£fl/+ a : Encyclopaedia Britjnnicj; Or, A DICTIONARY O F ARTS, SCIENCES, &c. On a Plan entirely New: By Which, THE DIFFERENT SCIENCES AND ARTS Are digefted into the Form of Diftind TREATISES or SYSTEMS, COMPREHENDING The History, Theory, and Practice, of each, according to the Lateft Difcoveries and Improvements; AND FULL EXPLANATIONS given of the VARIOUS DETACHED PARTS OF KNOWLEDGE, WHETHER Relating to Natural and Artificial Objeds, or to Matters Ecclesiastical, Civil, Military, Commercial, &c. TOGETHER WITH A DESCRIPTION of all the Countries, Cities, Principal Mo u n t a i n s, Seas, Rivers, throughout the World; A General HISTORY, Ancient and Modern, of the different Empires, Kingdoms, and States; An Account of the LIVES of the mofl Eminent Persons in every Nation, from the earlieft ages down to the prefent times. THE WHOLE COMPILED FROM THE WRITINGS OF THE BEST AUTHORS, IN SEVERAL LANGUAGES; THE MOST APPROVED DICTIONARIES, AS WELL OF GENERAL SCIENCE AS OF PARTICULAR BRANCHES; THE TRANSACTIONS, JOURNALS, AND MEMOIRS, OF LEARNED SOCIETIES, BOTH AT HOME AND ABROAD; THE MS. LECTURES OF EMINENT PROFESSORS ON DIFFERENT SCIENCES; AND A VARIETY OF ORIGINAL MATERIALS, FURNISHED BY AN EXTENSIVE CORRESPONDENCE. The Second Edition; greatly Improved and Enlarged. ILLUSTRATED WITH ABOVE TWO HUNDRED COPPERPLATES. VOL. II. INDOCTI D1SCANT, ET AMENT ME MINISSE PERT EDINBURGH: Printed for J. Balfour and Co. W. Gordon, J. Bell, J. Dickson, C. Elliot, W. Creech, J. McCliesh, A. Bell, J. Hutton, and C. Macfarqj; h ar. MDCCLXXVIII. y ‘T ^ 14^MYo " %. 1965 ^ lW A NEW Dictionary of Arts, Sciences, See. ASTRONOMY IS the knowledge of the nature and properties of the heavenly bodies; their magnitudes, diftances, and motions, both real and apparent; together with the na¬ tural caufes by which their revolutions are performed. Hiftory of JJlronomy. It is probable that aftronomy has exifted almoft from the beginning of the world. As there is nothing more furprifing than the regularity of thofe great luminous bodies, that feem to turn inceffantly round the earth, it is eafy to judge, that one of the firft curiofities of mankind was to confider their courfes, and obferve their periods. But it was not curiofity only that in¬ duced men to apply themfelvea to aftronomical fpecu- lations : neceffity itfelf may be faid to have obliged them to it; for if the feafons are not obferved, which are diftinguilhed by the motion of the fun, it is im- poffible to fucceed in agriculture. If the times proper for making voyages were not previoufly known, com¬ merce could not be carried on. If the duration of the month and year were not determined, a certain order could not be eftablifhed in civil affairs, nor the days allotted to the exercife of religion be fixed. Thus, as neither agriculture, commerce, polity, nor religion, could difpenfe with the want of aftronomy, it is evi¬ dent that mankind were obliged to apply themfelves to that fcience from the beginning of the world. What Ptolemy relates of the obfervations of the hea¬ vens, by which Hipparchus reformed aftronomy almoft 2000 years ago, proves fufficiently, that, in the moft ancient times, this fcience was much ftudied. It is agreed that aftronomy was cultivated in a par¬ ticular manner by the Chaldeans. The height of the tower of Babel, which the vanity of men ere&ed about 150 yearsafter the flood, the level and extenfive plains of that country, the nights in which they breathed the freih air after the troublefome heats of the day, an un¬ broken horizon, a pure and ferene Iky, all confpired to engage that people to contemplate the vaft extent of the heavens, and the motions of the ftars. From Chaldea aftronomy paffed into Egypt, and foon after was carried into Phoenicia, where they began to apply its fpeculative obfervations to the ufes of navigation, by which the Phoenicians foon became mailers of the fea and of commerce. What made them bold in undertaking long voyages, was their cuftom of fteering their Ihips by the obfer- vation of one of the liars of the little bear, which, be¬ ing near the immoveable point of the heavens called the pole, is the moft proper to ferve as a guide in naviga¬ tion. Other nations, lefs Ikilful in aftronomy, obfer¬ ved only the great bear in their voyages : but as that conftellation is too far from the pole to be capable of ferving as a certain guide in long voyages, they did not dare to Hand out fo far to fea as to lofe fight of the coafts ; and if a ftorm happened to drive them into the ocean, or upon fome unknown Ihore, it was impofiible for them to know by the heavens into what part of the world the tempeft had carried them. Thales, having at length brought the fcience of the ftars from Phoenicia into Greece, taught the Greeks to know the conftellation of the little bear, and to make ufe of it as their guide in navigation. He alfo taught them the theory of the motion of the fun and moon, by which he accounted for the length and Ihortnefs of the days, determined the number of the days of the fo- lar year, and not only explained the caufe of eclipfes, but Ihewed the art of predicting them, which he even reduced to praftice, foretelling an eclipfe which hap¬ pened foon after. The merit of a knowledge fo un¬ common in thofe days, made-him pafs for the oracle of his times, and occafioned his being reckoned the firft of the feven fages of Greece. Anaximander was his difciple, to whom Pliny and Diogenes Laertius aferibed the invention of the terre- ftrial globe ; or, according to Strabo, geographical maps. Anaximander is faid alfo to have ereCted a gnomon at Sparta, by the means of which he obferved the equinoxes and folftices, and to have determined the obliquity of the ecliptic more exaClly than had ever been done before ; which was neceffary for dividing the terreftrial globe into five zones, and for diftinguifn- ing the climates, that were afterwards ufed by geogra¬ phers for Ihewing the fituation of all the places of the earth. The Greeks, affifted by the inftru&ions they had received from Thales and Anaximander, ventured to make confiderable voyages, and'planted feveral colo¬ nies in remote countries. Commerce having induced the learned rrwn of Greece to vifit other nations, they greatly increafed their aftro¬ nomical knowledge from converfing with the Egyptian priefts, who had long made the fcience of the ftars their profeflion. They alfo learned many things from the Phythagorean philofophers in Italy, who by fomc are faid to have made fo confiderable a progrefs in this fcience, that they had rejected the common opinions. 743 ASTRONOMY. HJftory. and afferted that the earth and planets moved round the fun, which was at reft in the centre of the fyftem. But others affirm that Pythagoras only mentioned this as a conje&ure, which he did not pretend to eftablifh as a fyftem. Melon greatly diftinguiihed himfelf at Atheps by his profound knowledge in aftronomy. He lived in the time of the Peloponnefian war ; and was the inventor of the golden number, ftill placed in the calendar. The Greeks alfo improved their knowledge from converfing with the druids, who, according to Julius Caefar,. inftructed their pupils in the knowledge of the liars, and of the magnitudes of the heavenly bodies. This fpecies of learning was more ancient in Gaul than is generally imagined. Strabo has preferved a famous obfervation, made by Pytheas at Marfeilles a- bout 2000 years ago, with regard to the proportion of the fun’s fhrdow to the height of a gnomon at the time of the folftice. Were the circumftances of this, ob¬ fervation exactly known, it would be fufficient to re- folve the important queftion, Whether the obliquity of the ecliptic be or be not fubjedl to variation ? Pytheas was not contented with making obfervations in his own country. His paffion for aftronomy and geography induced him to travel through Europe, from the pillars of Hercules to the mouths of the Tanais. He alfo advanced along the ftiore of the weftem ocean, towards the north pole, and obferved that the days grew longer about the fummer folftice, in proportion as he travelled; fo that, in the ifland of Thule, the fun rofe almoft as foon as it fet, the tropic continuing entirely above the horizon. By this means he proved the fallacy of what fome philofophers had advanced, namely, that thofe climates were not habitable ; and at the fame time fhewed the method of diftinguiftiing the climates by the length of the days and nights. About the time of Pytheas, feveral of the Greeks applied themfelves to aftronomy in emulation of each other. Eudoxus, the difciple of Plato, not being fatif- fied with what was taught on that fubje£l in the fehools of Athens, repaired to Egypt, to cultivate aftronomy at its fource ; and having a letter of recommendation from Agefilaus king of Sparta, to Nedlanebus king of Egypt, he remained 16 months with the aftronomers of that country. At his return, he compofed feveral books upon aftronomy ; and, among others, a defcrip- tion of the conftellations, which Aratus, fome timeaT ter, turned into verfe, by order of Antigonus. Ariftotle, the difciple of Plato, and the contemporary of Eudoxus, made ufe of aftronomy for improving phyfics and geography. He attempted to determine, by means of aftronomical obfervations, both the figure and magnitude of the earth. He demonftrated, that it was of a fpherical form, by the circular appearance of its fhadow on the difk of the moon in eclipfes ; and from the inequality of the meridian altitudes of the fun, which are different in different latitudes. CalliftheSes, who attended Alexander the Great, having been fent to Babylon, found there aftronomical obfervations made by the Babylonians during thefpace of 1903 years, and fent them to Ariftotle. The princes who fucceeded Alexander in the king¬ dom of Egypt were very careful to draw the moft fa¬ mous aftronomers to their courts by their liberality ; fo that'Alexandria foon became the feat of aftronomy. The famous Conon'made a vaft number of obfervations; but they have not reached our hands. Ariftyllus and Timochares obferved the places of the fixed liars, in order to improve navigation and geography. Eratoft- henes meafured a degree of the meridian, in order to determine the magnitude of the earth. Hipparchus, who alfo refided at Alexandria, laid the foundation for a methodical fyftem of aftronomy ; for a new liar hap¬ pening to appear, he made a catalogue of the fixed ftars, confifting of 1022. He alfo defcribed their mo¬ tion round the poles of the ecliptic, and at the fame time applied himfelf to eftablilh a theory of the folar and lunar motion. The Romans, who alpired to the empire of the world, encouraged aftronomy, and endeavoured to car¬ ry it nearer to perfection ; and in the-reign of Anto¬ ninus it began to affume a new face : for Ptolemy, who may be called the reftorer of this fcience, impro¬ ving from the lights of his predeceffors, and adding the obfervations of Hipparchus, Timochares, and thofe of the Babylonians, to his own, compofed a fyftem of aftro¬ nomy, entitled, “ The Great Syntaxis.” It contain¬ ed the theory and tables of the motion of the fun, moon, and other planets, and of the fixed ftars. But as the beginning of great works are never per¬ fect, it is no wonder that Ptolemy’s work was not free from errors and defeCts. Many ages, however, elap- fed without any one’s prefuming either to correCt or complete it. At laft, the Arabian princes, having conquered the countries where aftronomy had long flourilhed, procured the work of Ptolemy to be tranf- lated into their own language, and called it the Alma* gejl. Nor did they ftop here ; they caufed many ob¬ fervations to be made, by which it appeared, that the greateft declination of the fun was one-third of a de¬ gree lefs than what Ptolemy had made it; and that the motion of the fixed ftars was not fo flow as he be¬ lieved it. They alfo ordered a large extent of country under the fame meridian to be meafured, in order to de¬ termine the length of a degree. This example of the khalifs excited the princes of Europe to promote the improvement of aftronomy. The emperor Frederic the fecond, willing that the Chriftians Ihould underftand aftronomy as well as the Barbarians, caufed the Almageft of Ptolemy to be tranflated from the Arabic into Latin ; and Alphonfo king of Caftile affembled the moft able aftronomers from all parts. By his orders they applied themfelves to reform aftronomy, and compofe new tables, which from him were called the Alpkonfme Tables. This work awakened the curiofity of the learned of Europe : they applied themfelves to invent inftruments for facilitating the obfervations of the heavenly bodies; they calculated ephemerides, and compofed tables for finding the declinations of the planets ; and laboured fuccefsfully to facilitate the calculation of eclipfes. The noble Dane Tycho Brahe was a far more accurate ob- ferver than any that preceded him. He publifhed from his own obfervations, a catalogue of ^70 fixed ftars; and Nicholaus Copernicus revived the ancient Pythago¬ rean fyftem. John Kepler, a moft excellent aftronomer, difcover- ed, by the help of Tycho’s labours, the true fyftem of the world, and the laws that regulate the motion of the celeftial bodies. Galileo, the Florentine philofo- pher. 749 Sea. I. ASTRONOMY. pher, is commonly feid to have firft direfted a telefcope to the heavens, and by the affiftance of that inftru- ment difcovered a great many new and furprifing phe¬ nomena, as the fatellites of Jupiter, and their motion, the various phafes of Saturn, the increafe and decreafe of the light of Venus, the mountains and uneven furface of the moon, the fpots of the fun, and the revolution of that luminary about its own axis. This honour, however. Dr Prieftley is of opinion, belongs to Zacha- rias Janfen, who was undoubtedly the firft inventor of telefcopes. He fays, that, having dire&ed his tele- fcope toward the celeftial bodies, he plainly difcovered the-fpots of the moon, and feveral new ftars ; and that the full moon evidently appeared through this inftru- ment not to be flat, but fphericalj the middle part be¬ ing prominent. Jupiter appeared, round, and rather fpherical. Sometimes, he faid, he faw one, fometimes two, three, artd at the moft four finall ftars a little a- bove or below him; and which, as far as he could ob- ferve, performed their revolutions round him; but that, he faid, he left to the confideration of aftronomers. Hevelius has given us a catalogue of the fixed ftars, much larger than that of Tydm, compofed from his own curious obfervations. Huygens and Caffini firft faw the fatellites of Saturn, and difcovered his ring. The indefatigable Mr Flamftead watched the motion of the ftars for more than 40 years ; and has obliged the world with fuch accurate obfervations on the mo¬ tions, fac. of the different luminaries, as will tranfmit his name to the lateft poftcrity : he alfo publiflied what is called the Britannic catalogue of fixed Jlars, containing above 3000 ftars, from his own obfervations. The merit of Sir Ifaac Newton is too well known to need any encomium, and his difcoveries too numerous to be particularly mentioned in this place. It is fuffi- cient to fay, that the fcience is perhaps more indebted to him than to all the other aftronomers that ever exifted. The great Dr Halley has obliged the world with the aftronomy of^ comets, a catalogue of the fouthern ftars, aftronomical tables, t^e*r apparent magnitudes are very different nets differ- at different times. Every perfon muft have obferved ent at differ- that Venus, though fhe conftantly appears with great ent times, fplendour, is not always equally big. But this appa¬ rent difference of magnitude is moft remarkable in the planet Mars, which fometimes appears no lefs than 25 times larger than at others. This increafe of magni¬ tude is likewife very remarkable in Jupiter, but lefs .5. fo in Saturn and Mercury. Jillat^mo-" Though we have thus defcribed the motions of the fion, planets, with refpeft to their apparent diftances from the fun, they by no means appear to us to move regu¬ larly in the heavens, but, on the contrary, in the moft complex and confufed manner that can be imagined, fometimes going forward, fometimes backward, and fometimes feeming to beftationary. PlateXLIV. fig. 2. reprefents the apparent paths of Mercury and Venus, as traced by Caffini and Mr Fergufon. They all feem to defcribe looped curves; but it is not known when any of thefe curves would return into themfelves, ex¬ cept that of Venus, which returns into itfelf every eighth year. In the figure referred to, that which has the feweft loops is the apparent path of Venus, the other that of Mercury. On each fide of the loops they appear ftationary ; in that part of each loop near the earth, retrograde; and in every other part of their path, direft. Thefe, however, are not the only moving bodies which comers, are to be obferved in the celeftial regions. The five above mentioned are indeed the only ones which appear almoft conftantly, or difappear only at certain intervals, and then as certainly return. But there are others which appear at uncertain intervals, and with a very " different afpefib from the planets. Thefe are called Comets, from their having a long tail, fomewhat refem- bling the appearance of hair. This, however, is not always the cafe; for fome comets have appeared which were as well defined and as round as planets : but in general they have a luminous matter diffufed around them, or proje&ing out from them, which to appear¬ ance very much refembles the Aurora Borealis. When thefe appear, they come in a diredt line towards the fun, as if they were going to fall into his body ; and after having difappeared for fome time in confequence of their proximity to that luminary, they fly off again on the other fide as faft as they came, projedting a tail much greater and brighter in their recefs from him than when they advanced towards him; but, getting daily at a farther diftance from us in the heavens, they continually lofe of their fplendour, and at laft totally difappear. Their apparent magnitude is very differ¬ ent: fometimes they appear only of the bignefs of the fixed ftars; at other times they will equal the diameter of Venus, and fometimes even of the fun or moon them¬ felves. So, in 1652, Hevelius obferved a comet which feemed not inferior to the moon in fize, though it had not fo bright a fplendour, but appeared with a pale and dim light, and had a difmal afpedl. Thefe bodies will alfo fometimes lofe their fplendour fuddenly, while their apparent bulk remains unaltered. With refpeft to their apparent motions, they have all the inequalities of the planets; fometimes feeming to go forwards, fome¬ times backwards, and fometimes to be ftationary. g Though the fixed ftars are the only marks by which Fixed ftars aftronomers are enabled to judge of the courfes of the fcemingly moveable ones, and though they have never been ob- dc^n,ft*,,le ferved to change their places, yet they feem not to be ^ ®encr‘1“ endued with the permanency even of the earth and planets, but to be perifhable or deftrudtible by accident, and likewife generable by fome natural caufe. Several ftars obferved by the ancients are now no more to be feen, but are deftroyed; and new ones have appear¬ ed, which were unknown to the ancients. Some of them have alfo difappeared for fome time, and again become vifible. Of thefe, a very remarkable one is mentioned by Dr Keil, in that part of the heavens called Se£t. T. ASTRONOMY. 75! called the neck of the whale : which for eiVht or nine cedes the firft liar of Aries t° 40', with 150 57' fouth months of the year withdrew itfelf from the fight, and for the other three or four months was conftantly chan¬ ging its luftre and bignefs. Its appearances were at¬ tended with the greateft irregularities; fometimes it ap¬ peared much fmaller than at others; fometimes it dif- appeared in three months, and fometimes appeared for four months; nor did the increafe or decreafe of its magnitude anfwer to the difference of the times of its appearance. We are alfo afiured from the obfervations of aftrono- mers, that fome ftars have been obferved which never were feen before, and for a certain time they have di- flinguifhed themfelves by their fuperlative luftre ; but afterwards decreafing, they vanifhed by degrees, and were no more to be feen. One of thefe ftars being firft feen and obferved by Hipparchus, the chief of the an¬ cient aftronomers, fet him upon compofing a catalogue of the fixed ftars, that by it pofterity might learn whether any of the ftars perifh, and others are produ¬ ced afrefh. After feveral ages, another new ftar appeared to Ty¬ cho Brahe and the aftronomers that were cotemporary with him; which put him on the fame defign with Hipparchus, namely, the making a catalogue of the fixed ftars. Of this, and of the other ftars which have $ appeared fince that time, we have the following hiftory Dr Halley’s by Dr Halley: “ The firft new ftar in the chair of new°tfarsf Caffiopeia, was not feen by Cornelius Gemma on the eighth of November 1572, who fays, he that night con- fidered that part of the heaven in a very ferene Iky, and faw it not: but that the next night, November 9, it appeared with a fplendour furpaffing all the fixed ftars, and fcarce lefs bright than Venus. This was not feen by Tycho Brahe before the 1 Ith of the fame month : but from thence he allures us that it gradually decrea- fed and died away, fo as in March 1574, after fixteen months, to be no longer vifible ; and at this day no figns of it remain. The place thereof in the fphere of fixed ftars; by the accurate obfervations of the fame Tycho, was os g° 17' a iraa -#■ fY5'8, with 530 45' north latitude. “ Such another ftar was feen and obferved by the fcholars of Kepler, to begin to appear on Sept. 30. Jl. vet. anno 1604, which was not to be feen the day be¬ fore: but it broke out at once with a luftre furpaffing that of Jupiter; and like the former, it died away gra¬ dually, and in much about the fame time difappeared totally, there remaining no footfteps thereof in Janu¬ ary i6o|-. This was near the ecliptic, following the right leg of Serpentarlus ; and by the obfervations of Kepler and others, was in 7s 20° 00' a ima * 'Y’, with north latitude i° 56’. Thefe two feem to be of a diftindl fpecies from the reft, and nothing like them has appeared fince. “ But between them, viz. in the year 1596, we have the firft account of the wonderful ftar in Collo Ceti, feen by David Fabricius on the third of Auguft,y?. vet. as bright as a ftar of the third magnitude, which has been fince found to appear and difappear periodically, its period being precifely enough feven revolutions in fix years, tho’ it returns not always with the fame Ijuftre. Nor is it ever totally extinguiflied, but may at all times be feen with a fix-foot tube. This was Angular in its kind, till that in Collo Cygni was difcovered. It pre¬ latitude. “ Another new ftar was firft difcovered by William Janfonius in the year 1600, in peflore, or rather in eduttione. Colli Cygni, which exceeded not the third magnitude. This having continued fome years, became at length fo fmall, as to be thought by fome to have difappeared entirely: but in the years 1657, 58, and 59, it again arofe to the third magnitude; though foon af¬ ter it decayed by degrees to the fifth or fixth magni¬ tude, and at this day is to be feen as fuch in 9s i8° 38' a imz moon> and tears, as they would appear to us idcopes. if we were brought as many times nearer to them as the telefcope magnifies, provided the light proceeding from the luminary we view was diminifhed in the fame proportion. Thus, fuppofing we view the moon thro’ a telefcope magnifying 1000 times, her face will ap¬ pear 1000 times bigger to us than it does to our naked eye, and we will perceive vaft numbers of fpots in it which are imperceptible to our naked eye : but then fhe will alfo appear 1000 times more dim through the telefcope than Ihe does to the naked eye; fo that thofe who look at her through a telefcope for the firfl: time, will be greatly difappointed, if they are not warned of this diminution of light. The reafon of this is, that the telefcope cannot increafe the quantity of light which falls upon itfelf from the moon, and by which ' only fhe is or- can be vifible to us. This quantity of light, however, is by the magnifying powers of the telefcope fpread over a proportionably large furface ; and therefore the moon or other body appears magni¬ fied indeed, but with a fplendor vaftly inferior to that with which flie appears to the naked eye. Hefice we may fee, that the advantages arifing from telefcope, with regard to the giving us a near view of the celeftial bodies, are not near fo great as one would at firft imagine : for though we fltould fuppofe a tele¬ fcope capable of magnifying 240,000 times, by which we could fee the moon at the dillance of only a fingle mile, we could only have this view with a light 240,000 times lefs than the moon atprefent affords us; and how imperfe&ly obje&s at the diftance of a mile could be feen by fuch a light, anyone may eafily ima¬ gine. We are not, however, to imagine, that the light here fpoken of would be 240,000 times lefs than moon-light; it would only be 240,000 times lefs than that wherewith fhe appears illuminated at prefent. The moon, we know, is enlightened by the fun, as well as the earth is ; and could the telefcope increafe the quan¬ tity of light as much as it does the apparent furface, we would, with a telefcope magnifying 240,000 times, fee all objefts in the moon as well as we do terreftrial objefts at a mile’s diftance when the fun fhines bright- eft : but by reafon of this incapacity of the telefcope to increafe the quantity of light, we could only fee the lunar obje&s as well as we would do terreftrial ones a mile diftant, with a light 240,000 times lefs than the light of the fun, or with a light little better than moon¬ light ; for the light of the fun is computed to be 300,000 times ftronger than that of the moon. What we have here faid of the moon is applicable to all the other celeftial bodies, but not in the fame de- f;ree. If we look at the fun, for inftance, whofe uftre is too great for our naked eye to bear, fuppofing the telefcope-to magnify as many millions of times as there are miles between us and the fun, we would per¬ ceive objects in him as diftin&ly as could be done at the diftance of a mile, not with the light of funfhine upon this earth, but with that intenfe light which is emitted from the fun at a mile’s diftance from his body. This is on the fuppofition that the telefcope could increafe the quantity of the light as well as magnify the apparent furface; but, being deftitute of this power, the light with which the fun would be feen through fuch a telefcope is as many millions of times lefs than the abovementioned intenfe light, as the number of times the telefcope magnifies, and which, according to the lateft calculations, behoved to be upwards of 95 millions. It muft be obferved, however, that no tele¬ fcopes ever have been, or probably ever will be, invented, whofe magnifying powers are fo great as either of thofe we have mentioned. The greateft magnifying power we have yet heard of in any telefcope is in one made by the late Mr James Short, which magnifies 12,000 times; and, by having another applied to it, is faid to mag¬ nify 60,000 times. From thefe confiderations it will be apparent, that our telefcopic views of the Celeftial objects can be but imperfedf, and that conjectures drawn from them muft be very vague and uncertain. It is alfo plain, that, ceteris paribus, bur views of Venus and Mercury, the planets neareft the fun, ought to be more ditlinft than of the more remote ones. Mars, Jupiter, and Saturn ; becaufe Venus and Mercury are much more ftrongly illuminated than the others: but this is not found to hold in fa£t; which is very furprifing, as a ftrong light ought to be much farther and more ftrongly refledled from any objedl than a weak one. The general ap¬ pearances of each of the celeftial bodies, when viewed with the beft telefcopes, the large one by Mr Short abovementioned excepted, are as follow. 1. The fun, when viewed with but an ordinary inftru¬ ment, and fometimes through a piece of fmoked glafs without any telefcope, difcovers on his furface numbers of black, or rather lefs bright, fpots, of various fhapes and fizes. Sometimes thefe fpots will vanifh in a very ftiort time from their firft appearance; fometimes they travel over his whole dilk, or vifible furface, from weft to eaft, when they difappear; and in 12 or 13 days they appear again, fo as to be known, by their fize and fhape, to be the fame that formerly difappeared. Thofe, how¬ ever, which are of. the longeft continuance, never ap¬ pear to have any durability or folidity of confiftence, but foon vanifli and become bright like the reft of the furface. The moft remarkable phenomena of thefe fpots have been remarked by Scheiner and Hevelius, and are as follow. 1. Every fpot, which hath a nucleus, or con- fiderably 14 Spots on the fun difco- vered by te¬ lefcopes. Account of their pheno¬ mena by Scheiner 8c Hevelius* 7S3 Sed. II. ASTRO fiderably dark part* hath alfo an timbra, or fainter (hade, furrounding it. 2. The boundary betwixt the nucleus and umbra is always diftindt and well defined. 3. The inereafe of a fpot is gradual, the breadth of the nucleus and umbra dilating at the fame time. 4. In like manner, the decreafe of a fpot is gradual, the breadth of the nucleus and umbra contradting at the fame time. 5. The exterior boundary of the umbra .never confifts of (harp angles ; but is always curvilinear, how irregular foever the outline of the nucleus may be. 6. The nucleus of a fpot, whilft on the decreafe, often changes its figure by the umbra encroaching irregular¬ ly upon it, infomuch that in a fmall fpace of time new encroachments are difcernible, whereby the boundary betwixt the nucleus and umbra is perpetually varying. 7. It often happens, by thefe encroachments, that the nucleus of a fpot is divided into two or more nuclei. 8. The nuclei of the fpots vanilh fooner than the um¬ bra. 9. Small umbrae are often feen without nuclei. 10. An umbra of any‘confiderable fize is feldom feen without a nucleus in the middle of it. 11. When a fpot which confided of a nucleus and umbra is about to difappear, if it is not fucceeded by a faculcty or fpot brighter than the reft of the dilk, the place where it ifi was is foon after not diftinguiftiable from the reft. By Dr Wil- In the Philofophical Tranfadtions, Vol. LXIV. Dr Glaf" Wilfon, profeffor of aftronomy at Glafgow, hath given 2°W' a differtation on the nature of the folar fpots, and men¬ tions the following appearances. 1. When the fpot is about to difappear on the weftern edge of the fun’s limb, the eaftern part of the umbra firft contradls, then vanifti- es, the nucleus and weftern part of the umbra remain¬ ing ; then the nucleus gradually contradls and vanifties, while the weftern part of the umbra remains. At laft this difappears alfo; and if the fpot remains long enough to become again vifible, the eaftern part of the umbra firft becomes vifible, then the nucleus ; and when the fpot approaches the middle of the dilk, the nucleus appears environed by the umbra on all Tides, as already mentioned. 2. When two fpots lie very near to one another, the umbra is deficient on that fide which lies next the other fpot: and this will be the cafe, though a large fpot ftiould be contiguous to one much fmaller; the umbra of the large fpot will be totally wanting on that , fide next the fmall one. If there are little fpots on each fide of the large one, the umbra does not to¬ tally vanilh; but appears flattened, or prefled in to¬ wards the nucleus on each fide. When the little fpots difappear, the umbra of the large one extends itfelf as ufual. This circumftance, he obferves, may fometimes prevent the difappearance of the umbra in the manner abovementioned ; fo that the weftern umbra may dif¬ appear before the nucleus, if a fmall fpot happens to 17 break out on that fide. By Mr Wol- In the fame volume, p. 337. Mr Wollaftcn obferves, lafton. t}je appearances mentioned by Dr Wilfon are not conftant. He pofitively affirms, that the faculte or bright fpots on the fun are often converted into dark ones. • “ I have many times (fays he) obferved, near the eaftern limb, a bright facula juft come on, which has the next day ftiewn itfelf as a fpot, though I* do not recolleft to have feen fuch a facula near the weftern one after a fpot’s difappearance. Yet I believe, both thefe circumftances have been obferved by others ; and perhaps not only near the limbs. The circumftance N O M Y. of the faculae being converted into fpots I think I may be fure of. That there is generally (perhaps always) a mottled appearance over the face of the fun, when carefully attended to, I think I may be as certain. It is moft vifible towards the limbs, but I have undoubt¬ edly feen it in the centre ; yet I do not recolledt to have obferved this appearance, or indeed any fpots, to¬ wards his poles. Once I faw, with a 12 inch reflec¬ tor, a fpot burft to pieces while I was looking at it. I could not expert fuch an event, and therefore cannot be certain of the exadl particulars : but the appearance as it ftruck me at the time was like that of a piece of ice when daftied on a frozen pond, which breaks to pieces and Aides in various dire&ions.” He alfo ac¬ quaints us, that the nuclei of the fpots are not always in the middle of the umbrae; and gives the figure of one 3d Plate feen November 13th 1773, which is a remarkable in- XLII. fig.fi. ftance to the contrary. Mr Dunn, however, in his new Mr p8nnn>s Atlas of the mundane fyftem, gives fome particulars account, very different from the above. “ The face of the fun (fays he) has frequently many large black fpots, of va¬ rious forms and dimenfions, which move from eaft to weft, and round the fun, according to fome obferva- tions in 25 days, according to others in 26, and accord¬ ing to fome in 27 days. The black or central part of each fpot is in the middle of a great number of very fmall ones, which permit the light to pafs be¬ tween them. The fmall fpots are fcarce ever in con- taft with the central ones: but what is moft remark¬ able, when the whole fpot is near the limb of the fun, the furrounding fmall ones form nearly a ftraight line, and the central part projedts a little over it, like Sa¬ turn in his ring.” The fpots are by no means confined to one part of the fun’s dilk; though we have not heard of any be¬ ing obferved about his polar regions: and though their diredtion is from eaft to weft, yet the paths they de- fcribe in their courfe over the dilk are exceedingly different; fometimes being ftraight lines, fometimts curves, fometimes defcending from the northern to the fouthern part of the dilk, fometimes afcending from the fouthern to the northern, &c. This was obferved by Mr Derham, (Philof. Tranf. N°. 330.) who hath given figures of the apparent paths of many different fpots, wherein the months in which they appeared, and their particular progrefs each day, are marked. 2d Phfe Befides thefe fpots, there are others which fpmetimes XLII.fig.r. appear very round and black, travelling over the dilk of the fun in a few hours. They are totally unlike the o- * thers, and will be Ihewn to proceed from an interpofition of the planets Mercury and Venus between the earth and the fun. Excepting the two kinds of fpots .above- mentioned, however, no kind of objedl is difcoverable on the furface of the fun, but he appears like an im- menfe ocean of elementary fire or light. | Ig 2. With the moon the cafe is very different. Many Telefcoplc darkilh fpots appear in her to the naked eye; and,'view of the' through a telefcope, their number is prodigioully in- n10011- creafed: Ihe alfo appears very plainly to be more pro¬ tuberant in the middle than at the edges, or to have the figure of a globe, and not a flat circle. This pro¬ tuberance, or globular figure, may alfo be perceived in fome degree by the naked eye, when Ihe does not Ihine very bright. When the moon is horned or gib¬ bous, the one fide appears very ragged and uneven. '54 ASTRONOMY. but the other always exaftly defined and circular. The fpots in the moon always keep their places exactly; never vanifhing, or going from one fide to the other, as thofe of the fun do. The aftronomers Florentius, Langrenus, John Heve- lius of Dantzic, Grimaldus, and Ricciolus, have drawn the face of the moon as {he is feen through telefcopes magnifying between two and 300 times. Particular care has been taken to note all the fhining parts in her furface; and, for the better diftinguifhing them, each has been marked with a proper name. Langrenus and Ricciolus have divided the lunar regions among the 3 Venus puts on the fame appearances with the Moon, or with Mercury; only fhe is not fo bright as the latter. Spots have fometimes been feen in her, which appeared of the nature of thofe in the fun ; o- thers have been obferved more permanent. In 1666, Odtober 14th, MrCaffini obferved, towards the middle of the dilk of this planet, a part more luminous than the reft, and two dark fpots to the weftward of it. He could make nothing of his obfervations at that time, nor had he any opportunity of obferving the fpots a- gain till April 28th 1667. At this time Venus had an horned appearance, and a quarter of an hour before fun-rifing the bright fpot was diftant from the fouthern horn little more than £ of its diameter. Near the eaft- ern part of the circumference he faw an oblong fpot, which was nearer to the northern than the fouthern horn. At the rifing of the fun, he perceived the bright part diftant from the fouthern horn by 4- of its diame¬ ter ; fo that it was plainly moving from fouth to north. He was very much furprifed, however, to rind, that though this fpot appeared to move from fouth to north while it remained on the fouthern part of the difle, yet, when he faw it on the northern part, it was plainly moving from north to fouth. From frequent obferva- tion, he imagined that this bright fpot finiihed its mo- Sea. If. tion, and returned to the fame place of the diik from whence it fet out, in about 23 hours, though not with¬ out fome irregularity. The fame irregular motion he obferved in the obfeure fpots. In 1672 and 1686, with a telefcope 54 feet long, Mr Caflini thought he faw a moon, or fatellite, belonging to this planet, and having the fame phafes with Venus herfelf, but not fo well defined. It was not above f- of the diameter of Venus diftant from her body. Mr Short alfo made the like obfervation fome years ago. 5. Mars viewed through a telefcope, fometimes ap- Mars* pears gibbous, but never horned. He has one very large fpot in his dilk, which at different times changes its figure, as well as Ihifts its place. Dr Hook ob¬ ferved this fpot carefully in 1665, and Mr Caffini in 1666 ; and the latter, by diligently continuing his ob¬ fervations, found it to return to the fame place in the fpace of 24 hours and 40 minutes. Through a 36 foot telefcope, made ufe of by Dr Hook, this planet appeared almoft as big as the full moon. The phafes are reprefented 4th Plate XLII. fig. 3. Befides thefe fpots. Mars fometimes appears to have darkilh fillets or belts along his dilk, which are called fafeia, and appear parallel to his equator. The fixed ftars which he approaches, fuffer the fame diminution of their light when obferved through a telefcope, as when feen by the naked eye. 6. Jupiter has the fame general appearance with Jupiter. Mars; only that the fixed ftars never fuffer any dimi¬ nution of light by his approaching them. In 1666, Mr Hook obferved the body of Jupiter through a tele- Phil. Tranf. fcope 60 feet long, and found the apparent diameter n°i4.p.»4j,, to be about four times as great as that of the full moon. This planet has more remarkable and more diftindt fa- feiae than Mars, which are terminated by parallel lines. They do not, however, appear to be permanent fub- ftances, being often interrupted and broken, and fome¬ times vanilhing entirely. In fome of thefe belts large black fpots have appeared, which moved fwiftly over the dilk from eaft to weft, and returned again to the fame place ; thofe neareft the planet’s equator in nine hours 56 minutes; and thofe nearer the poles, accord¬ ing to Dr'Smith, in 9 hours and 50 minutes. In 1665, Mr Cafiini faw a very large fpot in one of Jupiter’s belts, which he obferved continually for the fpace of two years, and determined exadtly the time of its ap¬ pearance and difappearance. In 1672, he obferved it to return to the fame place from whence it had fet out in one night, having had an opportunity of viewing Jupiter during 10 hours that night, and thus thorough¬ ly afeertained himfelf of the exadt time it took. In 1677, this fpot vanilhed, and was not feen again till 1679: afterwards, for the fpace of almoft three years, it continually Ihewed itfelf, and then gradually vanilhed ; and fince that time has frequently appeared and difap- peared. From the year 1665, in which it firft appear¬ ed, to the year 1708, it appeared and vanilhed no fewer than eight times. The apparent motion of thefe fpots over the dilk is very unequal when they firft appear: on the eaftern limb of the dilk, their motion is flow, aneNkey appear narrow; but, as they advance towards the middle, they grow broader, and their motion be¬ comes much quicker; and when they approach the weftern limb, they again change their figure, and move more flowly. The belts undergo feveral changes with¬ out 755 ASTRONOMY. Seft. II. out being broken ; fometimes becoming narrower, fometimes increafing in breadth, fometimes advancing 54 towards each other, fometimes receding, &c. His four What is mod remarkable of this planet is, that it moons. js attended by four fatellites or moons, which evident¬ ly circulate round it; as they fometimes recede a little from, and then come nearer to, the difk ; fometimes are hid behind the body of the planet, and then appear again on the other fide, receding to a little diftance, then come near and pafs over it: and it is obferved that when they pafs between us and the difk of Jupiter, they often refemble black round fpots, like Mercury and Venus pafiing over the difk of the fun ; their fhadows in the mean time travelling over the difk like other fpots, having the fame appearance, and marching either before or behind the fatellites according to the fit na¬ tion of the earth and Jupiter with refpeft to the fun. With good telefcopes, black fpots have alfo been ob¬ ferved on the difks of thefe fecondary planets them- lS felves. Saturn,with 7. Saturn, the moft remote of all the planets, makes his ring and a ftill more remarkable appearance ; being encompaffed fatelliies. with a luminous ring, reprefented 2d Plate XLII. fig. j. and PlateXLIII. fig.5. This ringappears double when feen thro’ good telefcopes, as reprefented in'the figures, and keeps always parallel to itfelf; by which means it difappears once every fifteen years, according to the fi- tuation of the earth and Saturn with regard to the Sun. The reafon of its difappearance is, that it is turned edge- wife to us, when its thinnefs renders it invifible. When it begins to turn its edge towards us, it appears fome- what thicker on one fide than another, and the thickefl: edge has been obferved on different fides of the planet. On the body of Saturn, fafcise have likewife been fome¬ times feen, but much more obfcurely than thofe of Mars or Jupiter ; and no dark fpots have ever been obferved on his furface. This planet is attended by five moons, whofe general appearances refemble thofe of Jupiter ; only they are more obfcure, and require better telefcopes to difcover them on account of their 2« great diflance. Of the co- 8. The Comets, viewed through a telefcope, have a wets. very different appearance from any of the planets. The nucleus, or ftar, feems much more dim. Sturmius tells us, that obferving the comet of 1680 with a te¬ lefcope, it appeared like a coal dimly glowing ; or a rude mafs of matter illuminated with a dufky fumid light, lefs fenfible at the extremes than in the middle; and not at all like a ftar, which appelrs with a round difk and a vivid light. Hevelius obferved of the comet in 1661, that its body was of a yellowifh colour, bright and confpicu- ous, but without any glittering light. In the middle was a denfe ruddy nucleus, almoft equal to Jupiter, encompaffed with a much fainter, thinner matter.— Feb. jth. The nucleus was fomewhat bigger and brighter, of a gold colour, but its light more dufky than the reft of the ftars ; it appeared alfo divided in¬ to a number of parts—Feb. 6th. The nuclei ftill ap¬ peared, though lefs than before. One of them on the left fide of the lower part of the difk appeared to be much denfer and brighter than the reft; its body round, and reprefenting a little lucid ftar; the nuclei ftill encompaffed with another kind of matter.—Feb. iotb. The nyclei more obfcurc and confufed, but brighter at top than at bottom.—Feb. 13th. The head dimi- nifhed much both in brightnefs and in magnitude.— March 2d. Its roundnefs a little impaired, and the edges lacerated.—March 28th. Its matter much dif- perfed; and no diftinft nucleus at all appearing. Weigelius, who faw through a telefcope the comet of 1664, ^ moon, and a little cloud illuminated by the fun, at the fame time; obferved that the moon ap¬ peared of a continued luminous furface, but the comet very different, being perfeftly like the little cloud en¬ lightened by the fun’s beams. The comets, too, are to appearance furrounded with Their at- atmofpheres of a prodigious fize, often rifing ten times mofplieres higher than the nucleus. They have often likewife and P*iaies< different phafes, like the moon. Thofe of 1744 and 1769 had both of them this appearance,5th Plate XLII. fig. 6. The latter alfo, when viewed through a tele¬ fcope, feemed to turn fwiftly round on its axis, and to emit flafhes or fparks of eleftric light from all parts of it; which fparks were inftantly impelled with great vio¬ lence towards the tail. As for the tails of comets, they refemble the ftreams of ele&ric light more than any thing elfe. That of 1769 feemed perpetually to fhoot out in ftraight lines of a pale filver hue, lengthening and fhortening at each inftant, and forming frequently fome of the configu¬ rations affumed by the Aurora Borealis. Dr Halley, about the year 1716, hath obferved the fame thing. Speaking of a remarkable Aurora Borealis, he fays, “ That the great ftreams of light fo much refembled the tails of comets, that at firft fight they might well be taken for fuch and afterwards adds, “ This light feems to have a great affinity with that which ele&ric bodies emit in the dark.” 53 . 9. The fixed ftars, when viewed through the beft te- ,Numl>er lefcopes, appear not at all magnified, but rather (^iis€.he nifhed. in bulk, by reafon that the telefcope takes off fe!irSh’ntCe[CcV that twinkling appearance they make to the naked eye, fcopes. and which increafes their apparent magnitude. Their number, however, appears increafed fo prodigioufly, that 70 ftars have been counted in the conftellation call¬ ed the Pleiades, and no fewer than 2000 in that of O- rion. The galaxy, or milky-way, appears in a manner made up of ftars fo fmall and fo clofe, that they cannot be difcerned fingly. To thefe ftars it owes a good part of its light, though not the whole, as fome fmall fpecks called nebula are difcovered in the heavens, having much the fame appearance, and which have not always ftars within them. Of thefe we have the following ac- ||g count in the Philofophical Tranfaclions, N° 347. “ Some of thefe bright fpots difcover no fign of a ftar in the Accomn of middle of them ; and the irregular form of thofe that the nebula;, have, fhews them not to proceed from the illumination of a central body. Thefe are fix in number, all which we will deferibe in the order of time, as they were difco¬ vered ; giving their places in the fphere of fixed ftars, to enable the curious, who are furnilhed with goodte- lefcopes, to take the fatisfa&ion of contemplating them. “ The firft and moft confiderable is that in the middle of Orion’s fword, marked with 8 by Bayer in his Ura- nometria, as a fingle ftar of the third magnitude ; and is fo accounted by Ptolemy, Tycho Brahe, and Heve¬ lius : but it is in reality two very contiguous ftars,envi¬ roned with a very large tranfparent bright fpot, thro’ which they appear with feveral others. Thefe are cu- rioufiy 756 ASTRONOMY. Sea. III. rloufly defcribed by Huygenius in his Syjlema Saturni- num, p. 8. who there calls this brightnefs Portentum cui certe ftmile aliud ■ nujquam apud reliquas fix as potu.it anmadvertere : affirming, that he found it by chance in the year. 1656. The middle of this is at prefent in n 190. with fouth lat. 28° “ About the year 1661, another of this fort was difcovered by Bullialdus, in Cingula Andromedse. This is neither in Tycho nor Bayer, having been omitted, as are many others, becaufe of its fmall- nefs : but it is inferted in the catalogue of Heve- lius, who has improperly called it Nebulofa inftead of Nebula ; it has no lign of a liar in it, but appears like a pale cloud, and feems to emit a radiant beam into the north-eaft, as that in Orion does into the fouth-eaft. It precedes in right afcenfion the northern in the girdle, or v Bayero? about a degree and three quarters, and has longitude at this time 240, with north lat. 33°t* “ The third is near the ecliptic, between the head and bow of Sagittary, not far from the point of the winter folftice. This, it feems, was found in the year 166; by a German gentleman M. J. Abraham Ihle, whilil he attended the motion of Saturn then near his aphelion. This is fmall, but very luminous, and emits a ray like the former. Its place at this time is vy 4°-j-, with about half a degree fouth lat. “ A fourth wfas found by Dr Edm. Halley in the year 1677, when he was making the catalogue of the fouth- ern ftars. It is in the Centaur, that which Ptolemy calls ° w t«v t^ic moon any atmofphere fimilar to what we jnufphere. breathe. Againft the exiftence of this it hath been urged, that the moon conftantly appears with the fame luftre when there are no clouds in our air ; which could not beexpefted, were (he furroundedwith an atmofphere like ours, the different changes of which behoved fome¬ times to diminilh and at other times to increafe her luftre. The ftrongeft argument, however, is drawn from the refraftive power of our atmofphere, which is well known to have a great influence on the rays of light pro¬ ceeding either from celeftial or terreftrial bodies, fo as to caufe them deviate from a ftraight line, and of confe- quence behoved to have the fame effedt on thofe which paffed through the atmofphere of the moon, if any fuch there was. But no fuch effedf has ever been perceived. The fmalleft fixed ftars, as hath been already obferved, preferve their luftre undiminifhed till they are fuddenly covered by the moon’s limb, and as fuddenly appear on N O M Y. Sea. nr. the other fide, without being at all affedied by their approach to her, as they are by the planet Mars. For this reafon, too, the fame philofophers maintain, that there can neither be clouds nor rain in the lunar regi¬ ons, but that fhe enjoys a perpetual and uninterrupted ferenity. To thefe arguments it hath been replied, I. That the appearances on which they are founded are not conftant. Hevelius writes, that he has feveral times found in Ikies perfedtly clear, when even ftars of the fixth and feventh magnitude were vifible, that at the fame altitude of the moon, and the fame elongation from the earth, and with one and the fame telefcope, the moon and its maculae do not appear equally lucid, clear, and confpicuous, at all times ; but are much brighter and more dittindf at fome times than at others. From the circumftances of this obfervation, fay they, it is evident that the reafon of this phenomenon is not either in our air, in the tube, in the moon, or in the fpe&ator’s eye ; but muft be look¬ ed for in fomething exifting about the moon. Along with this, the phenomena already mentioned of the different appearances of the moon in the total eclipfes are alfo urged, and are derived from the different con- ftitutions of the lunar atmofphere at that time. Caffini frequently obferved Saturn, Jupiter, and the fixed ftars, to have their circular figure changed into an elliptical one, when they approached either the illuminated or dark edge of the moon’s limb, and in other occultations found no change of figure at all. Mr Dunn particular¬ ly viewed Saturn at his emerfion from behind the moon, in order to determine this queftion; and the appearance which he then obferved, inclined him to think there was an atmofphere about the moon. 2. In the total eclipfes of the fun, we find the moon encompaffed with a lucid ring parallel to her periphery. Of this we have too many obfervations to doubt: In the great eclipfe of 1713, the ring was very vifible at London and elfe- where. The fame was obferved by Kepler in 1605, at Naples and Antwerp. Wolfius relates the fame of an eclipfe in 1606 at Leipfic, which is deferibed at large in the A&a Eruditorum, with this notable circumftance, that the part next the moon was vifibly the moft illu¬ minated, being confiderably brighter than that furtheft from it; which is alfo confirmed by the obfervations of the French aftronomers in 1706. For thefe reafons, it is concluded by many, that there is about the moon fome fluid, which conforms itfelf to her figure, and is more denfe near her furface than at a diftance from it, and that it both refle&s and re- fra&s the rays of the fun. It is alfo concluded, that the lunar atmofphere is not always in the fame ftate, as it fometimes changes the figures of the ftars, and feme- times not: and in the feveral eclipfes juft mentioned, there was obferved a trembling of the moon’s limb im¬ mediately before immerfion, with an appearance of thin fmoke flying over it during immerfion, very vifible in England. Hence, as thefe phenomena are obferved to happen in our atmofphere when full of vapours, it is con¬ cluded that the lunar atmofphere has been in a fimilar ftate at thefe times: andfince at other times thefe ap¬ pearances are not to be obferved, it is thought that then the lunar air has been clear and tranfparent, by reafon of rain, dew, or fnow, having fallen. The ftrongeft argument for a lunar atmofphere is that drawn from the luminous ring around her in folar eclipfes j Sea. III. A S T R < eclipfes; and this feems fo conclufive, that Dr Halley himfelf was almoft convinced of the exiftence of fuch an atmofphere by it. He tells us, however, that feve- ral very great aftronomers did not think fuch a thing at all probable. Other aftronomers have imagined that it proceeded from a folar atmofphere, becaufe it follow, ed the centre of the fun, and not that of the moon. Some there are who ridicule both thefe opinions, and take this appearance to be an undeniable proof of the moon’s being included within the atmofphere of the earth. We obferve, fay they, at all times, when the Iky is clear, the body of the fun to be furrounded with a very bright and dazzling circle, which extends to a confide- rable diftance, and whofe centre always coincides with that of the fun. This circle we never afcribe to the atmofphere of the fun, but to that of the earth. If the fun is hid by a mountain, part of this circle con¬ tinues vilible after his body difappears ; and that part of the luminous circle is brighteft, which is apparently neareft the mountain : yet we never afcribe this to the atmofphere of the mountain, but to the common atmo¬ fphere of the earth which lies beyond the mountain. In the cafe of folar eclipfes, therefore, why fhould we ima¬ gine the luminous circle to proceed from any thing elfe than that part of the earth’s atmofphere which lies be¬ yond the moon ? In this cafe, it will alfo follow the centre of the fun ; and the tremulations of that part of our atmofphere lying between us and the moon being obferved by means of the fun’s ftrong light, will occa- fion the apparent tremulation of the moon’s limb al¬ ready taken notice of at the time of immerfion. To have recourfe in this cafe, fay they, to fuch an ens ra- tionis as the folar or lunar atmofphere, is truly folving obfcunm per obfcurius. As for the fmall refradf ions of the light of the planets and fixed ftars which have fome- times been obferved when they approach the moon’s limb, the abovementioned perfons think they may be reafonably accounted for on the fame principles with the tides ; namely, by an accumulation of the aqueous vapours floating in our atmofphere under that part of the heavens where the moon is; and confequently the light of the ftars will be more or lefs refracted, accord¬ ing as it pafies through a larger pr leffer quantity of the accumulated vapour. This laft hypothefis might give fome encouragement to bifliop Wilkins’s fcheme of flying to the moon, which the want of a continued atmofphere between that luminary and the earth would be an effe&ual baragainft : but tho’ we fliould fuppofe the art of flying poffible, and likewife that the air all the way up were fit for fupporting our life, the bifhop’s journey would be rather too long ; for allowing him to fly 60 miles an hour, and to proceed day and night without intermiffion, he muft be five months before he came to his journey’s end, fuppoiing the moon’s mean diftance 340,000 miles, as it is commonly thought to 44 be. Conjcflures With regard to the planets Mercury, Venus, Mars, the planets JuP'ter’ anc^ Saturn, few conjectures have been formed i only that the temperature of the two inferior ones, Mercury and Venus, muft be much hotter, and that of the fuperior ones, Mars, Jupiter, and Saturn, confider- ably colder, than that of the earth. That the latter are much lefs enlightened than this earth, we are abfolutely certain: but whether they are not encompaffed with atmofpheres, which prevent them from differing any D N O M Y. 76r violent excefs of cold, we are entirely ignorant; as well as whether thofe of Mercury and Venus may not dimi- ni/h the heat which at firft would appear to be fo vio¬ lent at the fmall diftances from the fun at which they circulate. Mars, we are certain, has an atmofphere a- bout his body which is capable of refrafting and ab- forbing the light of other ftars; but what its other pro¬ perties are, we have no means of knowing. The fpots on Venus, Mr Dunn informs us, are thought to be feas. The belts obfervable on the fuperior planets have occa- fioned much fpeculation. Some have thought that they were inherent on the furfaces of the planets ; and thus fome philofophers have faid, that greater changes take place on the body of the planet Jupiter, than what would happen to this earth were the ocean and dry¬ land to change places. Others, however, have imagin¬ ed that they are fimilar to our clouds: but their con- ftant appearance feems contradi&ory to fuch a fuppofi- tion; for tho’ it is true that they are but of a tranfitory nature, they are vaftly more permanent than any clouds to be obferved on earth. For this reafou, fome have imagined that they are only certain parts of the atmo¬ fphere of the planet of a different conftitution, and lefs capable of tranfmitting the fun’s light than others. Thefe dark zones, they think, may have arifen from the fuperior planets enjoying a perpetual equinox, the rea- fon of which ihall be given when we come to fpeak of the caufes of the different feafons on earth. The moons of Jupiter and Saturn, and the ring with which the lat¬ ter is furrounded, are thought to be defigned for re- fledting the fun’s light upon thefe planets; and their number is imagined to be fufficient to compenfate in fome meafure for the great diftance at which they are placed from the fun. 4? We cannot conclude this article without taking no- Uncertainty tice, that aftronomers feem not to be agreed whether the appeaf- the belts of Jupiter are really darker or brighter than ance of Ju- the reft of his diflc. In Chambers’s Diftionary, we piter’sbclts. have the following account of them, under the word Fafcia-. “ Fafcite, in aftronomy, two rows of bright fpots, obferved on Jupiter’sbody; appearing like fwaths, or belts. The fafcise or belts of Jupiter are more lu¬ cid than the reft of his dilk, and are terminated by pa¬ rallel lines. They are fometimes broader, and fome- times narrower ; nor do they always poffefs the fame part of the difk. M. Huygens, likewife, obferved a very large kind of fafeia in Mars; but it was darker than the reft of the difk, and took up the middle part thereof.” Moft of the aftronomical writers, at leaft the more common ones, feem to be pretty filent on the fubjedf, but generally incline us to think that they are dark. In Mr Wollafton’s paper already quoted from Phil. Tranf. Vol. LXIV. he mentions both bright and dark belts. With regard to comets, innumerable conjedtures have of Comets, been formed. The ancients in general were of opinion that they were meteors formed in our atmofphere, only of a more permanent fubftance than the common ones ; and that they were figns of the wrath of God : which laft opinion hath been preferved among the vul¬ gar to this day. Some, however, thought otherwife; and afferted them to be a kind of planets that revolved round the fun, but in more extenfive circles than the others : but thefe were few in number; and the general opinion of their being figns of divine wrath, and pre- 762 ASTRO fages of ttrrible calamities to mankind, prevailed till 47 the revival of learning in the 16th century. Kepler and It was not, however, till feme time after people be- Bodin’s opi- gan to throw off the fetters of fuperftition and igno • them.°f rance l°ng held them, that any rational hypothefis was formed concerning comets. Kepler, in other refpedfs a very great genius, indulged the moft extravagant conje&ures, not only concerning comets, but the whole fyftem of nature in general. The pla- nets'he imagined to be huge animals who fwam round the fun by means of certain fins a&ing upon the ethe¬ real fluid, as thofe of fifhes do on the water : and agree¬ able to this notion, he imagined the comets to be mon- ftrous and uncommon animals generated in the celefiial fpaces ; and he explained how the air engendered them by an animal faculty. A yet more ridiculous opinion, if poflible, was that of John Bodin, a learned man of France in the 16th century. He maintained that co¬ mets “ are fpirits, which having lived on the earth in¬ numerable ages, and being at laft arrived on the con¬ fines of death, celebrate their lall triumph, or are re¬ called to the firmament like fhining ftars ! This is fol¬ lowed by famine, plague, &c. becaufe the cities and people deftroy the governors and chiefs who appeafe the wr^th of God.” This opinion, ridiculous as it is, he fays he borrowed from the philofopher Democritus, who imagined them to be the fouls of famous heroes : but this opinion being irreconcileable with Bodin’s Chriflian fentiments, he was obliged to fuppofe them to be a kind of genii, or fpirits fubjedl to death, like thofe fo much mentioned in the Mahometan fables. Others, again, have denied the exiftence of comets, and maintained that they were only falfe appearances occa- fioned by the refradiion or refledlion of light; as if the light could refradt or refledt of itfelf, without any fub- ftance from whence it was fo refledted or refradted. 48 The firft rational conjedlure we meet with is that of Beniqulli’s James Bernoulli, an Italian aftronomer, who imagined ©pinion. them to be the fatellites of fome very diftant planet, which was invifible to us on account of its diftance, as were alfo the fatellites unlefs when in a certain part of their courfe. But though we call this a rational con- jedture, in comparifon of the others, it is neverthelefs very abfurd ; as it fuppofes a fatellite to leave its pri¬ mary planet in darknefs, in order to enlighten-other 49 orbs with which it has nothing to do. 'Opinions of rate aftronomers of England, as Newton, Flamfteed, Halley, 6c. have been perfedtly fatisfied that the comets were a kind of planets which revolved round the fun in very excentric ellipfes ; and have ac¬ cordingly calculated the returns of fome of them, and made conje&ures concerning the ufe they may proba¬ bly be of in the general fyitem of nature. Caflini and fome of the French philolbphers thought this opinion highly probable; but De La Hire and others oppo- fed it. The whole event of the difpute, however, it is . plain, behoved to turn on the obfervation of the return of comets, and the calculation of their periodical times like thofe of other planets; which, whenever it was fully done, behoved to put the matter beyond a doubt : but until this was done, not at once or twice, but a great number of times, there behoved ftill to be an un¬ certainty, let the arguments for their return be fuppo- fed as probable as we pleafe. This Sir Ifaac Newton and Dr Halley attempted to do. Having obferved the N O M Y. Sea. m. | accounts of comets in hiftory, and found fome of them to appear at equal intervals of time, it was concluded that thefe were the fame comets, and would appear again after an interval of equal length. Thus, Sir Ifaac Newton having obferved a comet in the year 1680, of great apparent magnitude ; and found that fuch an one had appeared before, at an interval of 575 years be¬ tween each appearance ; he concluded that thefe were different appearances of the fame comet, and that it would again appear 575 years afterwards. The fame of j celebrated pmloiopher, aiong with Dr Halley and thecomet in! others, calculated the period of the comet which ap- 1684. ! peared in 1682, and found that it ought to appear again in the year 1758, its periodical time being about 754- years. The molt important objedtion that arofe to the return of this comet, was the inequality of its periods, which were as follows : “ That from Auguft W/?. of Co-, 25rh 1531, to the 26th of Odtober 1607, was performed mets>f'6l' 1 in 76 years and two months ; that from Odtober 26th 1607, to September 14th 1682, was rather lefsthan 75 years ; and its lalt period, from the 14th of September 1682, to the 13th of March 1759, which was the longeit of all, was 76 years and fix months, or 27,937 days, amounting to 583 days more than in the pre¬ ceding period. “ Dr Halley was aware of thefe differences, and at firft confeffed himfelf to be a little flaggered by them ; nor would he have had the courage to pronounce its re¬ turn fo pofitively, if hiftory had not informed him, that comets had appeared in 1456, 1380, and 1305, which put their identity out of all doubt. “ The appearances happening alternately in 75 and 76 years, and as the preceding period was only of 75 years, it was natural to fuppofe that the next would amount to 76. But as the difficulties arifing from thefe inequalities in the periods were forefeen and ob¬ viated by Dr Hailey, we cannot do better than to in- fert his own words. “ Perhaps fome may objedt to the diverfity of their Inequalities inclinations and periods, which is greater than what of its mo- is obferved in the revolutions of the fame planet; fee-t!on ac' ing one period exceeded the other by more than the £ou^ed^°r fpace of one year, and the inclination of the comet of iX. r ^ * the year 1682 exceeded that of the year 1607 by 22 entire minutes. But let it be confidered what I men¬ tioned at the end of the tables of Saturn, where it was proved that one period of that planet is fometimes longer than another by 13 days ; and that is evidently occafioned by the force of gravity tending towards the centre of Jupiter, which force indeed in equal diftances is only the 1000th part of that force tending to the fun itfelf, by which the planets are retained in their or¬ bits. But by a more accurate computation, the force of Jupiter upon Saturn, for example, in the great con¬ junction as they call it, January 26, in the year 1683, was found to be to the force of the fun upon the fame Saturn, as 1 to 186; the fum of the forces therefore is to the force of the fun, as 187 to 186. But at the fame diftance from the centre, the periodic times of bodies revolving in a circle are in the fabduplicate ratio of the forces with which they are urged: wherefore the gra¬ vity being increafed by the 186ch part of itfelf, the peri¬ odictime will be ihortened by about the 3 74th part, that is by a whole month in Saturn. How much more is a comet liable to thefe errors, which makes its excurfion near 7^3 ASTRONOMY. Sed. III. near four times higher than Saturn ; and whofe velo¬ city being increafed by lefs than the I io'*1 part of it- felf, would change its elliptic orbit into a parabolic traje&ory ? ‘ But it happened in the fummer of 1681, that the comet feen in the following year, in its defcent towards the fun, was in conjunction with Jupiter in fuch a man¬ ner, and for feveral months fo near him, that during all that time it muft have been urged likewife towards the centre of Jupiter with near the 50th part of that force by which it tended towards the fun: whence, ac¬ cording to the theory of gravity, the arc of the elliptic orbit, which it would have defcribed had Jupiter been abfent, muft be bent inwards towards Jupiter in an hy¬ perbolic form winding, and have affumed a kind of curyg very compounded, and as hitherto not to be ma¬ naged by the geometers; in which the velocity and di¬ rection of the moving body, in proportion to the caufe, would be very different from what it otherwife had been in the ellipfes. ‘ Hence a reafon may be affigned for the change of its inclination: for as the comet in this part of its path had Jupiter on the north almoft in a perpendicular di¬ rection to its path, that portion of its orbit muft be bent towards that quarter ; and therefore its tangent being inclined to a greater angle towards the plane of the ecliptic, the angle of the inclination of the plane it- felf muft be necefiarily increafed. Befides the comet continuing long in the neighbourhood of Jupiter, after it had come towards him from parts much more remote from the fun with a flower motion, and now being ur¬ ged with the joint central forces of both, muft have acquired more accelerated velocity, than it could lofe in its recefs from Jupiter by forces aCting a contrary way, its motion being more fwift, and the time being lefs.’ “ When the comet of 1682 defcended towards the fun and became viiible, Europe had fcarce recovered from the terrible panic into which it had been thrown but 18 months before by the great comet. However, this was comparatively too inconfiderable to be much re¬ garded ; for it was little imagined then, that the leaft of the two would become the moft interefting, and that it would be for ever celebrated by pofterity for having taught mankind how to know all the reft. . But how¬ ever inferior to the other this comet may have appeared in vulgar eyes, aftronomers obferved it with the greateft attention. Hevelius at Dantzick, Kitch at Leipfic, Flamfteed and Halley in England, Zimmerman at Nu- renburg, Baert at Toulon, Montanori at Padua, and Picard, Caflini, and la Hire, at Paris. This lift of names will fuffice to fttew that there can be no fcarcity of good obfervations upon this comet during that ap¬ pearance. “ In 1607 it was obferved by the famous Kepler, who publifhed his obfervations together with his general theory. The C6zh of September old ftyle, the Iky be¬ ing very clear, Kepler firft faw this comet upon the bridge at Prague ; and though it had no tail when he firft difcovered it, yet afterwards it had one of a confi- derable length and fplendour. It was likewife obferved by Longomontanus, September 18. He fays it appear¬ ed as large as Jupiter, though with a very obfcure and pale light; that the tail was pretty long and more denfe than the tails of comets ufually are, but as pale in co¬ lour as the comet itfelf. “ In the preceding revolution of 1531, we find our comet obferved by theaftronomer Appian at Ingoldftadt, the fame who firft remarked that the tails of comets were always in an oppofite direflion to the fun; which to him was an evident proof that the fun was the caufe of fuch eruptions. “ In 1456, there was a very remarkable exhibition of the fame comet. Comet a inaudit ee magnitudinis tot 9 menfe Jvnii cum pralonga cauda, it a ut duo fere fgna coeli comprehenderit., (Theatrum Comet.) “ It is difficult to comprehend how the comet, whofe tail was fo inconfiderable in its laft appearance, fliould in this have one of fixty degrees : but M. de la Lande, in his Theory of Comets, p. 127. accounts for this difference in the following manner. “ I find, fays this aftive aftronomer, that if the comet reached its perihe¬ lion in the beginning of June, it ought to have appear¬ ed at night towards the middle of the month with 60 degrees of elongation and a very northern latitude, its diftance from the earth being lefs than the femidiameter of the fun : fo that in this pofition, which of all others is the moft favourable, it muft have appeared in all the fplendour allowed to it by the old chronicles. Perhaps, by duofgna they only mean the extent of two conftel- lations, which is often much lefs than two figns of the ecliptic.” “ In 1379 and 1380 we find two comets mentioned, by Alftedius and Lubienietzki, but without any par¬ ticulars as to the time or form of their appearance. “ In 1305, our comet again appears, according to the hiftorians of that time, in all its terrors. Cometa bor- rendec magnitudinis vifits. eji circa ferias pafckatis, quem fecuta ejipeflilentia maxima. It is very likely that the horror occafioned by the plague had augmented the terrible impreffion left by the comet; however, upon calculation, it does appear that the comet muft this year have paffed very near the earth. “ The hiftory of this comet might be traced much higher by confulting Eckftormius, Riccioli, Alftedius,. and Lubienietzki. Among the 415 comets mention¬ ed by this laft writer, we find one for the year 1230, which appears to be the very comet in queftion; ano¬ ther 1005, three periods before it is found in 930, and higher up in the year 550, marked by the taking; of Rome by Totila. All the hiftorians of the empire fpeak of a great comet in the year 3'99> which may have been the fame. Cometa fuit prodigiofee magnitu¬ dinis, horribilis afpettu, comam ad ter ram, nfque.demit- tere vifus. “ In 323, that is to fay,.76 years before, a comet al« fo appeared in Virgo and in fhort it would be eafy to mount, without quitting the fame periods, as high as 130 yearsbefore-Chrift, when, according to Juftin, one appeared at the birth of Mithridates. But, in thefe early periods, there would be great danger of meeting with fome of thofe fabulous comets with which it was thought neceffary-perhaps to embellilh every famous reign.” As this comet engaged the attention of the moft ce¬ lebrated aftfonomers more than any other, and as its courfe was calculated by Newton, Halley, Maupertuis, Clairault, De Lifle, Le Monnier, La Caille, Meffier, La Lande, Pingre, &c. who unanimoufly determined that it ought to appear in 1758, it is not to be wonr dered 764 ASTRONOMY. Seft. III. dcrcd that thofe wlio refpcifted thefe illuftrious names, fhould expeft its return in that year, with abfolute cer¬ tainty, and even with no fmall degree of fear, as Dr Halley himfelf had thought that thefe bodies might poflibly ftrike upon the earth in fome of their revolu¬ tions, and occafion its utter deftrudlion, or come fo near as at leaft to occafion terrible calamities to thofe parts which were moft expofed to their malignant influences. A mortal fear accordingly feized the minds of great numbers ; and this panic feems to have been kept up by fome aftronomers of inferior note pro¬ bably with views not entirely difinterefted. Of this the following fentence in a book entitled The Theory of Comets illuftratedy &c. by B. Martin, 1757, is a re¬ markable inftance. “ It is well known to aftronomers how near that dreadful comet of 1680 approached to the earth’s orbit. Alfo the comets of 1472, 1618,1684, and the comet which we now expeft, with many others, pafs fo near the orbit of the earth, that it will not be without reafon if our fears and apprehenfions are conft- derably raifed thereat. However, the reader need not be under any needlefs terror about the return of this comet: for if it appears before the beginning of next May, it can do no harm; as he may be ealily convinced by the view of the comet’s orbit which Ipublifoed fome time St ago” ©ifappoint- The fatal period at length arrived; but no comet ap- ftronomers" Peaict^' This Was a prodigious difappointment to a- in i7j8. ftronomers ; and as they were now in danger of being turned into ridicule, it became abfolutely neceflary to find out fome reafons for the retardation of this comet which had been fo certainly expected. Thefe were happily found out by Mr Clairault; who accounted for its nonappearance, from retardations occafioned by the attra&ion of jhpiter and Saturn, which two planets are Why the found to have an effetft upon each other’s motions, comet did and mull have the fame upon any other body that comes rot then ap- near as pjg]] fui]y explained in the next peai‘ feftion. He found, “ that the action of Jupiter upon the comet, during the whole revolution of 1531 to 1607, had occafioned a diminution of 19 days in its period, which wmuld not have happened by the mere force of the fun; and at the fame time had altered its elements fo as to produce an acceleration of near 31 days in the following period. “ Proceeding afterwards to the revolution from 1607 to 1682, the adtion of Jupiter turns out much more oonfiderable : for it occafions an acceleration of about 420 days, which added to the 31 refulting ffom the a£tion of the fame planet during the preceding period, amounts in all to 451 days of diminution in the time of its period ; which would not have happened merely by its inclination to the fun. “ Now^ if we take the difference of thefe two accele¬ rations, in order to know how much ftiorter the fecond period was than the fir ft, it appears to be 432 days ; which differs only 37 days from the time refulting from the obfervations. “ And this period appears to be (till diminiftied by the action of Saturn. Indeed this diminution is not much, becaufe the effe&s of Saturn’s force are almoft reciprocally deftroyed in the two firft periods. “ Hence we fee that the theory gives within a month the difference fo remarkable between the two known re¬ volutions of this comet. Now if we confider the length of thefe periods, the complication of the two caufes of their irregularity, and the nature of the problem by which they are mtafured; this new demonftration of the Newtonian fyftem will perhaps be found as linking as any one that has hitherto been given. “ By comparing, in like manner, the force of the a&ion of Jupiter, during the fecond period of the co¬ met, with that which will be terminated at its approach¬ ing return ; I find the revolution about which we are at prefent interefted will be 518 days longer than the preceding, occafioned by the a&ion of Jupiter upon the comet, from its laft mean diftance to its perihelion ; that is, for the laft feven or eight years ; an interval, during which there can hardly be more than 15 days alteration. “ As to Saturn, the refult of its aftion on the comet is much more confiderabk compared with the two firft revolutions; for I find the prefent period protradled more than too days by it, independent likewife of its adlion lince 1751, and another fmall objeft which I have not had time to determine. From thefe confide- rations, then, it appears to me, that the expe&ed comet ought to arrive at its perihelion about the middle of the month of April next enfuing.” This period of M. Clairault was found to be fome- A ^rs jJ what too long; for on the 21st of January 1759, a January I comet made its appearance, and was feen at different 17JP. times, to the third of June the fame year. The fol¬ lowing is Mr Meffier’s defcription of it, as viewed April Ist. “ When I fawthis comet again on the firft of April, I could very plainly difcern its tail; but could not afcertain its length, becaufe of the morning twi¬ light, which was then beginning, and foon encreafed much: it filled the field of the telefcope; and muft have extended far beyond : according to what I have obferved, the tail of the comet muft have fpread to more than 25 degrees : the nucleus was confiderable, but not well terminated, and it apparently exceeded the fiz.e of ftars of the firft magnitude ; it was of a pale whitifh colour, not unlike that of Venus. The nebulofity which furrounded the nucleus, and went on leffening, fhewed reddifh colours; and thefe colours grew more vivid towards the brighteft part of the tail. The morning twilight, which increafed apace, foon put an end to thefe appearances, and afterwards made the comet itfelf difappear : however, I had been able to perceive it with the naked eye, when it was fomewhat difengaged from the vapours of the horizon.” On the firft of May it appeared to the naked eye larger than ftars of the firft magnitude, the nucleus being furround¬ ed with a great coma. Its light was but faint, like that of the planets feen thro’ the thick vapours of the horizon. It would have appeared brighter but for the light of the moon. In this laft appearance the comet was in the fextant, or 6o° from the fun, and was ob¬ ferved by moft of the aftronomers of Europe. The triumph of the aftronomers feemed now to be complete; and accordingly Dr Bevis exultingly fays, Phil. Tranf. Vol. LI. “ I think I may now venture to pronounce this to be the fame as the comet of 1682 ; and am about making out its future tra an(i f°r wbat ufe defigned. Sir Ifaac Newton, conjectures confidering the near approach of that of 1680 to the concerning fun, has computed the heat they fometimes undergo to em' be inconceivably great. That one, in particular, he thought to be heated to a degree 2000 times greater than red-hot iron. In confequence of this calculation he naturally imagined that they were bodies of extreme folidity, in order to fuftain fuch an intenlity of heat; arid that, notwithftanding their running out into the vaft regions of fpace, where they were expofed to the mod intenfe degrees of cold, they would hardly be cool on their returning again to the fun. Indeed, ac¬ cording to his calculation, the comet of 1680 muft be forever in a violent ftate of ignition. He hath compu¬ ted that a globe of red-hot iron of the fame dimenfions with the earth would fcarce be cool in 50,000 years. If then the comet be fuppofed to cool 100 times fatter than red-hot iron, as its heat was 2000 times greater, it muft require upwards of a million of years to cool it. In the fhort period of 575 years, therefore, its heat would be in a manner fcarce diminifhed; and there¬ fore, in its next and every fuccceding revolution it muft acquire an increafe of heat; fo that, fmce the creation, having received a proportional addition in every fucceeding revolution, it muft now be in a ftate of ignition very little inferior to that of the fun himfelf. Sir Ifaac hath farther concluded, that this comet muft be confiderably retarded in every revolution by the at- mofphere of the fun within which it enters ; and thus muft continually come nearer and nearer his body, till at laft it falls into it. This, he thinks, may be one ufe of the comets : viz. to futnifii fuel for the fun, which Vot. I. O N O M Y. would otherwife be in danger of wafting, from the con¬ tinual emifiion of its light. As to the tails with which comets are almoft con- His account ftantly attended, Sir Ifaac Newton fliews, that the at- mofpheres of comets will furnifti vapour fufficient to tJys form their tails. This he argues from that wonderful rarefadiion obferved in our air, at a diftance from the earth : a cubic inch of common air, at the diftance of half the earth’s diameter, or 4000 miles, would expand itfelf fo as to fill a fpace larger than the whole region of the ftars. Since then the coma or atmofphere of a cometis ten times higher than the furface of the nucleus, counting from the centre thereof; the tail, afcending much higher, muft; needs be immenfely rare: fo that it is no wonder the ftars Ihould be vifible through it. Now, the afcent of vapours into the tail of the comet, he fuppofes to be occafioned by the rarefadlion of the matter of the atmofphere at the time of the perihelion. Smoke, it is obferved, afcends a chimney by the im- pulfe of the air wherein it floats ; and air, rarefied by heat, afcends by the diminution of its fpecific gravity, taking up the fmoke along with it : why then ftiould not the tail of a comet be fuppofed to be raifed after the fame manner by the fun ? for the fun-beams do not adl on the mediums they pafs through, any otherwife than by refledlion and refradlion. The refledting particles, then, being warmed by the adlion, will again warm the ether wherewith they are compounded ; and this, rarefied by the heat, will have its fpecific gravity, whereby it before tended to de- fcend, diminiftied by the rarefadtion, fo as to afcend, and to carry along with it thofe refledling particles whereof the tail of the comet is compofed. This afcent of the vapours will be promoted by their circular motion round the fun ; by means whereof they will endeavour to recede from the fun, while the fun’s atmofphere,and the other matters in the celeftial fpaces, are either at reft, or nearly fo, as having no motion but what they receive from the fun’s circumrotation. Thus are the vapours raifed into the tails of comets in the neighbourhood of the fun, where the orbits are moft curve; and where the comets, being within the denfer atmofphere of the fun, have their tails of the greateft length. The tails thus produced, by preferving that motion, and at the fame time gravitating towards the fun, will move round his body in ellipfes, in like manner as their heads ; and by this means will ever accompany and freely adhere to their head. In effedl, the gravi¬ tation of the vapours towards the fun will no more occafion the tails of comets to forfake their heads, and fall down towards the fun, than the gravitation of their heads will occafion them to fall off from their tails ; but, by their common gravitation, they will either fall down together to the fun, or be together fufpended or retarded. This gravitation, therefore, does not at all hinder, but that the heads and tails of comets may receive and retain any pofition towards each other, which either the above-mentioned caufes, or any other, may occafion. The tails therefore, thus produced in the perihelion of comets, will go off along with their head into re¬ mote regions ; and either return thence, together with the comets, after a long feries of years ; or rather be there loft, and vanilh by little and little, and the comet 5 D be 766 ASTRO be left bare ; till, at its return, defcending towards the fun, fome little fliort tails be gradually and (lowly pro¬ duced from the heads ; which afterwards, in the peri¬ helion, defcending down into the fun’s atmofphere, will be immenfely increafed. The vapours thus dilated, rarefied, and diffufed thro’ all the celeftial regions, the fame author obferves, may probably, by little and little, by means of their own gravity, be attrafted down to the planets, and become intermingled with their atmofpheres. He adds, that for the confervation of the water and moifture of the planets, comets feem abfolutely requifite ; from whofe condenfed vapours and exhala¬ tions all that moifture which is fpent in vegetations and putrefa&ions, and turned into dry earth, &c. may be refupplied and recruited. For all vegetables grow and increafe wholly from fluids ; and, again, as to . their greateft part, turn by putrefa&ion into earth, an earthy (lime being perpetually precipitated to the bottom of putrefying liquors. Hence, the quantity of dry earth mu ft continually increafe, and the moifture of the globe decreafe, and at laft be quite evaporated ; if it have not a continual fupply from fome part or other of the univerfe. And I fufpe&, adds our great author, that the fpirit, which makes the fineft, fubtileft, and beft part of our air, and which is abfolutely requifite for the life and being of all things, comes principally from the comets. On this principle, there feemstobe fome foundation for the popular opinion of prefages from comets : fince the tail of a comet, thus intermingled with our atmo¬ fphere, may produce changes very fenfible in animal j8 and vegetable bodies. Controvert- This account of the formation of the tails of comets, ed by Dr we find controverted by Dr Hamilton of Dublin, in a Hamilton, fmall treatife intitled Conjectures on the nature of the Aurora Borealis, and on the tails of Comets. His hy- pothefis is, that the comets are of ufe to bring back the ele&ric fluid to the planets, which is continually difcharged from the higher regions of their atmo¬ fpheres. Having given at length the abovementioned opinion of Sir Ifaac, “ We find (fays he) in this ac¬ count, that Sir Ifaac afcribes the afcent of comets tails to their being rarer and lighter, and moving round the fun more fwiftly, than the folar atmofphere, with which he fuppofes them to be furrounded whilft in the neigh¬ bourhood of the fun ; he fays alfo, that whatever po- fition (in refpeft to each other) the head and tail of a comet then receive, they will keep the fame after¬ wards moft freely; and in another place he obferves, ‘ That the celeftial fpaces muft be entirely void of any power of refitting, fince not only the folid bodies of the planets and comets, but even the exceeding thin vapours of which comets tails are formed, move thro’ thofe fpaces with immenfe velocity, and yet with the greateft freedom.’ I cannot help thinking that this account is liable to many difficulties and objections, and that it feems not very confiftent with itfelf or with the phenomena. “ I do not know that we have any proof of the ex- iftence of a folar atmofphere of any confiderable ex¬ tent, nor are we any where taught how to guefs at the limits of it. It is evident that the exiftence of fuch an atmofphere cannot be proved merely by the afcent of comets tails from the fun, as that phenomenon may N O M Y. Sect. III. poffibly arife from fome other caufe. However, let us fuppofe, for the prefent, that the afcent of comets tails is owing to an atmofphere furrounding the fun, and fee how the effe&s arifing from thence will agree with the phenomena. When a comet comes into the folar atmofphere, and is then defcending almott direft- ly to the fun, if the vapours which compofe the tail are raifed up from it by the fuperior denfity and weight of that atmofphere, they muft rife into thofe parts that the comet has left, and therefore at that time they may ap¬ pear in a dire&ion oppofite to the fun. But as foon as the comet comes near the fun, and moves in a direction nearly at right angles with the direction of its tail, the vapours which then arife, partaking of the great velo¬ city of the comet, and being fpecifically lighter than the medium in which they move, and being' vaftly ex¬ panded through it, muft neceflarily fuffer a refiftance immenfely greater than what the fmall and denfe body of the comet meets with, and confequently cannot pof¬ fibly keep up with it, but muft be left behind, or, as it were, driven backwards by the refiftance of that me¬ dium into a line diredted towards the parts which the comet has left, and therefore can no longer appear in a direClion oppofite to the fun. And, in like manner, when a comet pafles its perihelion, and begins to afcend from the fun, it certainly ought to appear ever after with its tail behind it, or in a direction pointed to¬ wards the fun; for if the tail of the comet be fpecifi¬ cally lighter than the medium in which it moves with fo great velocity, it muft be juft as impoffible it ftiould move foremoft, as it is that a torch moved fwiftly thro’ the air Ihould projeCt its flame and fmoke before it. Since therefore we find that the tail of a comet, even when it is afcending from the fun, moves foremoft, and appears in a direction nearly oppofite to the fun, I think we muft conclude that the comet and its tail do not move in a medium heavier and denferthan the mat¬ ter of which the tail confifts, and confequently that the conftant afcent of the tail from the fun muft be ow¬ ing to fome other caufe. For that the folar atmofphere ftiould have denfity and weight fufficient to raife up the vapours of a comet from the fun, and yet not be able to give any fenfible refiftance to thefe vapours in their rapid progrefs through it, are two things inconfiftent with each other. And therefore, fince the tail of a co¬ met is found to move as freely as the body does, wc ought rather to conclude that the celeftial fpaces are voidof all refilling matter, than that they are filled with a folar atmofphere, be it ever fo rare. “ But there is, I think, a further confideration which will Ihew, that the received opinion, as to the afcent of comets tails, is not agreeable to the pheno¬ mena, ancl may at the fame time lead us to fome know¬ ledge of the matter of which thefe tails confift; which I fufpeCl is of a very different nature from what it has been hitherto fuppofed to be. Sir Ifaac fays, the va- ours, of which the tail of a comet confifts, grow hot y refle&ing the rays of the fun, and thereby warm and rarefy the medium which furrounds them; which muft therefore afcend from the fun, and carry with it the refle&ing particles of which the tail is formed; for he always fpeaks of the tail as Alining by refle&ed light. But one.would rather imagine, from the phe¬ nomena, that the matter which forms a comet’s tail has pot the leaft fenfible power of reflefting the rays of ■67 Sea. III. A S T R light. For it appears from Sir Ifaac’s obfervation, which I have quoted already, that the light of the fmalleft ftars, coming to us through the immenfe thick- nefs of a comet’s tail, does not fuffer the leaft diminu¬ tion. And yet, if the tail can refledl the light of the fun fo copioufly, as it muft do if its great fplendour be ow¬ ing to fuch refledfion, it muft undoubtedly have the fame effedt on the light of the ftars; that is, it muft refledf back the light which comes from the. ftars be¬ hind it, and by fo doing muft intercept them from our fight, confidering its vaft thicknefs, and how exceed¬ ingly flender a ray is that cofties from a fmall ftar ; or if it did not intercept their whole light, it muft at leaft increafe their twinkling. But we do not find that it has even this fmall effect, for thofe ftars that appear through the tail are not obferved to twinkle more than others in their neighbourhood. Since therefore this fadt is fupported by obfervations, what can be a plainer proof that the matter of a comet’s tail has no power of refledting the rays of light ? and confequently that it muft be a felf-lhining fubftance. But the fame thing will further appear, from confidering that bodies refledt and refradt light by one and the fame power; and therefore if comets tails want the power of refradting the rays of light, they muft alfo want the power of re¬ fledting them. Now, that they want this refradting power appears from hence : If that great column of tranfparent matter which forms a comet’s tail, and moves either in a vacuum, or in fome medium of a dif¬ ferent denfity from its own, had any power of refrac¬ ting a ray of light coming through it from a ftar to us, that ray muft be turned far out of its way in paf- fiog over the great diftance between the comet and the earth ; and, therefore, we fhould very fenfibly perceive the fmalleft refraction that the light of the ftars might fuffer in palling through a comet’s tail. The confe- quence of fuch a refradtion muft be very remarkable: the ftars that lie near the tail would, in fome cafes, ap¬ pear double; for they would appear in their proper pla¬ ces by their diredt rays, and we Ihould fee their images behind the tail, by means of their rays which it might refradt to our eyes; and thofe ftars that were really be¬ hind the tail would difappear in fome fituations, their rays being turned afide from us by refradfion. In Ihort, it is eafy to imagine what ftrange alterations would be made in the apparent places of the fixed ftars by the tails of comets, if they had a power of refrac¬ ting their light, which could not fail to be taken no¬ tice of, if any fuch ever happened. But fince aftrono- mers have not mentioned any fuch apparent changes of place among the ftars, I take it for granted that the ftars feen through all parts of a comet’s tail appear in their proper places, and with their ufual colours, and confequently I infer that the rays of light fuffer no re¬ fraction in palling through a comet’s tail. And thence I conclude (as before) that the matter of a comet’s tail has not the power of refradting or refledting the rays of light, and muft therefore be a lucid or felf-lhining fubftance.” 59 But, whatever probability the Dodtor’s conjedture account0Je concein*nS t^e materials whereof the tails are formed feuded. C" may ^ave **> his criticifm on Sir Ifaac Newton’s ac¬ count of them feems not to be juft : for that great phi- lofopher fuppofes the comets to have an atmofphere pe¬ culiar to themfelves; and confequently, in their neareft O N O M Y. approaches to the fun, both comet and atmofphere are immerfed in the atmofphere of that luminary. In this cafe, the atmofphere of the comet being prodigioully heated on the fide next to the fun, and confequently the equilibrium in it broken, the denfer parts will con¬ tinually pour in from the regions fartheft from the fun; for the famereafon, the more rarefied part which is be¬ fore, will continually fly off oppofite to the fun, being difplaced by that which comes from behind ; for tho’ we muft fuppofe the comet and its atmofphere to be heated on all fides to an extreme degree, yet ftill that part which is fartheft from the fun will be lefs hot, and confequently more denfe, than what is neareft to his bo¬ dy. The confequence of this is, that there muft be a conftant ftream of denfe atmofphere defcending to¬ wards the fun, and another ftream of rarefied vapours and atmofphere afcending on the contrary fide; juft as, in a common fire, there is a conftant ftream of denfe air defcending, which pulhes up another of rarefied air, flame, and fmoke. The refiftance of the folar atmofphere may indeed be very well fuppofed to occa- fion the curvature obfervable in the tails of comets, and their being better defined in the fore part than be¬ hind ; and this appearance we think Dr Hamilton’s Dr Hamil- hypothefis is incapable of folving. We grant, that ton’s hypo- there is the utmoft probability that the tails of comets infuf’ are ftreams of eledtric matter; but they who advance a theory of any kind ought to folve every phenome¬ non, otherwife their theory is infufficient. It was in¬ cumbent on Dr Hamilton, therefore, to have explained how this ftream of eledtric matter comes to be bent into a curve, and alfo why it is better defined and brighter on the outer fide of the arch than on the in¬ ner. This indeed he attempts, in the following man¬ ner : “ But that this curvature was not owing to any refilling matter appears from hence, that the tail muft be bent into a curve though it met with no refiftance; for it could not be a right line, unlefs all its particles were projedled in parallel diredlions, and with the fame velocity, and unlefs the comet moved uniformly in a right line. But the comet moves in a curve, and each part of the tail is projedled in a-diredlion oppofite to the fun, and at the fame time partakes of the njotion of the comet; fo that the different parts of the tail muft move on in lines which diverge from each other; and a line drawn from the head of a comet to the ex¬ tremity of the tail will be parallel to a line drawn from the fun to the place where the comet w'as when that part of the tail began to afcend, as Sir Ifaac obferves; and fo all the chords, or lines drawn from the head of the comet to the intermediate parts of the tail, will be refpedlively parallel to lines drawn from the fun to the places where the comet was when thefe parts of the tail began to afcend. And therefore, fince thefe chords of the tail will be of different lengths, and parallel to different lines, they muft make different a'ngles, with a great circle palling through the fun and comet, and confequently a line palling through their extremities will be a curve. “ It is obferved, that the convex fide of the tail which, is turned from the fun is better defined, and Ihines a little brighter, than the concave fide. Sirlfaac accounts for this, hy faying, that the vapour on the convex fide is frelher (that is, has afcended later) than that on the concave fide; and yet I cannot fee how the particles on 5 D 2 the 768 ASTRO the convex fide can he thought to have afcended later than thofe on the concave fide which may be nearer to the head of the comet. I think it rather looks as if the tail, in its rapid motion, met with fome flight re¬ finance juft fufficient to caufe a fmall condenfation in that fide of it which moves foremoft, and which would occafion it to appear a little brighter and better defined than the other fide ; which flight refiftance may arife from that fubtfle ether which is fuppofed to be dif- perfed through the celeftial regions, or from this very ele£tric matter difperfed in the fame manner, if it be different from the ether.” On the la ft part of this obfervation we muft remark, that though a flight refiftance in the etherial medium would have ferved Sir Ifaac Newton’s turn, it will by no means ferve Dr Hamilton’s; for though a ftream of water or air may be eafily deftroyed or broken by re¬ fiftance, yet a ftream of ele&ric matter feems to fet every obftacle at defiance. If a (harp needle is placed on the conduftor of an ele&ric machine, and the machine fet in motion, we will perceive a fmall ftream of ele&ric matter iffuing from the point; but though we blow a- gainft this ftream of fire with the utmoft violence, it is impoffible either to move it, or to brighten it on the fide againft which we blow. If the celeftial fpaces then are full of a fubtile ether capable of thus affefting a ftream of ele&ric matter, we may be fure that it al- fo will refift very violently ; and we are then as much difficulted to account for the proje&ile motion conti¬ nuing amidft fuch violent refiftance; for if the ether refifts the tail of the comet, it is impoffible to prove «i that it doth not refift the head alfo. Electric This obje&ion may appear to fome to be but weak- matter not jy. founded as we perceive the eledlric fluid to be en- five^5 Pa " ^owe and the univerfal power of attraction of the fame forces or powers. which draws them off from thefe lines, the curvilineal “ A double projetile force will always balance a PlateXLlV. motions of all the planets arife. If the body A be quadruple power of gravity. Xjet the planet at B have % 3* projeted along the right line A B X, in open fpace, twice as great an impulfe from thence towards X, as it where it meets with no refidauce, and is not drawn a- had before ; that is, in the fame length of time that it fide by any other power, it will for ever go on with was projected from B to b, as in the lad example, Jet the fame velocity, and in the fame direction. For the it now be projected from B to c; and it will require force which moves it from A to B in any given time, four times as much gravity to retain it in its orbit: will carry it from B to X in as much more time, and that is, it mud fall as far as from B to 4 in the time fo on, there being nothing to obflruCt or alter its mo- that the projeCtile force would carry it from B to c; tion. But if, when this projeftile force has carried it, otherwife it could not defcribe the curve BD, as is evi- fuppofe to B, the body S begins to attraft it, with dent by the figure. But, in as much time as the pla- a power duly adjuded, and perpendicular to its motion net moves from B to C in the higher part of its orbit, at B, it will then be drawn from the draight line it moves from I to K, or from K to L, in the lower A B X, and forced to revolve about S in the circle part thereof; becaufe, from the joint aCtion of thefe B Y T U. When the body A comes to U, or any two forces, it mud always defcribe equal areas in equal other part of its orbit, if the fmall body «, within the times throughout its annual courfe. Thefe areas are fphere of U’s attraction, be projected as in the right reprefented by the triangles BSC, CSD, DSE, ESF, line Z, with a force perpendicular to the attraction of &c. whofe contents are equal to one another, quite U, then u will go round U in the orbit W, and ac- round the figure. company it in its whole courfe round the body S. Here, “ As the planets approach nearer the fun, and recede S may reprefent the fun, U the earth, and u the moon, farther from him, in every revolution } there may be “ If a planet at B gravitates, or is attraCled, toward fome difficulty in conceiving the reafon why the power the fun, fo as to fall from B toy, in the time that the of gravity, when it once gets the better of the pro- projeCIile force would have carried it from B to X, it jeftile force, does not bring the planets nearer and will defcribe the curve B Y by the combined a&ion of nearer the fun in every revolution, till they fall upon thefe two forces, in the fame time that the projeftile and unite with him; or why the projeftile force, when force fmgly would have carried it from B to X, or the it once gets the better of gravity, does not carry gravitating power fmgly have caufedit to defeend from the planets farther and farther from the fun, till it re- B toy; and thefe two forces being duly proportion- move them quite out of the fphere of his attraction, ed, and perpendicular to each other, the planet obey- and caufe them to go on in ftraight lines for ever af- ing them both will move in the circle B Y U. To terward. But by confidering the efteCts of thefe powers make the projeCtile force balance the gravitating power as deferibed in the two lall articles, this difficulty will fo exaCtly, as that the body may move in a circle, the be removed. Suppofe a planet at B to be carried by projeCtile velocity of the body muff be fuch as it would the projeCtile force as far as from B to b, in the time have acquired by gravity alone in falling through half that gravity would have brought it down from B to 1: the radius of the circle. by thefe two forces it wfill defcribe the curve B C. “ But if, whilft the projeCtile force would carry the When the planet comes down to K, it will be but half planet from B to b, the fun’s attraction (which confti- as far from the fun S as it was at B; and therefore, by tutes the planet’s gravitation) fhould bring it down gravitating four times as ftrongly towards him, it would from B to 1, the gravitating power would then be too fall from K to V in the fame length of time that it ftrong for the projeCtile force, and would caufe the would have fallen from B to 1 in the higher part of its planet to defcribe the curve B C. When the planet orbit, that is, through four times as much fpace; but comes to C, the gravitating power (which always in- its projeCtile force is then fo much increafed at K, as creafes as the fquare of the diitance from the fun S di- would carry it from K to C in the fame time ; being minifhes) will be yet ftronger for the projeCtile force; double of what it was at B; and is therefore too ftrong and, by confpiring in fome degree therewith, will acce- for the gravitating power, either to draw the planet to Jerate the planet’s motion all the way from C to K; the fun, or caufe it go round him in the circle K/w», caufing it to defcribe the arcs B C, C D, D E, E F, &c. which would require its falling from K to u, thro’ Vol. I. 5 E a 774 ASTRONOMY. Sed. IV. i a greater fpace than gravity can draw it, whilft the pro- jedlile force is fuch as would carry it from K to /h and therefore the planet afcends in its orbit KLMN, de- creafing in its velocity for the caufes already afligned. “ By the above-mentioned law, bodies will move in all kinds of ellipfes, whether long or fhort, if the fpaces they move in be void of refiftance. Only, thofe which move in the longer ellipfes, have fo much the lefs proje&ile force impreffed upon them in the higher parts of their orbits ; and their velocities, in coming down towards the fun, are fo prodigioufly in- creafed by his attraftion, that their centrifugal forces in the lower parts of their orbits are fo great as to overcome the fun’s attra&ion there, and caufe them to afcend again towards the higher parts of their orbits; during which time, the fun’s attradlion afting fo con¬ trary to the motions of thofe bodies, caufes them to move flower and flower, until their projectile forces are diminilhed almoft to nothing ; and then they are 85 brought back again by the fun’s attraClion, as before. Gravity and “ The fun and planets mutually attraft each other : projectile pOWer by which they do fo we call gravity. But counted "for whether this power be mechanical or not, is very much ' difputed. Obfervation proves, that the planets difturb one another’s motions by it; and that it decreafes ac¬ cording to the fquares of the diftances of the fun and planets; as light, which is known to be material, like- wife does. Hence gravity fhould feem to arife from the agency of fome fubtle matter preffing towards the fun and planets, and a&ing, like all mechanical caufes, by contaft. But, on the other hand, when we coniider that the degree or force of gravity is exa&ly in pro¬ portion to the quantities of matter in thofe bodies, without any regard to their bulks or quantities of fur- face, afting as freely on their internal as external parts ^ it feems to furpafs the power of mechanifm ; and to be either the immediate agency of the Deity, or ef- fe&ed by a law originally eftablifhedand imprefhdon all matter by him. But fome affirm that matter, being altogether inert, cannot be impreffed with any law even by almighty power ; and that the Deity, or fome fub- ordinate intelligence, muft therefore be conftantly im¬ pelling the planets toward the fun, and moving them with the fame irregularities and difturbances which gra¬ vity would caufe, if it could be fuppofed to exift. But, if a man may venture to publifh his own thoughts, it feems to me no more an abfurdity, to fuppofe the Dei¬ ty capable of infufing a law, or what laws he pleafes, into matter, than to fuppofe him capable of giving it exiflence at firft. The manner of both is equally in¬ conceivable to us; but neither of them imply a contra- di&ion in our ideas, and what implies no contradiftion is within the power of Omnipotence. “ That the projeftile force was at firft given by the Deity, is evident. For, fince matter can never put itfelf in motion, and all bodies may be moved in any dire&ion whatfoever; and yet the planets, Ijoth pri¬ mary and fecondary, move from weft to caft, in planes nearly coincident; whilft the comets move in all di¬ rections, and in planes very different from one another ; thefe motions can be owing to no mechanical caufe or neceffity, but to the free will and power of an intelli¬ gent Being. Mg “ Whatever gravity be, it is plain that it aCls every moment of time: for if its aCtion fhould ceafe, the projeftile force would inftantly carry off the planets in ftraight lines from thofe parts of their orbits where gravity left them. But, the planets being once put into motion, there is no occafion for any new projeClile force, unlefs they meet with fome refiftance in their orbits ; nor for any amending hand, unlefs they difturb one another too much by their mutual attractions. “ It is found that there are difturbances among the p}gner7s jj. planets in their motions, ariling from their mutual at- (hirh one traCtions when they are in the fame quarter of the hea- anothers vens ; and the beft modern obfervers find that our moti°ns* years are not always precifely of the fame length (a). Befides, there is reafon to believe that the moon is fome- what nearer the earth now than fhe was formerly; her periodical month being fhorter than it was in former ages. For our aftronomical tables, which in the pre- fent age fhew the times of folar and lunar eclipfes to great precifion, do not anfwer fo well for very ancient eclipfes. Hence it appears, that the moon does not move in a medium void of all refiftance; and there¬ fore her projeClile force being a little weakened, whilft there is nothing to diminifh her gravity, fhe muft be gradually approaching nearer the earth, deferibing fmaller and fmaller circles round it in every revolution, and finifhing her period fooner, although her abfolute motion with regard to fpace be not fo quick now as it was formerly: and, therefore, fhe muft come to the earth at laft ; unlefs that Being which gave her a fuffi- cient projeCtile force at the beginning, adds a little gg more to it in due time. And, as all the planets move Muft necef- in fpaces full of ether and light, which are material farify ap- fubftances, they too muft meet with fome refiftance. Proach tlie And therefore, if their gravities are not diminifhed,