ing the contrary has been given out by some, that these chymical oils mixed with the most highly rectified spirit of wine, produce no cold, either on their mixture or half an hour after. It results from these experiments, that though there are many liquids which can produce artifical cold, the nitrous acid is the most powerful; and mercury may be congealed by it, without any difficult process, at any time, when the heat of the atmosphere is not greater than 175 by the thermometer before-mentioned. And these experiments have not only succeeded with our author, but with many others; among whom, it may be sufficient to mention Messieurs Lomonosow, Zeiher, Aepinus, and Model, as these gentlemen have made themselves well known in the philosophical world. The nitrous acid was poured on the snow in no determinate quantity; sometimes a few drops were sufficient, sometimes it required a larger quantity. Snow seems to be more fit for these experiments than pounded ice; as the former, from its loose texture, is of more apt and easy solution. Hence it appears, that mercury is no longer to be ranked with the semi-metals, but as a perfect one, fusible, though with a much less degree of heat than any of the others. It agrees likewise with other metals; as their parts like it, when in fusion, attract one another, and run into globules, and, from a state of fluidity, pass into a solid state, not all at once, but successively, and vice versa. But it is worth inquiring, whether this metal, which agrees with all others, both in a solid and fluid state, has not the particular property of boiling at a certain degree of heat, which is by no means to be observed in other metals. The degree of heat, in which mercury begins to boil, is not at 600 of Fahrenheit's scale, as is generally imagined; but at least at 709 of the same scale, which corresponds with 414 of our author's, whose cypher is at the heat of boiling water. Both the boiling and freezing of mercury have this in common; that when it begins to boil, it rises with rapidity; and descends rapidly, when it begins to freeze. If therefore the mean term of the congealation of mercury is fixed at 650 below the cypher, and the term of its boiling at 414 above the cypher; its greatest contraction to its greatest dilatation, will be 1064 degrees of our author's thermometer, and 1237 of Fahrenheit's; as 212 is the point of boiling water in this last, and 32 the freezing one; which corresponds with 150, under the term of boiling water, in our author's. Hence every one will see the great alteration of specific gravity in frozen and boiling mercury, as, between one and the other, the 10th part of the olume is lessened. It may be asked, why the mixture of snow and nitrous acid does not run into a solid mass, and form itself into ice, but remain of a soft consistence, though actually much colder than what is required to freeze aquafortis? We have already mentioned that aquafortis freezes at 204 of our author's thermometer, which corresponds with 34 below the cypher of Fahrenheit's. The frigorific mass, in a degree of cold far below this, remained soft like a poultice. The cause of this extraordinary phenomenon seems to be no other than a continuation of the solution of the snow, and its mixing with the nitrous acid. For as the production of cold depends solely on the solution and mixture, it cannot happen that this mass, which constitutes a fluid of a hard kind, should run into a solid consistence, so long as the solution and mixture continue. XXXII. Observations on the Transit of Venus over the Sun, June 6th 1761. By the Rev. Nathaniel Bliss, M.A., Savilian Prof. of Geometry, Oxford, and F.R.S. p. 173. The present bad state of health of Dr. Bradley * his Majesty's Astronomer, preventing him from making the proper observations of the transit of Venus, he desired Mr. B. to attend at the Royal Observatory to supply his place in making the necessary observations. The instruments they proposed to use, were a reflecting telescope, of 2 feet focal length, to which was fitted Mr. Dollond's micrometer, both executed by Mr. Short. There were some additions necessary to be made to this instrument, which could not be completed before the 2d inst. But it is absolutely necessary that the telescope should be nicely adjusted to distinct vision for the observer's eye, otherwise the apparent angle, measured by the micrometer, will not be exactly true; and as the eyes of different observers may vary very much; the weather was so very unfavourable, that Mr. B. had not so much as one opportunity of seeing any celestial object, by which he might fit it to the proper focus of parallel rays for his eye. Mr. Green therefore, Dr. Bradley's assistant, was the only person who could use that instrument, having adjusted it to his eye some time before. The instrument Mr. B. made use of, was an exquisite micrometer, of the old form, made by the late Mr. Graham, adapted to an excellent refracting telescope of 15 feet focal length. The sky was so cloudy the morning of the transit, and the apparent probability of its clearing up so small, that they almost despaired of being able to make any observation; for they had but one glimpse of the sun, and that only for about half a minute, till half an hour after 7 o'clock. They then prepared to observe the distance of Venus from each limb of the sun, on the chords parallel to the equator, by Mr. Green, with the reflecting telescope, and its micrometer; and Mr. B., with the refracting telescope, and the old micrometer, observed differences of right ascension and declination from the consequent and southernmost limb of the sun. They observed the internal contact of Venus with the sun's limb, Mr. Green Dr. Bradley died the year following (1762), and was succeeded by Mr. Bliss, as Astronomer Royal. having taken off the micrometer, with the 2 feet reflector, Mr. Bird, mathematical instrument-maker in the Strand, with a reflector 18 inches focal length, ofhis own making, and Mr. B. with the refractor, the telescopes used by Mr. Bird and him magnifying about 55 times, that by Mr. Green 120 times, June 5th 1761, at 20h 19m 00o apparent time, all three agreeing to the saine second. The final egress by Mr. Green and Mr. Bliss, was only one second later than by Mr. Bird, at 20h 37m gs apparent time. At 20h 26m 563, by the mean of 5 observations, the centre of Venus was north of the sun's south limb in declination, by his micrometer, 3′ 20. The diameter of Venus was once measured by Mr. Green, with Dollond's micrometer, 57"; by Mr. Canton in Spital-Square, being the mean of 3 good observations, with the same kind of micrometer, 58". The sun's horizontal diameter was observed by Mr. Bird, with the reflector, 31′ 36", which Mr. B. suspects is 3 or 4 seconds too large, as the telescope was not accurately adjusted for parallel rays to his eye, The internal contact was observed by Mr. Hornsby, on the north side of the observatory at Shirburn-Castle, with an excellent 12 feet telescope and micrometer, made by Mr. Bird, of the old form; and by Mr. Phelps, on the south side, with a 14-feet telescope; the telescope used by Mr. Hornsby magnifying 68 times, and that by Mr. Phelps about 55 times; by Mr. Hornsby at 20h 15m 10s apparent time, by Mr. Phelps 4 seconds later, Mr. Bartlet counting the clock, which each observer could hear. Mr. Phelps lost the final contact by mistaking the teller of the clock. Mr. Hornsby makes it at 20h 33m 173; but supposes it to have happened a few seconds later; for at 20h 33m 125; it was not quite gone off the sun, when he was obliged to move his eye-stand, and at 20s after, it was certainly totally emerged. They make the diameter of Venus 56", and Mr. Hornsby, by a mean of 12 observations, made a little before and after the noon of the 5th, makes the diameter of the sun at right angles to the equator, with his micrometer, 31′32′′. At 20h 12m apparent time, Mr. Hornsby, by one observation, makes the centre of Venus north of the sun's south limb in declination, 3' 26". The latitude of the observatory at Shirburn-Castle is 51° 39′ 22′′, being to the north of the Royal Observatory 10′ 43′′. The difference of longitude between them has been determined, by some former observations, to be 4' 1", Shirburn being to the west. XXXIII. The Transit of Venus over the Sun, June 6, 1761, at Savile-House, about 8s of Time West of St. Paul's, London. By Mr. Ja. Short. p. 178. The instruments, made use of on this occasion, were a reflecting telescope of 18 inches focus, with a helioscope adapted to it, and having a field of more than the sun's diameter, proper for showing Venus on the sun's disk, with great ease 4 B VOL. XI. and satisfaction; and another reflector of 2 feet focus, with an achromic objectglass micrometer of 40 feet focus, being the same sort of instrument with those that were made, by order of the R. S., for Dr. Bradley, at the Royal Observatory at Greenwich; and for Mr. Maskelyne, who went to St. Helena; and Mr. Mason, who went to Bencoolen; differing in one particular from their instruments, which had only a common object-glass micrometer. They had no sight of the sun till a quarter of an hour before 6 o'clock, when through an opening, which lasted for about 2 minutes, Dr. Blair, Dr. Bevis, and Mr. S. plainly and distinctly saw Venus on the sun, and concluded that she was then considerably past the middle of her transit. About a quarter after 6, Mr. S. made the first observation, which was, in measuring the diameter of Venus; and soon after he measured her distance from the sun's limb, in the direction of a line going through the sun's centre; and so continued measuring in the same manner, and sometimes measuring the diameter of Venus, till near the internal contact; only about a quarter after 7, he measured the distance of Venus from the sun's limb, in a supposed direction of her transit line, or path over the sun. About half an hour after 7 the clouds dispersed, and they had the sun perfectly clear during the remainder of the transit. When Venus approached the internal contact, Mr. S. took off the micrometer, and changed the magnifying power of the telescope, which during the measurements had been that of 70 times, into another of 140 times, and with this magnifying power he observed the internal contact; in which he thinks he cannot have erred so much as 2 seconds, for the air was extremely clear, and at rest. With the same magnifying power he observed the total exit; and does not think he erred in this above 5 seconds, though this is a more uncertain observation than the former, and can by no means be determined so accurately as the internal contact; and what he erred in this last observation is rather in excess, in making the exit too late. Times and measurements taken at Savile-house, June 6, 1761. Mr. Short observing, and Dr. Bevis marking down the times. Internal contact by Mr. Short, through a reflector of 2 feet focus, magnifying 140 times.. 8h 18m 21+ ... Total exit by Dr. Blair, through a reflector of 18 inches focus, magnifying 35 times. Total exit by Mr. Short, through a reflector of 2 feet focus, magnifying 140 times ... 8 36 124 8 37 05 The diameter of the sun in a horizontal direction was measured just after the transit, and found to be = 31′ 30.8". The clock at Savile-house was several times compared with Mr. S.'s clock in Surry-street, from Friday evening the 5th June to Monday evening the 8th June; so that he was as sure of the time at street. XXXIV. On the Transit of Venus, June 6, 1761, made in Spital-Square; the Longitude of which is 4'11" West of the Royal Observatory at Greenwich, and the Latitude 51° 31' 15" North. By John Canton, M.A., F. R. S. p. 182. Having measured the diameter of Venus on the sun 3 times, with the objectglass micrometer, the mean was found to be 58 seconds; and but of a second, the difference of the extremes.* The diameter of the sun, from 4 observations very nearly agreeing with each other, was 31' 33" 24". The time, by the clock, of the internal contact, was These observations were all made with a reflecting telescope of 18 inches focal length, which magnified about 55 times. XXXV. Observations of the Planet Venus, on the Sun's Disk, June 6, 1761; and Certain Reasons for an Atmosphere about Venus. By Samuel Dunn. p. 184. The latitude of his place was 51° 29′ 5′′ N. and 41' of time west of the observatory at Greenwich, between the physic-garden and Chelsea hospital. With the 6-feet Newtonian reflector, and its magnifying power of 110, and also of 220 times, he carefully examined the sun's disk, to discover a satellite of Venus, but saw none; for he had a very clear dark glass next his eye, and the sun's limb appeared most perfectly defined; but a very narrow waterish penumbra appeared round Venus, by which its limb was not perfectly defined, and at the distance of about a 6th part of Venus's diameter from its edge, was the darkest part of Venus's phasis, from which to the centre an imperfect light increased, and illuminated about the centre. At 8h 16m by the clock, he was prepared to observe the internal contact; and as Venus drew nearer to the sun's limb, the penumbra near the limb of Venus became darker, and threatened to obscure the point of contact at the instant it would happen. r * With the same micrometer, the diameter of Venus was measured, off the sun, 12 times, March 29th, 1758, about noon; and the mean was 1′ 1′′ 42′′"; whence the diameter, at the time of the transit, ought by computation to have been 1′ 0′′ 19′′′′.-Orig. 1 |
