of the eclipse of the fun, October 25, 1772; of occultations of ftars by the moon; of eclipfes and occultations of Jupiter's fatellites, tranfits over his difc, conjunctions of the fatellites and appearances of his belts. Article 14. An Inquiry into the Quantity and Direction of the proper Motion of Arcturus: with fome Remarks on the Diminution of the Obliquity of the Ecliptic: By Thomas Hornsby, M. A. Savilian Profeffor of Aftronomy, at Oxford; and F. R. S. It has been generally apprehended, from a comparison of antient with the best modern obfervations, that some of the fixed ftars have a proper motion of their own, independent of any motion hitherto known in our fyftem; or, in other words, that the angular distances of the fixed ftars have not always continued the fame; and this variation has been most remarkable in the place of Ar&turus. Mr. Caffini, in the memoirs of the Academy of Sciences for 1738, p. 231, has fhewn, that there. is a variation of five minutes in the latitude of this ftar in the fpace of a century and a half, between his own time and that of Tycho; and Monf. le Monnier, in the memoirs of the Academy of Sciences for 1767, p. 417, proves, that the latitude of Arcturus varies at the rate of two feconds every year; and that the longitude decreases at the rate of 60" in a hundred years. This article contains a very elaborate and accurate inquiry into the quantity and direction of this motion, founded on a series of obfervations made with this view in the year 1767 and 1768 and compared with thofe of Mr. Flamstead in 1690. It appears from the refult of thefe obfervations, that the proper motions of Arcturus, for this period of 18 years, have been weftward in right afcenfion=1′ 33′′, 974, and 2′ 36′′, 81 in declination fouthward; and therefore that the real motion of Arcturus is inclined in an angle of 30°. 56′ to the weft of the meridian, or horary circle, and that its velocity is at the rate of 32", 81 in 78 years, or at the rate of 2", 343 in a year: and as the direction of this motion is nearly perpendicular to the plane of the ecliptic, the latitude of Arcturus must diminish yearly almoft in the fame proportion; and its longitude will alter lefs than that of the other stars, though not fo confiderably as its right afcenfion. It appears from hence, that the prover annual motion of Arcturus is 1", 205 in right afcenfion weftward, and 2′′, co5 in declination, and therefore its yearly preceffion in right afcenfion will be 41", 108, and in declination 19", 133; fo that the true right afcenfion of Arcturus on Jan. 1, 1773 is 211°. 19′ 47′′, 4, and declination north 20° 22:23, 3. " As this motion is the most confiderable in Arcturus, though not peculiar to it, Mr. Hornsby infers, that this is the nearest ftar to our fyftem vifible in this hemifphere: and if the annual parallax of the fixed stars can ever be difcovered, it is moft 3 likely likely to be derived from obfervations of Arcturus. He attributes the apparent change in fituation vifible from the planet which we inhabit, either to the motion of our own Syftem in abfolute space, or, if this is at reft, to a real motion in the stars themselves; fo that their angular diftances muft vary in proportion to the velocity or direction of these motions with refpect to ourselves. And there is reafon to expect, that the fmaller motions which are obfervable in other ftars will be gradually discovered by the industry and accuracy of modern obfervers. These obfervations must be of great importance in refolving a queftion, which has been the fubject of debate among modern aftronomers. It has been generally apprehended, that the obliquity of the ecliptic has been continually diminishing; but Mr. Caffini and Monf. le Monnier have ftrenuoufly maintained the contrary opinion, or at leaft that the quantity of its diminution has not been fo confiderable, as others have imagined. The ingenious Author of this article has fhewn, that the method they made ufe of to ascertain the fact is by no means fo accurate as it fhould have been. They have compared the altitude or zenith diftance of the fun's limb with Arcturus, without previously fettling the quantity of that ftar's motion in declination. This, it is plain, mult be determined and allowed for, before we can precifely obtain the quantity of the fun's ap proach to or recefs from any ftar nearly in the fame parallel at the time of the folftice, when the neceflary obfervations are to be made. The fequel of this paper contains a variety of obfervations for determining this quantity, together with the conclufions to be drawn from them: all which plainly indicate a decrease in the obliquity required. "By comparing (fays Mr. Hornby) the obfervations at the fummer folftices of 1771 and 1772 with those at the winter folftice of 1771, it appears that the mean obliquity was about the beginning of the year 177223°. 28′ 9′′, 4 and 23°. 28′ 8′′. I fuppofe therefore the mean obliquity to be 23°. 28 8" at the beginning of the prefent year: and confequently, the obliquity has diminished, by my obfervations, 47" in 81 years, fince Mr. Flamsteed's time, or at the rate of 58" in 100 years, a quantity which will be found nearly at a mean of the computations framed by Mr. Euler and Mr. de la Lande, upon the principles of attraction." Article 25. A Letter to the Rev. Nevil Mafkelyne, Aftronomer Royal, from Mr. Bailly, of the Royal Academy of Sciences at Paris: containing a propofal of fome new Methods of improving the Theory of Jupiter's Satellites. The Writer of this letter propofes to the Aftronomer Royal, in the most respectful and obliging terms, a very curious method REV. May 1774. A a of of measuring the diameters of Jupiter's fatellites, and of faci 1.tating the comparison of thole obfervations that are made in different places and with different inftruments. These two objects are of great importance towards perfecting the theory of this part of the folar fyftem. The obfervations of the eclipses of the third and fourth fatellite, made by different aftronomers, vary from each other feveral minutes; nor is the difference in those of the fecond infenfible. This inequality in the moment of the eclipfes depends on four different caufes, which M. Bailly has reduced to one principle, and fhewn how to adjust and obviate. It is to be confidered, that when any fatellite difappears, there is a fegment of its dife which remains uneclipfed and this fegment varies in the proportion of the fquares of the diftances of Jupiter from the fun and from the earth: which is the firft caufe of the inequality. Befide this, it has been discovered, that the light of the fatellite decreases in proportion to the proximity of Jupiter's difc.; the brightness of the planet weakening that of the fatellite and rendering those eclipses which happen near its oppofition to the fun apparently defective. To which may be likewife added, that the light of Jupiter and his fatellites is weaker or ftronger, according to their different elevations above the horizon; and whenever the planet is near the horizon, and confequently the light lefs, the uneclipsed or infenfible fegment increafes and occafions another inequality in the moment of eclipfes. And the aperture or power of the telescopes, which, as it is greater or lefs, gives more or less light, contributes to the variation of this fegment. On these four caufes depends the magnitude of the fore mentioned fegment. In order to determine the quantity of this fegment and confequently its verfed fine, our Author confidered, that, when the fatellite difappears, it is owing to the diminution of its light; and therefore contrived to imitate what happens in eclipfes, by di minifhing the light. For this purpose he applied to the outfide of the object glafs of an achromatic telefcope, 5 feet in length and of 24 lines aperture, fome diaphragms of pafteboard, the opening of which gradually leffened by half lines from 24 lines to 3. He then tried, whether fome of thefe might not be found, that would cause the fatellite to difappear; and he received great fatisfaction from the fuccefs of thefe experiments. At one time, an opening of 3 lines caufed the third fatellite to disappear, and an aperture of 6 lines produced the fame effect in the firft. And as the quantity of light in telescopes is proportional to the fquares of the apertures, he hence inferred, that the 6th part of the light of the 3d fatellite and the 16th of the first, were infenfible: and therefore in the moment of an eclipfe, the invifible fegment of the latter would be a 16th part of the dife and that of the former a 64th part, whence he was able to com5 pute ་ 1 pute the verfed fines of thefe fegments. Mr. B. then proceeds to estimate the variations to which these fegments are liable from the feveral causes above recited; and he has formed tables or deduced algebraic formula, by means of which these variations may be known. In order to determine the radius of the difc of the fatellite he finds, in the manner already propofed, the proportion which the invifible fegment bears to the whole difc of the fatellite; and having found the fize of that diaphragm which makes the fatellite difappear, he applies to his telescope a piece of pasteboard fomewhat larger, fo that he might juft perceive the fatellite, but of fuch a fize, that the leaft farther diminution of light would render it invifible. He marks the exact moment of its difappearance, and taking away the diaphragm counts the number of feconds that elapfe before the true immerfion; this gives him a great part of the diameter, from which he cafily computes the whole. This method is farther illuftrated by a figure and two examples. M. Bailly obferves, that by the tables he has conftructed, it will be poffible to compute the invifible fegment for all the obfervations, which have hi'therto been made; and, the fatellite's diameter being known, to reduce the inftant of the observed eclipfe to that of the paffage of the centre over the edge of the fhadow, which will be a fixed term for all the obfervations and all the obfervers, who but feldom agree in their accounts of the fame eclipse: And this method is not liable to the inconveniences and errors that arife from the different tranfparency of the air, and the inequality of fight in different obfervers or in the fame obferver at different times. From the agreement of fundry obfervations in various parts of the world great advantages would arife for perfecting the theory of the fatellites and the precifion of terrestrial longitudes. We have endeavoured to give our philofophical Readers as clear an idea as we could of the main fcope of this curious article: but for a more full and accurate acquaintance with the method here propofed for determining the diameters of the fatellites, and adjufting the different oblervations of their eclipfes, we must refer to the Author's diagrams, and to his own account in the paper itself. [To be continued.] R-B. ART. V. The Poetical Works of the late William Dunkin, D. D. To which are added, his Epistles, &c. to the late Earl of Chesterfield. 4to. 2 Vols. 1 1. I s. Nicol, &c. 1774.. D R. Dunkin, though not a poet, was a tolerable cutter of verses, and, operating on the fundry materials of Greek, Latin, and English, exhibited his goods in a variety of forms. A a 2 He He would, for inftance, take a fubject, work it up into a Greek poem, afterwards tranflate it into a Latin poem, then into Englith blank verfe, and laft of all into heroic rhyme; fo that he gave you the fame joint dreffed in four different ways, and you might feed upon that which beft fuited your palate. Yet, notwithstanding this uncommon affiduity to please, we fall into languor over the greatest part of the Doctor's labours, and fuffer under a kind of involuntary ingratitude. To divert thefe fentiments, we turn to the idea of his life and character, and enjoy the remembrance of a man who had wit and humour enough to recommend him to the familiar hours of Swift and Pope, the literary demi-gods of their day. Swift, too, was a verse cutter, like Dunkin; only with this difference, that the former put his hard-ware out of his hand more highly finished, and worked with sharper and finer tools. George Faulkner, the famous Dublin printer, the butt of all the wits of his time, makes a principal figure in Dr. Dunkin's poems: and as that hero is ftill living and well known, we can think of no extract that may be more entertaining to our Readers than that paffage in the poem, entitled, the Parfon's Revels, where he makes his appearance: The Prince of Printers, whom we dub Sir George, and Emperor of Grub, |