unavoidable errors of each may be greatly diminished. For the farther illuftration and ufe of this inftrument, we muft refer to the Author's own account of it.

An Effay on the Ufe of COMETS, and an Account of their LUMINOUS APPEARANCE; together with fome Conjectures concerning the Origin of HEAT. By Hugh Williamfon, M. D.

Comets are dark folid bodies, moving round the fun in very eccentric ellipfes, of various magnitudes, and in different periods. The distance of that which appeared in 1680 was twelve thousand millions of miles from the fun 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 aftronomy of comets lefs perfect than that of the other planets. This, however, is certain, that they receive their light and heat from the fun. In 1723 a comet was obferved by the help of a telescope, before it was bright enough to be difcovered by the naked eye; and the great comet which appeared in 1743 was no larger than a ftar of the fourth magnitude, when fint feen; but as it came down towards the fun, it increased in fize and luftre till it terrified half the world. In these respects comets evidently agree with other planets, but they have a luminous train on their approach to the fun, whence they are denominated blazing ftars, peculiar to themfelves. The Author in this paper endeavours to account for this appearance; and fhould he prove fingular in his opinion, he hopes for indulgence in a matter of meer hypothefis.

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

That comets fhould be defigned, amongst other purposes, as Sir Ifaac 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 reafon to allow. On this fuppofition the folar fyftem must be filled with an atmosphere fufficient for attracting and fufpending fluids, which, it is apprehended, would destroy the prefent fyftem of aftronomy: and, befide, there is great reafon to believe, that all the apparent changes in matter depend on combination and folution alone: fo that, fince the Creation,

Creation, this globe has not sustained the abfolute lofs of one ounce of water, or gained one ounce of earth: any nourishment from the vapour of comets is therefore unneceflary.

Comets are supposed to be the habitation of intelligent beings, greatly fuperior to the fhort-lived race of man, both in refpect 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 thofe of an antediJuvian. It is taken for granted that the comets are inhabited; and the Author endeavours to obviate the only plaufible objection against this opinion, deduced from the viciffitudes of climate. We shall give our Readers the substance of what he advances to this purpose.

If heat were a body proceeding immediately from the fun, the quantity of heat in any space would be inverfely as the fquare of its diftance from the fun: but this, he fuppofes, is not the cafe; and therefore the propofition founded upon the hypothefis 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 fame caufe, according to the medium on which it operates: and that the aptitude of a body to be heated is nearly as the elafticity of that body, or the cohesion of its parts. Heat depends on the tremulous motion of the conftituent 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 diffufed through all bodies, and which is excited by any cause that produces this tremulous motion. He lays down this fundamental propofition, 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. 1. by attrition; 2. chymical mixture; 3. fermentation; 4. inflammation; 5. by the fun. From his obfervations on these particulars he infers, that all the heat which is caused by the fun, 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 diftance from the fun, but according to the fitness of that body to retain and propagate the feveral vibrations which are communicated to its particles by the rays of light. And hence it is, that the air which is very elaftic, when well compreffed by the weight of the incumbent atmosphere, will receive a great degree of heat near the furface of the earth, while the light thin air, whofe particles

are removed to a confiderable distance, as on the top of a high mountain, is always in a freezing ftate 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 confifts of particles fmaller, more fubtile, elastic, and eafily heated than ours and therefore the fun's rays may produce a fufficient degree of heat in an atmosphere fo compreffed, at a distance, in which we fhould perceive the most intense cold. And in order to prevent the exceffive heat which would be occafioned by the comet's nearer approaches to the fun, he supposes that their atmosphere does not continue in all seasons of equal denfity and weight. In its approaches to the fun it is greatly rarefied, and hereby rendered lefs fit for generating or retaining heat. Befides this fource of relief, the atmosphere of a comet undergoes a change peculiar to itfelf. It is removed by the rays of light, and thrown off to a confiderable distance behind the planet, fo that the weight of its conftituent particles is hardly perceptible near the furface. This atmosphere will extend itfelf longitudinally behind the planet in the form of a fhadow; and as every particle near the furface of this ftream will be illuminated by the reflection and refraction of the fun's rays, it will exhibit the faint appearance of a blaze. Thus (he fays) we are apt to imagine that a comet is intenfely hot, and that a prodigious flame proceeds from it, while we fee nothing elfe than its enlightened fhadow. As the inhabitants of comets are not preffed by day, when they come near the sun, with a thousandth part of the atmosphere which ufually furrounds them, and which is doubtlefs the mediate and principal caufe of their perceiving heat, we may eafily fee how they may be tolerably cool at noon-day, on their nearest approach to the fun,'

We leave this fine fpun theory, liable as it is to a variety of objections, to the judgment of our Readers.

A Letter from David Rittenhouse, A. M. of Norriton, to William Smith, D. D. Provost of the College of Philadelphia; containing Obfervations 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 fmallness of its fize, and the fhortnefs of the time it continued vifible. At first its velocity was furprisingly accelerated, and before it difappeared again retarded, whence its near approach to the earth may be inferred. It continued vifible 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 refpect 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 this time. On the 1ft of July, it was about one-fixtieth part of, the fun's diftance from us. He farther conjectures, that if the apparent distance of the nucleus, from fome fixed ftar near which it paffed, had been meafured with a micrometer, at dif ferent places on the earth conveniently fituated, the fun'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 fo fhort a period, that we may hence probably conjecture, that there are numbers of thefe wandering bodies which traverse the vaft space encircled by the planets, entirely unperceived by us.

In a fecond letter it appears that M. Meffier difcovered the fame comet in France 10 or 12 days fooner than it was feen in America; and this information, together with the account he afterwards received of its having been feen in England, on its afcent from the fun, towards its aphelion, was peculiarly agree able to the Author of this article, as it confirmed his theory. Mr. Rittenhoufe's calculation agreed exactly with Mr. Six's ob fervation in declination, and differed from it only 20 in right afcenfion, which he does not think very material, unless he knew what method was taken to determine the right afcenfion of a heavenly body, out of the meridian.

Some Account of the fame Comet, 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 difcovered a new flar, about 78° diftant from the pole. Its appearance was larger than a ftar of the firft magnitude, of a dull light, with a bright fpeck or nucleus in the center. He fufpected it to be a comet, and waited for the evidence of farther obfervations. June the 30th he obferved it again, when it appeared rather larger than before, and 8 nearer the pole. On the ift of July it was advanced within 48° of the pole, and seemed to be increased in fize, its fhape rather oval than circular, with its nucleus removed towards the northern part of the whole appearance. It was feen the next evening at twelve o'clock, within about 8° of the pole, and then difappeared. The prodigious velocity of this comet, and its apparent fize, give reafon to believe that its real magnitude was small, and that its path lay at no great diftance from the earth. An eafy Method of deducing the true Time of the Sun's paffing the Meridian per Clock, from a Comparison of four equal Altitudes obferved on two fucceeding Days. By David Rittenhouse, A. M. The calculator is to be furnished with four fets of altitudes obtained on two fucceffive days (viz. one fet in the morning, and one in the afternoon of each day) the inftrument being

kept

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

Take the difference in time between the forenoon obfervations of the two days, and also between the afternoon obfervations. Call half the difference of the two differences X, and half the fum of the two differences Y. Let the half interval between the two obfervations of the fame day be Z. Then, if the times of the altitudes obferved on the fecond day be both nearer 12, or both farther from 12 per clock, than on the first day, X will be the daily variation of the clock from apparent time, and I will be the daily difference in time of the fun's coming to the fame altitude, arifing from the change of declination. And the proportion will be 24: :: Z: E the equation fought; which will be found the fame (without any visible difference) as the equation obtained from the tables,

But if one of the obfervations on the fecond day be nearer 12, and the other more remote from 12 than on the first day; then 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 fame altitude; and the proportion will be 24: X:: Z: E, which equation is to be fubtracted from the mean noon, if the fun's meridian altitude be daily increafing, but to be added, if it be daily decreafing.'

This rule is illuftrated by two examples.

Account of the Tranfit of Mercury over the Sun, Nov. 9, 1769, as obferved at Norriton in Pennsylvania, by W. Smith, D. D. John Lukens, Efq; Meffrs. David Rittenhouse, and Owen Biddle; the Committee appointed for that Obfervation, by the American PhilaJophical 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 fky, and improved the opportunity afforded them by feveral obfervations, of great ufe in fettling the theory of this planet's motion. They were likewife 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 obfervation of this phænomenon. It was firft obferved by Gaffendus at Paris, October 28, 1631, O. S. and twice by Dr. Halley, in 1677 and 1723. Upon a comparison of the theory with the obfervations, there is found a fmall difagreement; the latitude being increafed by the laft tranfit about one quarter of a minute more north than the theory would give, and the time of the middle falling about 4. too foon. It is fuggefted as a proper fubject of enquiry, whether this can be accounted for from a re-examination of the obfervations themselves, or by any correction in the motion of Mercury's nodes?