were occafionally introduced will Sidus, as they follow, in the or be mentioned in the obfervations der of time in which, they were and experiments on the Georgium made. Obfervations on the Light, Diameter, and Magnitude, of the Georgium Sidus. Oct. 22, 1781. The Georgium Sidus was perfectly defined with a power of 227; had a fine, bright, fteady light; of the colour of Jupiter, or approaching to the light of the moon. Nov. 28, 1781. I measured the diameter of the Georgium Sidus by the lamp-micrometer, and took one measure, which I was affured was too large; and one, which I was certain was too little; then taking the mean of both, I compared it with the diameter of the ftar, and found it to agree very well. Hence Image 2,4 inches Distance 431 inches tang.,0055684; and Angle = 10'8′′, Power 2276 the diameter 5",06. But the evening was foggy, and the ftar having much aberration, I was induced to try the above method of extreme and mean diameters, fuggefted by the method of altitudes, where two equally diftant extremes give us a true mean. Nov. 19, 1781. The diameter measured 324 parts of my micrometer, the wires being outward tangents to the difk. On fhutting them gradually by the fame light, they clofed at 24; therefore the difference is 8 parts, which, according to my fcale, gives 5" 2" for the diameter. This was taken with 227, and the measure feemed large enough. Not perfectly pleafed with my light, which was rather too ftrong, I repeated the meafure, and had 33 parts; then shutting the wires gradually, by this light they closed at 25: the difference, which is 8 parts, gives 5"11". Aug. 29, 1782. 15 h. I faw the Georgium Sidus full as well defined with 460, as Jupiter would have been at that altitude with the fame power. Sept. 9, 1782. Circumstances being favourable, I took a measure of the diameter of the Georgium Sidus with the power of 460, and filkthread micrometer. After a proper allowance for the zero, I found 4" II". Oct. 2, 1782. I had prepared an apparatus of lucid difks, and measured the diameter of the Georgium Sidus with it. Having only white oiled papers, I placed two of them together, and ufed only a fingle lam; but could not exactly imitate the light of the planet. When I firft faw the Sidus and luminous circle together, I was struck with the different colours of their lights; which brought to my r collection y Andromeda, Bootis, & Herculis, 6 Cygni, and other coloured ftars. The planet unexpectedly appeared blueish, while the lucid difk had a frong tincture of red; but neither of the colours were fo vivid and fparking as thofe of the juft mentioned stars. The distance of the luminous circle from the eye (which I always measure with deal rods) rods) was 588,25 inches. The circle measured 2,35 inches. Hence we have the angle 13' 44"; which, divided by the power 227, gives 3",63 for the diameter of the planet. I fufpected fome little fallacy from the want of a perfect refemblance in the light and colour of the artificial disk to the real appearance of the planet. Oct. 4, 1782. I meafured the diameter of the Georgium Sidus again, by an improvement in my apparatus, for I now ufed pale blue papers, both oiled and plain, instead of white; by which means I obtained a refemblance of colours; and by an affortment of one oiled and two dry papers with two lamps burning, I effected the fame degree of light which the planet had, and both figures were equally well defined. By first changing the difk, and, when I had one which came neareft, changing my diftance, I came at a perfect equality between the planet and disk. The measure was feveral times re2,8 peated with great precaution. The refult was =,0040283; 13′ 53′′,85 692,6 and =3",67. If any thing be wanting to the perfection of this measure, it is, perhaps that the Sidus fhould be in the meridian, in order to have all the advantages of light and diftinctness. Oct. 10, 1782. The measures of the planet by the lucid disk micrometer appearing to me very fmall, I refolved to ascertain the power of my telescope again moft fcrupuloufly, by an actual experiment, without any deduction from other principles. On a moft convenient and level plain I viewed two flips of white paper, and measured their images upon a wall. The diftances were measured by deal rods, every repetition whereof was certainly true to half a tenth of an inch; nor did the direction of the measure ever deviate, fo much as two inches, from a ftraight line. Distance of the object from the eye in inches 7255.5 80,2 Distance of the vertex of the fpeculum from the object 7335-7 Distance of the eye from the wall 2292,35 Diameter of the largest paper Diameter of the smallest 5075 Image of the largest paper on the wall 73, Image of the fmalleft on the fame 37,8 Angle fubtended by the large paper at the vertex of the fpe culum 27",87 Angle fubtended by its image on the wall, at the eye, 1° 49′ 26′′,4. Power of the telescope deduced from the large paper Angle fubtended by the fmall paper at the vertex of the fpeculum 14",27. 235,6 Angle fubtended by its image on the wall, at the eye, 56' 40",9. Power of the telescope deduced from the small paper 238,3 Mean Mean of both experiments, as being equally good Upon Capella And 237 diminished in the ratio of 85,2 to 86,1625 gives It appears then, from thefe experiments, that the power of the telescope has not been over-rated; and that, therefore, the measures of the Georgium Sidus cannot be found too fmall on that account.. That There is one cause of inaccuracy or deception in very small meafures, long fufpected, but never yet fufficiently investigated. there is a difperfion of the rays of light in their paffage through the atmofphere, we may admit from various experiments; if then the quantity of this difperfion be, in general, regulated by certain difpofitions of the air, and other caufes, it will follow, that a concentration may alfo take place: for fhould the rays of light, at auy time, be lefs difperfed than ufual, they might with as much reafon be faid to be concentrated, as the mercury of a thermometer is faid to be contracted by cold, when it falls below the zero. Oct. 12, 1782. The night was fo fine, that I faw the Georgium Sidus very plainly with my naked eye. I took a measure of its diameter by the lucid difk, and found, that I was obliged to come nearer, as the planet rofe higher, and gained more distinct light. At the altitude of 529 it was as follows: Oct. 13, 1782. 16h. I viewed the Georgium Sidus with several powers. With 227 it was beautiful. Still better with 278. With 460, after looking fome time, very diftinct. I perceived no flattening of the polar regions, to denote a diurnal motion; though, I believe, if it had had as much as Jupiter, I should have feen it. With 625 pretty well defined. Oct. 19, 1782. The inconvenience arifing from the quantity of light contained in the lucid difk, fuggefted to me the idea of taking only an illuminated periphery, inftead of the area of a circle. By this means I hoped to fee the circle well defined, and yet have but little light to interfere with the appearance of the planet. The breadth of my lucid periphery was one-twentieth of an inch. The 14" 15",60 refult of this measure proved- =,0041486; and 3,3 765,45 227 33"77. O. 26, 1782. In my laft experiment I found the lucid periphery much broader than I could have wifhed; therefore, I prepared one of no more than one-fortieth part of an inch in breadth, the outer circle measuring very exactly 4,00, and the inner circle 3,95. With this flender ring of light illuminated with only one fingle lamp, I meafured the Georgium Sidus, by removing the telescope to various dif tances 4, tances; and found at laft the following refult: =3′′,51. 227 and 13′ 18", 6 1033,05 =,00387203 Nov. 4, 1782. I was now fully convinced that light, be it in the form of a lucid circle, or illuminated periphery, would always occa fion the measures to be lefs than they fhould be, on account of its vivid impreffion upon the eye, whereby the magnitude of the object, to which the planet was compared, would be increased. It occurred to me then, that if a lucid circle encroached upon the furrounding darker parts, a lucid fquare border, round a dark circle, would in its turn advance upon the artificial difk. In my laft measures, where the planet had been compared to a lucid ring, I had plainly obferved that the Sidus, which was but juft equal to the illuminated periphery, was confiderably larger than the black area contained within the ring. This feemed to point out a method to difcover the quantity of the deception arifing from the illumination; and confequently, to furnish us with a correction applicable to fuch meafures; which would be plus, when taken with a lucid disk or ring; and minus, when obtained from a dark ring or circle. Having fufpended a row of pafte-board circles against an illuminated sheet of oiled paper, I caufed the Georgium Sidus to pafs by them feveral times, and felected from their number that to which the planet bore the greateft refemblance in magnitude. I produced a perfect equality by fome fmall alteration of my distance, and the refult was as follows: I was defirous of feeing what would be the effect of leffening the light of the illuminated frame, againft which the dark difks were fufpended, and alfo waited a fhort time that the planet might rife up higher. The measure being then repeated at a different diftance, and with a different black difk, I obtained the following particulars : 3,59 803,05 I intend to purfue thefe experiments ftill farther, especially in the time of the planet's oppofition, and am therefore unwilling as yet to draw a final conclufion from the feveral measures. In a fubject of fuch delicacy, we cannot have too many facts to regulate our judgemeat. Thus much, however, we may in general furmife, that the diameter of the Georgium Sidus cannot well be much lefs, nor perhaps much larger, than about four feconds. From this, if we will anticipate more exact calculations hereafter to be made, we may gather that the real diameter of that planet must be between four and five times that of the earth: for by the calculations of M. de la Lande, contained in a letter he has favoured me with, the distance of the Georgium Sidus is ftated at 18,913, that of the earth being 1. And if we take the latter to be feen, at the fun, under an angle of 17", > 4 it would fubtend no more than ,"898, when removed to the orbit of the Georgium Sidus. Hence we obtain ,894,454; which number expreffes how much the real diameter of the Georgium Sidus exceeds that of the earth. Extracts from an Account of the Earthquakes which happened in Italy, from February to May 1783; by Sir William Hamilton, Knight of the Bath, F. R. S. in a Letter to Sir Jofeph Banks, Bart. P. R. S. From the fame Work! IA Naples, May 23, 1783. AM happy now to have it in my power to give you, and my brethren of the Royal Society, fome little idea of the infinite damage done, and of the various phenomena exhibited, by the earthquakes (which began the 5th of February laft, and continue to be fenfibly, though lefs violently, felt to this day) in the two Calabrias, at Melfina, and in the parts of Sicily nearest to the continent. From the most authentic reports, and accounts received at the of fices of his Sicilian, majefty's fecretary of ftate, we gathered in general, that the part of Calabria, which has been most affected by this heavy calamity, is that which is comprehended between the 38th and 39th degree, that the greateft force of the earthquakes feemed to have exerted itself from the foot of thofe mountains of the Apennines called the Monte Deio, Monte Sacro, and Monte Caulone, extending weftward to the Tyrrene fea; that the towns, villages, and farm-houses, reareft thefe mountains, fituated either on hills or in the plain, were to tally ruined by the firft fhock of the 5th of February about noon; and that the greatest mortality was there; that in proportion as the towns and villages were at a greater diftance from this center, the damage they received was lefs confiderable; but that even those more diftant towns had been greatly damaged by the fubfequent fhocks of the earthquake, and efpecially by thofe of the 7th, the 26th, and 28th of February, and that of the ift of March ; that from the first fhock, the 5th of February, the earth continued to be in a continual tremor, more or lefs; and that the fhocks were more fenfibly felt at times in fome parts of the afflicted provinces than in others; that the motion of the earth had been various, and, according to the Italian denomination, vorticofo, orizontale, and ofcillatorio, either whirling like a vortex horizontal, or by pulfa tions, or beatings from the bottom upwards; that this variety of motion had increafed the apprehenfions of the unfortunate inhabitants of thofe parts, who expected every moment that the earth would open under their feet, and fwallow them up; that the rains had been continual and violent, often accompanied with lightning and irregular and furious gufts of wind; that from all thefe caufes the face of the earth of that part of Calabria (comprehended as abovementioned between the 38th and |