СНАР. Х.

QF COMETS; AND OF THE PROPORTION OF LIGHT AND HEAT ON THE PLANETS.

BESIDES the feven primary planets already enu- R merated, and their moons or attendants, there are other bodies that revolve round the Sun, and claim peculiar distinction on several accounts. These are called Comets, and appear occafionally in every part of the heavens; their motions being performed in very long ellipfes, whofe lower focus is in or near the Sun. By obfervations of parallax it is s found, that at their firft appearance they are nearer to us than Jupiter; whence it is concluded, that they are most commonly less than that planet; for if they were as large as Saturn, they would be feen as far off.

called its tail.

When a comet arrives within a certain distance r of the Sun, it emits a fume or vapour, which is This fhews that they contain a portion of matter confiderably more rare and volatile than any on the Earth; for the tail begins to appear while they are yet in a higher, and confequently colder region than Mars. The tail is always directed to that part of the heavens which is

directly

directly or nearly oppofite to the Sun; and is greater after the comet has paft its perihelium, than during its approach towards it; being greatest of all at the time when it has juft paffed the perihelium.

vance.

U That part of a comet's orbit which comes under our obfervation is fo fmall in proportion to the whole, that in most it does not differ from parabola, by quantities that obfervation can diftin- . guifh for which reason the dimenfions of their orbits and periodical times cannot be determined with any degree of precifion from a single appearBut from the re-appearance of comets after long intervals of time in the fame region of the heavens, and moving in the fame curve, it is decided that they revolve about the Sun in very long or eccentric ellipfes; being governed throughout by the fame law of defcribing equal areas in equal times, which is found to take place in the inferior part of their orbits. The comet that appeared in the year 1661 was feen before in the fame orbit, and under the fame circumftances, in the year 1532: which fhews its period to be 129 years. So likewife, the comet that appeared in the years 1456, 1531, 1607, 1682, and 1759, is determined to revolve in a period of about feventy-fix years. And that very remarkable comet which was observed in the year 1680, is fhewn to be the fame with that which appeared in the year 1106; its period being 575 years.

The

The number of comets is very much greater w than that of the planets which move in the vicinity of the Sun. From the reports of former hiftorians, as well as from the obfervations of late years, it is afcertained, that more than four hundred and fifty have been seen previous to the year 1771: and when the attention of aftronomers was called to this object by the expectation of the return of the comet of 1759, no fewer than feven were obferved in the course of seven years. From this circumftance, and the probability that all the comets recorded in ancient authors were of confiderable apparent magnitude, while the fmaller were overlooked, it is reasonable to conclude, that the num→ ber of comets is confiderably beyond any estimation that might be made from the observations we now poffefs. But the number of comets whofe x orbits are fettled with fufficient accuracy to ascertain their identity when they may appear again is no more than fifty-nine, reckoning as late as the year 1771. The orbits of most of these are inclined to the plane of the ecliptic in large angles, and the greater number of them approached nearer the Sun than the Earth ever does. Their motions z in the heavens are not all in the order of the figns, or direct, like thofe of the planets: but the number whose motion is retrograde is nearly equal to that of those whofe motion is direct.

It is not neceffary in this work, to enter fully a into the confideration of final caufes; more particularly as the fubject has embarraffed the greatest metaphyficians,

VOL. I.

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metaphyficians, and may with juftice be faid to be too extended for the human powers, In every thing we fee, the phenomena, confidered fingly, are neceffary confequences of certain general laws, to which the univerfe appears to be fubjected; but when they are confidered in a collective view, a certain relation, or fitnefs for producing fome general effects, is feen, which by no means depends on the fame laws, and by analogy is referred to the operation of an intelligent agent. To illuftrate this by an example, of the fimpleft kind, we may observe, that in the well-known inftrument called fciffars, it follows neceffarily from the laws of motion, already explained in the mechanical powers (60, G. 67, H.), that the blades will cut or divide certain fubftances expofed to their action; but if we confider the various circumftances that co-operate in producing this effect, we must difclaim all reafoning from analogy before we can refolve their connection into an effect of those laws, without fuppofing the agency of an intelligent being as the cause of their union, and concluding that it was intended they should jointly concur in one purpose. It is to this being that we refer, in order to decide, why the sharp edges were made on the inner, rather than on the outer, part of each blade: why the other extremities have annular terminations: why the inftrument is made of steel rather than lead; and fo forth. The purposes or motives which determine the actions of intelligent beings, and produce their effects in a manner fimi

lar

lar to the operation of the laws of nature or properties of matter, in cafes where thought is not fupposed to be concerned, are called final caufes. In the works of nature we behold enough of ex- B quifite contrivance, and can fee far enough into many final causes, to convincé us that the arrangement of the univerfe has been made, and probably is still occafionally adjusted, by a being whose intelligence and power is immenfely beyond that which we poffefs. To judge properly of his in- c tentions, or, in other words, to be equal to the task of exploring the fcience of final caufes, requires no less than a perfect knowledge and recollection of every purpose to which the objects around us may be applied, together with a clear conception of the ideas of fitness and order that form the prototypes in the mind of that great being who directs their motions. These confiderations fhew the abfurdity D of attempting to explain the final causes of every event we fee; but they by no means require that we should neglect them in cafes where we have reason to believe we understand the phenomena, and have fufficient experience to be affured that we difcern the principal, or at least one of the principal purposes to which things may have been deftined. Thus, it is fcarcely to be imagined that we can err in concluding, that the eyes, ears, legs, wings, and other parts of animals, were made for the purposes of seeing, hearing, walking, flying, and the like. Neither can we avoid inferring, that the power who conftructed living creatures with mouths,

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