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unavoidable errors of each may be greatly diminished. For the farther illustration and use of this instrument, we must refer to the Author's own account of it. An Esay on the Use of Comets, and an Account of their LUMI

NOUS APPEARANCE ; together with some Conjectures concerning the Origin of Heat. By Hugh Williamson, M. D.

Comets are dark solid bodies, moving round the sun in very eccentric ellipses, of various magnitudes, and in different periods. The distance of that which appeared in 1680 was twelve thousand millions of miles from the sun in aphelio, and only half a million in perihelio. Its period is 575 years. The period of that in 1661 is 120 years; and the comet of 1758 completes its revolution in 75 years.

The eccentricity of their orbits, and the length of their periods, render the astronomy of comets less perfect than that of the other planets. This, however, is certain, that they receive their light and heat from the sun. In 1723 a comet was observed by the help of a telescope, before it was bright enough to be discovered by the naked eye ; and the great comet which appeared in 1743 was no larger than a star of the fourth magnitude, when finit seen; but as it came down towards the sun, jt increased in size and luftre till it terrified half the world. In these respects comets evidently agree with other planers, but they have'a luminous train on their approach to the sun, whence they are denominated blazing fars, peculiar to themfelves. The Author in this paper endeavours to account for this appearance; and should he prove singular in his opinion, he hopes for indulgence in a matter of meer hypothesis.

Comets (says the Author) do not burn at all, nor is there any remarkable heat in that tail, which appears so terrible. In proof of this he alledges, that the comet of 1743 had acquired a tail of some thousands of iniles long, while he was three hundred millions of miles from the sun. If this is supposed to be a flame of fire kindled by the sun, comets must take fire in a place where every drop of water on this globe would instantly freeze,

That comets should be designed, amongst other purposes, as Sir Isaac Newton conjectured, to nourish and refresh this earth and all the neighbouring planets, and that their tails should be intended to scatter vapour through the planetary regions, our Author fees no reason to allow. On this supposition the solar fyfiem must be filled with an atmosphere sufficient for attracting and fufpending fluids, which, it is apprehended, would destroy the present system of astronomy : and, beside, there is great reason to believe, that all the apparent changes in matter depend on combination and solution alone: lo that, since the

creation, Creation, this globe has not sustained the absolute loss of one ounce of water, or gained one ounce of earth : any nourishment from the vapour of comets is therefore unnecessary.

Come:s are supposed to be the habitation of intelligent beings, greatly superior to the short-lived race of man, both in respect of capacity and duration. They have better opportunities for comprehending and admiring the works of their Creator; and the Author conjectures, from the annual periods of the worlds they inhabit, that their years must exceed those of an antediJuvian. It is taken for granted that the comets are inhabited; and the Author endeavours to obviate the only plausible objection against this opinion, deduced from the vicillitudes of 'climate. We Mall give our Readers the substance of what he advances to this purpose.

If heat were a body proceeding immediately from the sun, the quantity of heat in any space would be inversely as the square of its distance from the sun : but this, he supposes, is not the case; and therefore the propofition founded upon the hypothesis is not true. Heat is produced in the bodies on which the rays of light fall; hence we confound light and heat together, though it be demonftrable that light is not heat, and that heat is not light. Experience teaches us, that different quantities of heat are produced by the same cause, according to the medium on which it operates : and that the aptitude of a body to be heated is nearly as the elasticity of that body, or the cohesion of its parts. Heat depends on the tremulous motion of the constituent particles of the heated body: the Author imagines it does not confift merely in the rapid vibration of these particles, but in the action of that elementary fire which is disfused through all bodies, and which is excited by any cause that produces this tremulous motion. He lays down this fundamental proposition, that heat in every body is uniformly as the vibratory motion of the particles of that body; and then attempts to prove it by an examination of the five methods whereby heat is generated ; viz. I. by attrition ; 2. chymical mixture; 3. fermentation ; 4. inflammation ; 5. by the sun. Fro his observations on these particulars he infers, that all the heat which is caused by the sun, depends on a tremulous motion excited by the rays of light, in the particles of the body which is heated. And therefore that the heat of any body will not be according to its distance from the sun, but according to the fitness of that body to retain and propagate the several vibrations which are communicated to its partiçles by the rays of light. And hence it is, that the air which is very elaftic, when well compressed by the weight of the incum. bent atmosphere, will receive a great degree of heat near the surface of the earth, while the light thin air, whose particles

are

Aa 4

are removed to a considerable distance, as on the top of a high mountain, is always in a freezing state within the torrid zone.

Our Author applies this theory of the generation of heat to the comets. The atmosphere of a comet is 8 or 10,000 miles high, and consists of particles smaller, more fubtile, elastic, and easily heated than ours : and therefore the sun's rays may produce a sufficient degree of heat in an atmosphere so compressed, at a distance, in which we should perceive the most incense cold. And in order to prevent the excessive heat which would be occasioned by the comet's nearer approaches to the fun, he fupposes that their atmosphere does not continue in all seasons of equal density and weight. In its approaches to the sun it is greatly rarefied, and hereby rendered less fit for generating or retaining heat. Besides this source of relief, the atmosphere of a comet undergoes a change peculiar to itself. It is removed by the rays of light, and thrown off to a considerable diftance behind the planet, so that the weight of its constituent particles is hardly perceptible near the surface. This atmosphere will extend itself longitudinally behind the planet in the form of a fhadow; and as every particle near the surface of this stream will be illuminated by the relection and refraction of the sun's rays, it will exhibit the faint appearance of a blaze. Thus The says) we are apt to imagine that a comet is intensely hot, and that a prodigious flame proceeds from it, while we see nothing else than its enlightened shadow. As the inhabitants of comets are not pressed by day, when they come near the sun, with a thousandth part of the atmosphere which usually surrounds them, and which is doubtless the mediate and principal cause of their perceiving heat, we may easily see how they may be tolerably cool at noon-day, on their ncarest approach to the fun.'

We leave this fine spun theory, liable as it is to a variety of objections, to the judgment of our Readers. A Letter from David Rittenhouse, A. M. o Norriton, to William

Smith, D. D. Provost of the College of Philadelphia ; containing Observations of the Comet which appeared in June and July, 1770, with the Elements of its Motion, and the Trajectory of its Path.

The most remarkable circumstances in this comet were, its prodigious apparent velocity, the smallness of its size, and the Thortness of the time it continued visible. At first its velocity was surprisingly accelerated, and before it disappeared again retarded, whence its near approach to the earth may be inferred. It continued visible from the 25th of June to the morning of July the 3d ; and in the last 25 hours moved above 45°. The Author of this article obferves, with respect to this comet, that

in any

future returns, while it continues to move in the fame orbit, it can never approach the earth nigher than it did at of the sun's distance from us. He farther conjectures, that if

A was about one-sixtieth part the apparent distance of the nucleus, from some fixed ftar pear which it passed, had been measured with a micrometer, at different places on the earth conveniently situated, the sun's parallax might, by this means, have been determined nearer than we can ever hope for by any other'method. This comet was so small, and was visible for so short a period, that we may hence

probably conjecture, that there are numbers of these wandering bodies which traverse the vast space encircled by the planets, entirely unperceived by us.

In a second letter it appears that M. Messier difcovered the fame comet in France jo or 12 days sooner than it was seen in America ; and this information, together with the account he afterwards received of its having been seen in England, on its ascent from the sun, towards its aphelion, was peculiarly agrees able to the Author of this article, as it confirmed his theory: Mr. Rittenhouse's calculation agreed exactly with Mr. Six's ob. servation in declination, and differed from it only 20 in right ascension, which he does not think very material, unless he knew what method was taken to determine the right ascension of a heavenly body, out of the meridian. Some Account of the fame Cemet, in a Letter from the Right Hon.

WILLIAM EARL OF STIRLING to William Smith, Ď. D. June the 28th, about ten in the evening, his Lordship discovered a new far, about 78° distant from the pole. Its appearance was larger than a ftar of the first magnitude, of a dull light, with a bright speck or nucleus in the center. He sure peded it to be a comet, and waited for the evidence of farther observations. June the 30th he observed it again, when it appeared rather larger than before, and 8 nearer the pole. On the ist of July it was advanced within 48° of the pole, and seemed to be increased in size, its shape rather oval than circular, with its nucleus removed towards the northern part of the whole appearance: It was teen the next evening at twelve o'clock, within about go of the pole, and then disappeared. The prodigious velocity of this comet, and its apparent size, give reason to believe that its real magnitude was small, and that its path lay at no great distance from the earth. An easy Method of deducing the true Time of the Sun's passing the

Meridian per Clock, from a Comparison of four equal Altitudes observed on two fucceeding Days. By David Rittenhouse, A. M.

The calculator is to be furnished with four sets of altitudes obtained an two fucceffive days (viz, one set in the morning, and one in the afternoon of each day) the instrument being

kept kept exactly at the same height both days; then the exact time of the sun's passing the meridian per clock may be had by the following rule:

6 Take the difference in time between the forenoon obfer. vations of the two days, and also between the afternoon obfervations. Call half the difference of the two differences X, and half the sum of the two differences 7. Let the half interval between the two observations of the same day be z. Then, if the times of the altitudes observed on the second day be both nearer 12, or both farther from 12 per clock, than on the fri day, X will be the daily variation of the clock from apparent time, and Y will be the daily difference in time of the sun's coming to the same altitude, arising from the change of declination. And the proportion will be 24": Y::Z: E the equation fought; which will be found the fame (without any visible difference) as the equation obtained from the tables,

6 But if one of the observations on the second day be nearer 12, and the other more remote from 12 than on the first day; then r will become the daily variation of the clock from apparent time, and X will be the daily difference in time of the fun's being at the same altitude; and the proportion will be 24": X:: 2:E, which equation is to be subtracted from the mean noon, if the sun's meridian altitude be daily increasing, but to be added, if it be daily decreasing.'

This rule is illustrated by two examples. Account of the Transit of Mercury over the Sun, Nov. 9, 1769, as

observed at Norriton in Pennsylvania, by W. Smith, D. D. John Lukens, Esq; Messrs. David Rittenhouse, and Owen Biddle; the Committee appointed for that Observation, by the American PhilaSophical Society. Drawn up and communicated by Direction and in Behalf of the Committee, by Dr. Smith.

The gentlemen concerned in this business had the advantage of a clear sky, and improved the opportunity afforded them by several observations, of great use in settling the theory of this planet's motion. They were likewise happy in obtaining an accurate measure of the diameter of Mercury, which they found to be no more than 8". 22. This makes the fourth observation of this phænomenon. It was first observed by Gallendus at Paris, October 28, 1631, O. S. and twice by Dr. Halley, in 1677 and 1723. Upon a comparison of the theory with the observations, there is found a small disagreement; the latitude being increased by the last transit about one quarter of a mipute more north than the theory would give, and the time of the middle falling about 4. too soon. It is suggested as a proper subject of enquiry, whether this can be accounted for from a re-examination of the observations themselves, or by any correction in the motion of Mercury's nodes?

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