An Account of the Transit of Venus over the Sun, on Saturday Morning, 6th June 1761, at Savile-House, about 8^{′ ′} of Time West of St. Paul's, London

XXXIII. An Account of the Transit of Venus over the Sun, on Saturday Morning, 6th June 1761, at Savile-House, about 8'' of Time West of St. Paul's *, London. Read June 11, 1761. His Royal Highness the Duke of York, being desirous of observing the rare phenomenon of Venus's passage over the disk of the sun, I had the honour of being commanded by his Royal Highness, to attend him on that occasion at Savile-house, which was a place the most proper for that observation, on account of its remarkable elevation above all the neighbouring buildings, and consequently above the gross vapours of the town. In obedience to these orders, on Friday, 5th June, I carried thither the instruments proper for this observation, together with an astronomical clock, made by Mr. Shelton, and the fellow of that which was last made for the Royal Observatory at Greenwich, and which stands in the transit-room, and went to the said house on Saturday morning, at four o'clock, in company with the Reverend Dr. Blair, and Dr. Bevis, and immediately put the instruments in order. 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 shewing Venus on the sun's disk, with great ease and satisfaction; * N. B. St. Paul's is 22½'' west of Greenwich observatory. and another reflector of 2 feet focus, with an achromatic object-glass micrometer of 40 feet focus, being the same sort of instrument with those that were made, by order of the Royal Society, 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. I intended to have measured the distance of Venus from each limb of the sun, in chords parallel to the plane of the equator, and in chords parallel to the horizon, and also to have taken the appulses of the limbs of the sun and Venus to a vertical and horizontal wire, and had all the apparatus necessary for those observations; but the cloudiness of the morning prevented my putting any of those methods into practice, for the clouds continued so close, that we had no sight of the sun, till a quarter of an hour before six o'clock, when, through an opening, which lasted for about two minutes, Dr. Blair, Dr. Bevis, and I, 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 six, I made the first observation, which was, in measuring the diameter of Venus; and soon after, I measured her distance from the sun's limb, in the direction of a line going through the sun's center; 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 seven, I 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 seven, the clouds dispersed, and we had the sun perfectly clear during the remainder of the transit. When Venus approached the internal contact, I 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, I observed the internal contact; in which, I think, I cannot have erred so much as two seconds, for the air was extremely clear, and at rest. With the same magnifying power, I observed the total exit; and I do not think I have erred in this above five 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 I have erred in this last observation, is rather in excess, in making the exit too late. I have mentioned, above, the magnifying power of the telescope I used; because I have found, by experience, that the different lengths of telescopes, their different magnifying powers, and their different goodness, as well as the different goodness of eyes, want of practice, and different state of the air, will produce differences of times in those sort of observations. These observations were made in the presence of his Royal Highness the Duke of York, accompanied by their Royal Highnesses Prince William, Prince Henry, and Prince Frederick; her Royal Highness Lady Augusta was pleased likewise to do us the honour of looking at this uncommon appearance. Times Times and measurements, taken at Savile-House, on Saturday morning, 6th June 1761. Mr. Short observing, and Dr. Bevis marking down the times. 5 46 37 first saw Venus on the sun. 6 15 12 diameter of Venus = 0 59.8 6 20 44 nearest distance of the limbs 4 48.2 of the sun and Venus. 6 31 05 ditto - - - - - - - - 4 28.5 6 50 24 ditto - - - - - - - - 3 49.9 6 52 38 ditto - - - - - - - - 3 44.8 6 54 23 ditto - - - - - - - - 3 44.0 6 59 37 diameter of Venus - - = 1 00.7 These preceding observations were taken in the intervals of clouds, and somewhat in a hurry. 7 01 42 diameter of Venus = 0 58.9 7 05 36 nearest distance of the limbs 3 20.1 7 08 05 ditto - - - - - - - - 3 12.4 7 09 28 ditto - - - - - - - - 3 08.5 7 11 27 ditto - - - - - - - - 3 05.5 7 12 53 ditto - - - - - - - - 3 01.3 7 18 22 ditto - - - - - - - - 2 48.4 7 19 54 distance of the limbs in a 4 03.6 supposed line of Venus's path. 7 22 03 nearest distance of the limbs 2 37.3 7 24 23 ditto - - - - - - - - 2 32.2 7 26 09 ditto - - - - - - - - 2 27.0 7 31 58 ditto - - - - - - - - 2 13.7 7 37 24 ditto - - - - - - - - 1 58.9 7 40 59 ditto - - - - - - - - 1 49.5 7 41 30 diameter of Venus = 0 58.9 7 43 20 ditto - - - - - - - - 0 58.9 7 44 26 nearest distance of the limbs 1 39.2 7 47 30 ditto - - - - - - - - 1 30.2 7 52 01 ditto - - - - - - - - 1 16.9 7 55 41 ditto - - - - - - - - 1 08.4 8 01 08 ditto - - - - - - - - 0 52.1 8 04 32 ditto - - - - - - - - 0 42.7 Internal contact by Mr. Short, through a reflector of 2 feet focus, magnifying 140 times. 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. 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 my clock in Surry-Street, from Friday evening, the 5th June, to Monday evening, the 8th June; so that I am as sure of the time at Savile-House, as if the observation had been made at my house in Surry-Street. Ja. Short. XXXIV. Observations on the Transit of Venus, June the 6th, 1761, made in Spital-Square; the Longitude of which is $4^\circ 11''$ West of the Royal Observatory at Greenwich, and the Latitude $51^\circ 31' 15''$ North; by John Canton, M. A. and F. R. S. Read Nov. 5, 1761. Having measured the diameter of Venus, on the sun, three times, with the object-glass micrometer, the mean was found to be 58 seconds; and but $\frac{6}{10}$ of a second, the difference of the extremes *. * With the same micrometer, the diameter of Venus was measured, off the sun, twelve times, March the 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 $1' 9'' 19''$.