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 System 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.

Thefe 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 ftrenuously maintained the contrary opinion, or at least 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 use of to ascertain the fact is by no means fo accurate as it should 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, muft be determined and allowed for, before we can precifely obtain the quantity of the fun's approach to or recefs from any ftar nearly in the fanie parallel at the time of the folftice, when the necessary obfervations are to be made.

The fequel of this paper contains a variety of observations 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. Hornfby) the obfervations at the fummer folftices of 1771 and 1772 with thofe at the winter folftice of 1771, it appears that the mean obliquity was about the beginning of the year 1772=23°. 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. Flamfteed's time, or at the rate of 58" in roo years, a quantity which will be found nearly at a mean of the compu-. tations 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 proposes to the Aftronomer Royal, in the most respectful and obliging terms, a very curious method REV. May 1774.

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of

of meafuring the diameters of Jupiter's fatellites, and of faci 1.tating the comparifon of thofe 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 ecliples 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 difc 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 eclipfes 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 increases 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 segment 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 telescope, 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 these might not be found, that would cause the fatellite to disappear; 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 infenfiole: and therefore in the moment of an eclipse, the invifible fegment of the latter would be a 16th part of the difc and that of the former a 64th part, whence he was able to com

pute the verfed fines of these fegments. Mr. B. then proceeds to eftimate the variations to which thefe fegments are liable from the feveral caufes 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 invisible segment bears to the whole dife of the fatellite; and having found the fize of that diaphragm which makes the fatellite difappear, he applies to his telescope à 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 eafily 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 invisible segment for all the obfervations, which have hitherto been made; and, the fatellite's diameter being known, to reduce the inftant of the obferved eclipse 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 eclipfe: And this method is not liable to the inconveniences and errors that arise 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 obfervations of their eclipfes, we must refer to the Author's diagrams, and to his own account in the paper itself.

[To be continued.]

ART. V. The Portical Works of the late William Dunkin, D.D. To which are added, his Epistles, &c. to the late Earl of Chesterfield. 4to. 2 Vols. 11. I s. Nicol, &c. 1774

R. Dunkin, though not a poet, was a tolerable cutter of

on fundry materials

Latin, and English, exhibited his goods in a variety of forms.

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He

He would, for inftance, take a fubject, work it up into a Greek poem, afterwards tranflate it into a Latin poem, then into Englifh 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 these 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 verfe-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 still 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,
At end of all this hopeful club

Sat upper;

For he, right worshipful, could boast
His title from the rubric poft,
And was high honour'd by mine host
At fupper.

His heart for liberty was true,
Nor would he change his orange-hue,
Although he got a garter blue,

As Barrage.
And, though no star emblaz'd his breast,
The very falcon' in his creft,

With blood-portending beak expreft

His courage

Full many worthies, paffing bold,
By cunning heralds are enroll'd,
From Windfor to the knights of old
Jerufalem.
Each knight his proper order has,
No matter what his father was,
His blood's as good, and antient, as
Methufalem.
Sir George, fays B with a bow,

I beg your pardon, but I vow,
That you were made a knight, till now,
I never heard: