

îtances attending future meteors of this “ I should also conclude, from the kind, as will rend to explain their nagreatness of the report, that the fub- ture and properties, as well as ascerItance of the meteor was of a firmer tain their situation and motions.

a

a

P. Q.

LOND. MAG, Dec. 1783.

3 S

TO

W direc

[ocr errors]

a

[ocr errors]

TO THE READERS OF THE LONDON MAGAZINE.

E have been favoured with the following paper from Dr. Maskelyne tions and ingenious remarks of fo able an astronomer. This paper will serve as an appendix to the full account of the Fire-Balls, which precedes it, and cannot but attract the notice and attention of our readers in a high degree, both from the nature of the subject, and from the able manner in which it is treated. It is no easy tak to unite entertainment with philosophy. PLAN FOR OBSERVING THE METEORS CALLED FIRE-BALLS,

BY NEVIL MASKELINE, D.D. F.R.S. AND ASTRONOMER ROYAL. FIVE meteots, of the kind which altitude, compared with that of the

from their appearance are generally fun or moon at the same altitude; the called Fire-balls, have been seen of brightness and colours of its light, and late, in the space of a few weeks, viz. the degree of illumination which it on August 18th, Sept. 26th, October gave; and to make a sketch or drawing 4th, 19th, and 29th, which seems to of the appearances before and after it indicate that they appear more fre- . burst, or any other of its appearances. quently than is commonly imagined. 4th. Whether both the body and the The curious and extraordinary appear- tail burst; and how many parts this ances which they exhibited, shew them bursting produced; and whether this to be deserving more attention than happened before or after it arrived at has been hitherto given them. For its greatest apparent altitude; the length want of a series of proper observations, of the tail before the meteor burit; and little progress has been made towards indeed every alteration of its length accounting for their phenomena. The they observe; whether the meteor apgreater part of those who have seen peared very faint at first, and gradually them, not being previously acquainted grew brighter, or appeared very bright with the circumítances they onghe to at ence; and whether it was extinguishattend to, have made observations too ed suddenly, or by degrees. imperfect to answer that purpose. It stil. How long the appearance lasted. is, therefore, to be wished that all oth. Whether a sound or sounds (as persons who may happen to see a of an explosion) was heard some mimeteor would attend to the following nutes after its disappearance, and how particulars, and set down their remarks long, and from what point of the comas soon as they can after they see it, pass they thought it came. while the impresion made by the mic- 7th. The bearing and distance of the teor is full and fresh in their memory, place of observation from the nearest before it is vitiated by their own after- market-town should be

put down. thoughts, or the accounts received from N. B. As found moves only at the other observers. Such after-thoughts rate of 13 miles in a minute, the obmay be of great use: but their own server should patiently wait for at least genuine criginal observations are chiefly 8 or 10 minutes, listening for the found, to be willied for by any one who is to for all meteors appear to be very many calculate the track of the meteor. miles indeed nearer to the observer than

The particulars to be attended to they really are. are there.

REM ARK S. 11t. The precise time ofits appearance. Curious persons may avail themselves

zd. Its apparent altitudes and bears of observations made even by the most ings at its first appearance, at its greatest illiterate, by causing them to trace with cleration, at its buriting, and at its a itick the path which the meteor dedisappearance. .

scribed in the Heavens, according to 34. Its tigore, and the diameter of the best of their recollection. The the body when at the greatuit apparent observations would be better made, if you accompany the person to the absolute velocity of the meteor, the very spot where he saw the meteor, for velocity of the sound propagated to us there the neighbouring objects, such from the higher regions of the atmoas roads, houtes, or trees, will much sphere, and the longitudes of places aflift his memory:

might be determined. The apparent altitudes of the meteor Even in cloudy weather it might be are best found by a quadrant (a com- useful to note the times of accidental mon wooden one of three inches radius explosions, or any unusual sounds divided into degrees will sutlice) which heard, with the points of the compass the perfon should direct to the points in from which they are thought to come, the Heavens where the meteor appeared whether in the day or night, and of to him, if he saw it, or even to such sudden illuminations of the sky in the points where the illiterate person above- night, as they may prove afterwards to mentioned pointed. In like manner have been owing to meteors, and will its bearings Thould be found by a ferre fome of the purposes abovecompass.

mentioned. To ascertain how long the appearance These meteors generally leave a lasted, he should trace over its path in visible tract of faint light behind them, the Heavens with its proper velocity, which gives time to observers to ascerwhile another person observes the time tain the path, either by the stars near by a watch or clock that shews feconds; it, or the observations of altitudes and or by the number of fwings of a tem- bearings. Meteors are sometimes seen porary pendulum made by a mulket in the day-light. ball, or any small weight suspended by It may not be amiss to apprize oba string of 39 inches long from the servers, that estimations of altitudes centre of the ball or weight, which will made without an instrument are very fiving seconds. Without some fuch uncertain, owing to the apparent figure method as this, they will be apt to of the sky being the segmentof a sphere, eitimate the time much longer than whose centre is greatly below the surit is.

face of the earth; so that persons will It would be well if those persons, be apt to judge an object which is near who happen to see a meteor, would the horizon to be much higher than it put down the time by their watch when is; at 23° of altitude, they may think it first appeared, or was at its greatest it at 45°; and to be in or near the altitude, or burst, or disappeared, and zenith, when with an instrument it again when they hear the found; and, would be found 10 or 20 degrees from as common watches are liable to vary it. This points out the necessity for much in a few hours, that they would, observers to mention, whether they as foon after as may be, find the error estimated their altitudes, or observed of their watch by comparing it with a thein with an instrument. good regulator; for, it the exact times could be had at different places, the Greenwich, November 6th 1783.

a

a

a

TO THE EDITOR OF THE LONDON MAGAZINE. SIR, A S the aërostatical experiments lately made in France and in London have

justly engaged the attention of the public, I am willing to imagine that an account of the principle upon which they are performed will be acceptable to the readers of your useful and entertaining miscellany. If your opinion co

. incides with mine, I shall be glad to fee the following pages inserted. N. A MONG other distinctions by fuidity is one of the most remarkable.

which natural bodies are clafiei, The best derinition of a fuid is, that that which respects their folidity or it is a body whose parts yield to any

3 S 2

force

a

force impressed upon them, and in often deride, as the visionary offspring yielding are eatily moved amongit each of a deluded brain. Sir Francis Baother. From this effential property of con was one of the first philosophers a fluid it happens that any pressure who made it an object of enquiry, whatsoever which is made upon any whether bodies might not be construct. part is communicated in all directions ed so light as actually to become buoy. to the rest, and even that pressure ant or float in air; and the enquiry has which arises from the gravity or weight been renewed since the more perfect of the parts of any fluid is made to discovery of the air-pump, and the act in the same manner. So that if weight of the atmosphere. . The result any substance be immersed, for exam- of this was, that the attempt by means ple, in water, its upper part will be of exhausting the internal air is in its prefied by, or sustain the weight of all own nature incapable of being attended the water which lies perpendicularly with success. For an hollow metallic above it, and this pressure will there. sphere, of strength sufficient to support fore be in proportion to its depth be- the external preflure of the atmosphere neath the surface: and again, because when the internal air is exhausted, will any lower part of the fluid is pressed in all cases be heavier than an equal by the weight of the upper parts, from bulk of air, and, therefore, will not which, by its own nature as a fluid, it ascend. The archives of the Royal will endeavour to recede in every di- Society contain the first, and perhaps rection, the parts beneath the substance the only attempt to float bodies in immersed will likewise press upwards condensed air. “This is mentioned in in a like proportion. But the pressure Spratt's History of the Royal Society, beneath will exceed the pressure above p. 218, in a manner which does not in the proportion of the diference be. clearly determine whether it was attween the depths of the under and up- tended with success or not. fer surfaces. That is to say, all cir- It has already been laid before the cumstances being considered, any body public* what were the preliminary whatsoever immersed in a fluid will discoveries among the English, which be pressed upwards by a force which led their neighbours and rivals, the is equal to the weight of a mass of the French, to that successful experiment fluid equal in bulk to the body im- which future ages will mention to their mersed. This very useful hydrosta- honour. Happy would it be for both tical theorem is applicable to the mat- nations, if we might indulge the Utoter before us.

pian with that all other rivalry beFor it is clear, that if the whole iween them might cease, except that weight of the body immersed be not of mutually striving to go beyond each equal to a mass of the fluid of the fame other in promoting those sciences on magnitude, the upward presure will which the welfare of society depends ! exceed the gravity of the body, and Let us throw away the mean and inconsequently it will ascend to the fur- terested ambition which prompts us to face. This is the cause why some bo- decry the merit of men who happen dies swim, ard the converse of this not to have been born in our inand; reasoning shews why others link. and while we enjoy the produce of

We are not at all surprised to see their industry and abilities" let us albodies swin in water, because we are low them their share of fame. Engfamiliarized to experiments with this land can boast of Newton, who is with dense fluid. The vulgar behold facts, justice called the first of men; but if and rest contented, without farther en- France had never produced Descartes, quiry. Eut the philosopher pursues the great and immortal Sir Isaac would diftant analogies, and forms remote perhaps have been no more than the combinations, in the extended arrange- first scholastic fophifter of his time. ments that occupy his attention, which The atmosphere or body of air which the less thoughtful part of mankind environs the globe is composed of parts

* See our Magazine for September last, p. 281.

12

58

[ocr errors]

which gravitate towards the earth like imposible; because no veffel of that other bodies, at the same time that magnitude and weight can be strong they mutually repel each other. That enough to support the preffure of the is to say, the air is an heavy and elaf- atmosphere without yielding inwards. tic fluid. Its weight is familiarly Let it, therefore, be filled with inshewn by exhauiting the air out of a flammable air. The spring of this air bot:le, which will then weigh less being equal to the pretture of the atthan before, and its elasticity appears mosphere, will act on the internal surin a blown bladder, which being made face of the vellel and susiain it; though to give way inwards, by pressure, re

at the same time, on account of its covers its figure when the pressure is extreme lightness, it will add no more removed. Now the air near the sur- than t'o of 1 oz. that is too oz. to face of the earth being subject to the the weight. The vessel consequently pressure of the whole superincunkent will fill ascend with a force equal atmosphere, is denser tian at greater to be of an oz. In this estimate the heights, and at the height of 44 miles inflamınable air is supposed to be pure the air is so extremcly rarefied that it and unmixed. ceases to reflect the solar light in any It is hardly practicable to make so fenfible degree, as is fufliciently proved small a veslol as we have mentioned by observations upon the twilight. sufficiently light; but it is well known

The spring or elasticity of the dif- that by increasing its magnitude its ferent kinds of air, with the know- comparative weight may be diminished. ledge of which the fortunate skill of in any proportion to its balk. Those Dr. Prientley has enriched the modern who are unacquainted with the mathephilosophy, is very different in inten- matics may ealily understand this, by confity. The elasticity of inflammable sidering that the whole weight of the air is the strongest, and therefore its vessel is in proportion to the surface of bulk, equal weights being supposed, filk, or other material employed in its is the greatest, or, in other words, its contruction, and that the fane piece of specific gravity is the leart of any. But tilk which would make two bags of a the action of the spring of all the given size, will make a single bag of kinds of air must be equal while they much more than double the former are subject to the fame common prej fize, if the whole be employed together. fure; namely, that of the atmosphere: But it will be easy to apply the mebecause, if the expression may be used, thod of computation to a vessel of any the springs are all equally bent. Up- other magnitude, where weight is on these two circumitances all the ad- known. vantage of the French experiment de- The pressure of the atmosphere bepends.

coming less and less while the height For the weight of a cubic foot of increases, but the spring of the included common air being 10 oz. averdupois, infiammable air remaining the same, it follows from the hydrostatical prin- except so far as it may be affected by ciple laid down at the beginning of the temperature of those regions this paper, that every body of one cu- through which the vessel may pass, bic foot solidity is presled upwards by it is clear that the equilibriuin which the air with a force equal to that subsisted between the spring of the inweight, and if the weight of the body cluded, and the pressure of the external be less than the upward pressure, it air, will not continue as the vessel will ascend. Suppole now an hollow afcends, but the foriner will prevail, vessel of one cubic foot capacity, and and the vessel at length burst, if there weighing half an ounce in the air, be not some provision made in its conwere to have its internal air exhausted, ftruction to prevent it.

The only it would then ascend with a force equal effectual method to do this, seems to to the surplus of 1 beyond that is be that of adapting a valve that shall to say equal to fo of an ounce. But open outwards, with some difficulty. this we have already observed to be By this contrivance, the surplus of air

a

« VorigeDoorgaan »