At 8h 16m 41 No diminution of light between the limbs of Venus and the 8 8 sun. 16 42 Slight penumbra, or diminution of light, near where the con 16 43 8 16 44 16 45 16 8 16 47 tact was to be. Penumbra of a grey colour, near the same place. Penumbra almost brown, and the thread of light very narrow. 46 Penumbra more brown, and the touch the smallest possible. 8 16 48 8 16 49 True black in the point of contact, and the edges a little broader. Here he guessed that observers would differ in their judgment about the moment of contact some seconds of time, or that some would estimate the contact sooner than others. From these observations he concluded that the thread of light in the point of contact was so obscured as to be undiscernible at 8h 16m 46s, and that true black did not succeed in the same point till 3* after, namely, 8h 16m 49s; and from both of these properties he concluded that the real internal contact was at 8h 16m 47 by the clock; which makes 8h 16m 11s equal time, and 8h 18m 2s apparent time at Chelsea; and 8h 18m 43s apparent time at Greenwich. While Venus was on the sun's limb no other penumbra appeared between the limb of Venus and the sun, than had appeared before on the sun's disk; and therefore he concluded there must be an atmosphere about Venus, which receiving weak im.pressions of light between the limbs of Venus and the sun, occasioned the uncertainty of ascertaining the exact instant of the internal contact as above described. * As the 6-feet Newtonian telescope magnified 4 times as much as that of the 2-feet Gregorian telescope, and the vanishing of the thread of light, from its least degree of duskishness to a true black, was about 3 seconds of time by the 6-feet telescope, the time in which the thread of light was vanishing from the least degree of duskishness to a true black, by a 2-feet Gregorian reflector, may be supposed to have been 4 times 3 = 12 seconds of time; and hence an error, or rather difference of pronunciation, but not of judgment, may have arisen among good observers, if some estimated the contact by the invisibility of the thread of light, and others by an apparent blackness. in the point of contact; or, which is the same thing, the time when the planet had made the least apparent dent in the sun's limb, of the same colour, through a dark glass, as the sky. This was verified by a 2-feet Gregorian reflector, in the contact above mentioned, and possibly may have occasioned greater differences in estimating the contact, with lesser Celescopes, to no less than half a minute of time.-Orig. At 8h 35m per clock, the external contact was near, and not incumbered with such a penumbra or partial light as the internal contact had been. At 8h 35m 4o the least dent possible, quite black, appeared in the sun's limb. And at 8k 35m 63, the limb was restored to its perfect form, there having been a small trembling light between the narrow watery border of Venus, and the vanishing point of contact in the sun's limb, for these 2 seconds of time. From which the external contact at Chelsea was 8h 34m 30s equal time, and 8h 36m 21s apparent time; which makes 8h 37m 2s apparent time at Greenwich. From the foregoing circumstances, it appeared that the external contact was more easily to be determined than the internal one, which was contrary to what he had before expected. And considering the aforesaid penumbra, or border of partial light, surrounding Venus as an atmosphere of that planet, with the time of its vanishing, 24 seconds of time; and reducing this to the diameter of Venus, with due allowance for the oblique direction over the sun's limb, the atmosphere of Venus comes out 84 thirds of a degree, which is nearly about part of Venus's diameter; which diameter being nearly equal to the earth's, the atmosphere of Venus comes out nearly 50 geographical miles.. XXXVI. Account of the Observations made on the Transit of Venus, June 6, 1761, in the Island of St. Helena. By the Rev. Nevil Mashelyne, M.A., F.R. S. p. 196. From the very cloudy weather which prevailed here for the whole month preceding the transit, Mr. M. almost despaired of obtaining any sight of it at all. He was however fortunate enough to obtain two fair views, though but of short continuance, of this curious celestial phenomenon. The first was a few minutes after sun-rise, when he was surprized not only at seeing Venus so very large, but also so much nearer the sun's limb than he had reason to expect from the best-grounded calculations; which last circumstance foreboded that she would make a more speedy exit from the sun's body than the same calculations allowed; which accordingly happened. At this time her limb, as well as the sun's, appeared exceedingly ill defined, which was no more than what one might naturally. expect, from their great proximity to the horizon. This alone was sufficient to prevent making any observations at that time, which could admit of any exactness, if the clouds had not presently come up, and totally deprived him of the sight both of Venus and the sun. In this manner the skies continued unfavourable for about an hour, when they became again. extremely clear, and he had the pleasure of seeing Venus appear as an intensely black spot on the sun's body, and perfectly well defined. At this time he measured the distance of the nearest limbs of Venus and the sun from each other, with the curious object-glass micrometer adapted to the reflecting telescope, ac cording to Mr. Dollond's ingenious invention. This distance was 1' 44", at 7h 31m 7s apparent time, or 7h 29m 15 mean time. Mr. M. remarks, that though Venus's limb and the sun's appeared as well defined as could be desired; yet when the artificial internal contact of Venus's limb with the sun's was made, in order to measure their distance, Venus's limb alternately dilated itself over, and contracted itself within the sun's limb by a small space. He endeavoured to take it in the middle of this vibration; but he doubts whether, if the real internal contact had happened at this time, it could have been observed, in such circumstances, to that degree of exactness which Dr. Halley hoped for; and whether on occasion of the next transit, which is to happen 8 years hence, it might not be convenient that the observers should endeavour to place themselves on such parts of the globe, as that they may not see Venus on the sun's body, very near the horizon, but rather when they are both elevated to considerable heights; which will afford them a greater chance of making their observations free from clouds, which usually skirt the horizon, as well as of making them to advantage. Presently after measuring the distance of Venus from the sun's limb, the clouds returned again, and prevented him not only from making any more observations of the same kind, or measuring Venus's diameter, but also, what was of much more consequence, from observing the last internal contact of Venus with the sun's limb, which was the principal observation of all. About 23 minutes after 8 the clouds separated again, and the sun appeared very bright and clear; but there was not the least appearance to be seen of Venus, though he thought himself in a manner sure of observing at least the external contact, as all the calculations make the end to happen much later. Mr. Waddington took the passages of Venus and the sun's limbs across the horizontal and vertical wire of the equal altitude instrument. All the observations which he was able to make are as follow: 33 5 34 53 O's western and subsequent limb at vertical wire. O's upper limb at horizontal wire. The observation of this N.B. As the telescope inverts, the observations, as usual, are set down according to the appearance. 1 As it is to be feared that our other observers, Mr. Mason and Mr. Dixon, by the misfortunes they have met with, have not been able to make their observations at Bencoolen, as was proposed; Mr. M. humbly hazards an opinion, whether the difference in the total duration of the transit of Venus over the sun's disk, observed in any two places, where it is likely observations have been made, will be great enough to enable us to infer the sun's parallax with sufficient exactness, or even nearer than it is known already. So that he is afraid we must wait till the next transit in 1769, which is, on many accounts, better circumstanced than this, before astronomers will be able to do justice to Dr. Halley's noble proposal, and to settle, with the last and greatest degree of exactness, that curious and nice element in astronomy, the sun's parallax, and thence determine the true distance of all the planets from the sun, and from each other. Mr. Maskelyne excuses himself that he does not at present attempt to deduce any consequences from the above observations, not only as he was in want of others corresponding to them made in other places, but also as he was not yet able to settle the longitude of this place to sufficient exactness; though he was of opinion it cannot differ much. Mr. M. had not been able to get one observation of an eclipse of Jupiter's satellites, though he was ready to seize any opportunity, if it had offered; the very cloudy weather, which prevailed at the time, which is the winter there, depriving him not only of these, but almost all other observations. XXXVII. Account of the same Transit. By the Rev. Richard Haydon. p. 202. Mr. Haydon's latitude, at Leskeard, was 50° 26′ 55′′, and his longitude west of London in time, 16m 10; though he, from a memorandum he made some years ago, supposed it near 2 minutes more. By correspondent altitudes of the upper limb of the sun, June 5th, 1761, his clock was too fast in apparent time 9m 30s. By the same, June 6, it was 9m 40s. June 6th, 1761, the diameter of was. 31′ 31′′.5 The nearest distances of Venus from the sun's limb; the diameter of Venus included. Angular value. } 5′ 51′′.6 Semidiameter of Venus to be deducted for the path of 5' 38.7 the centre. By the clock. At 5h 34m 54s 5 53 Mr. Haydon, in a subsequent letter, says that on comparing his observations with those made in London, his interval between the internal contact and total egress, was considerably longer than any of the others. Therefore he examined his notes again, but could not find he had made any mistake in transcribing them, but thinks he may at first have set down 1m too many in the total egress. XXXVIII. Observations on the same Transit; and on an Eclipse of the Moon, May 8, and of the Sun, June 3, 1761. By Mr. Peter Wargentin, F. R.S., and Sec. R. Acad. of Sciences, Sweden. From the Latin. p. 208. Mr. W. says the lat. of Stockholm, his situation, is 59° 20′ 31′′, and its long. east of Greenwich 1h 12m 1s. 9 32 30 A Total Eclipse of the Moon observed May 18, 1761. 10 41 12 15 13 21 8 A Solar Eclipse observed June, 5, 1761. At 3h Om os Sun rose eclipsed. 8 12 32 -The Transit At 3h 21m 37 3 39 23 8 The end as much as could be seen. of Venus over the Sun, June 6, in the Morning. Interior contact, or total immersion. Beginning of emersion, or interior contact. 9 48 9 The beginning of the first contact could not be taken, on account of the undulation of the sun's border. The observations were made with a tube of 20 Swedish feet, of 3 inches focal distance. It was remarkable that on the exit, when a part of Venus was quite emerged, it was visible faintly illuminated. But whether this was owing to an inflection of the sun's rays, or to a refraction in the atmosphere of Venus, he leaves to others to determine. End of the emersion, or exterior contact. XXXIX. An Account of the Observations made on the same Transit in Sweden. In a Letter from Mr. Peter Wargentin. Translated from the French. p. 213. At Torneo in Lapland, Messrs. Lagerborn and Hellant very happily observed |