| Description | Present: The President; Sir Joseph Ayloffe; Mr John Belchier; Lord Charles Cavendish; Dr Knight; Mr Maskelyne; Dr Maty; Dr Morton; Dr Parsons; John Ross DD; Dr Watson; Samuel Wegg Esq.
Matters discussed;
Counterpart Lease from William Pennel to Roger life for 11 years at £65 where every piece of land of the Acton Estate specified - delivered to the Treasurer.
House and effects of the Society to be insured, Treasurer to ask for quotes
Permission from his Executors regarding opening of Mr Short's paper deposited at the Society and publishing it.
Observations of Astronomer Royal for 1768 including nine sheets of Transit observations ordered to be printed
Ordered
Mr Robertson to deliver volumes of Transaction from 1758 to present to Mr Bernoulli of the Royal Academy of Berlin for volumes they do not have.
Astronomer Royal to have tables made by himself and his assistants at the Royal Observatory to use in settling the places of the fixed stars and planets to be printed together with his 'Observations'
' The Astronomer Royal produced the directions he had given to the Observers, sent to Ireland and to the North Cape, relative to their Observations of the ensuing Transit of Venus and other Astronomical business.
Ordered that the said directions be copied into the Council books.
Directions to be observed by Mr Charles Mason, with respect to his making Astonomical observations in the North West parts of Ireland: drawn up by the Revd. Nevil Maskelyne Astronomer Royal Feb 22 1769
I. You are expected by the Council of the Royal Society to proceed with the utmost dispatch to Londonderry, or to any place; towards the North West coast of Ireland, lying between Londonderry and Galway Bay, which you shall find to be most proper for the purpose of making, astronomical observations. You will therefore chuse a place free from fogs arising from bogs or lakes or very high land; but a moderate elevation will be very advantagious. You will endeavour to settle at such a places by the 8th of March next, in order to be ready to observe the eclipses of Jupiter's satellites for two months before Jupiter's opposition to the Sun; and you will continue your observations there for the space of four months.
II. Immediately upon your arrival prepare some place for receiving your instruments, or build an occasional observatory, if necessary; and get your instruments ready to begin making observations without any loss of time, remembering to fix the clock very firmly to the solid wall of a house, or to a massy post let into the ground, and put the upper part of the bob of the pendulum against the lower of the two scratches, and the index to point to the No. 3 1/2, in order to determine the difference of gravity between that place and Greenwich; With this length the clock will keep near mean time at Londonderry.
III. Observe corresponding altitudes of the sun every day, in order to find the going of your clock: And if there be a defect of observations of the Sun, take corresponding altitudes of proper fixed Stars, especially about the time of the transit of Venus, or the beginning of June next.
IV. Make a variety of Observations in order to settle the longitude of the place (see page 41 of my printed instructions relative to the Observations of the transit); particularly the immersions and emersions of Jupiter's first satellite; the recultation [signs ] by the [sign], April 11th next; that of [signs] by the Moon May 9th and the eclipse of the Sun which happens early on the morning immediately after the transit; and any other occultations of smaller Stars than those of the 4th magnitude which are not set down in the Ephemeris.
V. To determine the latitude of the places, observe meridian altitudes of the Sun, and also of Stars, both to the morth and South of the Zenith,; and set down the altitudes of the barometer and Thermometer (the latter being placed without doors in the shade), to all altitudess taken by the Quadrant.
VI. In observing the transit of Venus over the Sun on June 3rd be very careful to catch the first least visible impression of Venus upon the sun's limb, or the first external contact: In order to which be looking out for it 10 or 15 minutes before the time calculated for Dublin in my instruction, allowing for your differences of Longitude, West of the same: Also calculate at what part of the sun's circumference the first external contact will be made. Calculate the angle of position at the sun, from the latitude of the place, and the apparent time of the external contact at the place, previously computed. The difference of the angle of position, and 36 degrees 45 ' will shew how many degrees, the external contact will happen, as measured along the Sun's circumference, from the Sun's zenith or uppermost point in altitude, and will be to the East or West of the said point, as the angle of position is less or greater than 36 degrees 45 ' The external contact happens at Greenwich, within a degree of the Sun's zenith point; and at N.W. parts of Ireland it cannot be very different. Observe the external and internal contacts, with the magnifying power 128, which is obtained by using the smallest little speculum, or that of the shortest focus, and the longest eye-piece of the two, which properly belong to the Telescope; [for the eye-piece with the parallactic wires, is an additional one, as is the 3rd or broadest speculum, which is surrounded by a rim of card, also additional, being designed to be used only with the eye-piece containing the parallactic wires]. Set down in what manner you observe the contacts, particularly the internal contact; whether by the coincidence of the limbs of the Sun and Venus, or by the breaking in of the Sun's light behind the posterior limb of Venus. Take notice also, whether any crescent is to be seen about that part of Venus's body, which is not entered upon the Sun, rendering it visible before the internal contact. Note also, after the internal contact, whether any thin faint and diluted ring is to be seen surrounding the circumference of Venus, or any other signs of an Atmosphere. After the internal contact is over, put on Dollond's micrometer, applying the broadest little speculum, which has a rim of card about it, and the eye-piece with the parallactic wires, and adjust the Vernier scale, which is on the side of the tube, to that division, which shews the process adjustment of distinctnesse of the Telescope, with this speculum, eye-piece, and the Micrometer on, as found by several trials of previous days on the limb of the Sun, and red spots in the Sun; Then measure the diameter of Venus several times with (o) of the Vernier, placed alternately to the right and left of the beginning of the divisions, which will at the same time give the correction of the adjustment of the scale, see p. 36 of my instructions; Measure also the horizontal diameter of the Sun, 4 or 5 times, which will serve to give the value of the Scale. Then proceed to take several measures of the differences of declination of the north limbs of the sun and Venus, in the manner prescribed in my written account, which may be compared with the measures of the nearest distance of the centers, that may be observed in the South Seas or at Hudson's bay for determining Venus's parallax in declination whence the parallax of the Sun may be inferred.
VII. Observe the immersions and emersions of Jupiter's satellites, occultations of fixed Stars by the Moon, and the eclipse of the Sun, with the largest of the two little speculums, properly belonging to the Telescope [N.B. it has a small flaw, a piece being broke out of it at the edge] and the shorter of the two proper eye-pieces, which will magnify 82 times.
VIII. Observe the beginning and end of the eclipse of the Sun, which happens a few hours after the transit. In order to catch the beginning better, compute at what part of the Sun's limb the Moon will make the first impression. Take a series of measures of the distance of the cusps with Dollond's Micrometer, especially about the middle of the eclipse, using the largest of the two speculums, proper to the Telescope, and the largest of the two eye-pieces proper to the same, which will magnify 62 times; and take care to measure the Sun's horizontal diameter for determining the value of the scale with this speculum and eye-piece, and so find the correction of its adjustment at the time. '
' Directions to be observed by Mr Jeremiah Dixon and Mr William Bayley, with respect to their making Astronomical observations at the north cape of Europe, or near it, and in the voyage out and home again.
I. In your voyages out and home again, make frequent observations with the Hadley's sextant, or a Hadley's Quadrant, of the latitude of the Ship, and of the distance of the Moon from the Sun and fixed Stars, in order to determine that Longitude of the Ship from time to time; but more especially when in sight of land, or soon after leaving land, or when you are expecting to make land soon, and keep a Ship's reckopning in the usual manner, and also corrected by the said Observations. Observe also frequently, or assist at the observations of the Variation, by the Azimuth compasses of the Ship.
II. It is intended that you and the other observer appointed by the Royal Society should be separated to two different Stations; one of which will be the North Cape, and the other, some other place at some distance, to be fixed by Capt. Douglas, in order to afford a greater chance of obtaining one observation of the transit of Venus, at least, free from the accidents of cloudy weather. A double set of instruments is accordingly appointed for this purpose; and you and the other Observer will settle between yourselves which shall observe at one place, and which at the other. The Island of Maggeroe (on the northernmost point of which this North Cape stands) extending half a degree of latitude, it will be rather more convenient that the Observer should be stationed towards the northern point, or near the North Cape, when the Sum will be highest at the transit of Venus. You are expected to to stay at the place of Observation during two months after the transit, in order to settle the latitude and Longitude of the place; except you should miss of observing all the contacts of Venus with the Sun's limb thro' bad weather; in which unlucky case, which it is to be hoped will not happen, you might depart the place immediately, only first observing carefully the eclipse of the Sun, which happens a few hours after the transit is over. A moderate elevation above the level of the Sea, and a few miles distance from thence, will probably afford the most convenient air, for making Astronomical Observations.
III. Immediately upon your arrival at the North cape, or the place destined for your observations, set up your wooden Observatory with the dwelling house adjacent, and fix up the clock firmly and truely perpendicular to the upright posts of the Observatory, and put the upper part of the bob of the pendulum against the horizontal scratch, and the index to point to No 12 B clock [and] 14 D clock [12 B clock is placed directly above 14 D clock in the text] in order to determine the difference of gravity between that place and Greenwich; With this length of the pendulum the clock will keep near sidereal time at and near the North cape.
IV. Observe several corresponding altitudes of the Sun every day in order to find the going of your clock; Hence you will soon be enabled to draw a meridian line; and thence to bring the Observatory in the proper direction, that the Transit instrument may move nearly in the meridian. By the equal altitudes of the Sun you will also by degrees bring your transit instrument still nearer to the meridian, and finally you will correct it, if necessary, by observations of the transit of the pole Star and other circum polar stars above and below the the pole. N.B. The Stars which are opposite to the Sun in [blank] or nearly so, will probably be the best for your purpose. To which you may add Capella, a Star of the first magnitude. Fix up a Meridian mark at a convenient distance; perhaps a quarter of a mile might be as well as any; Or if you had two, one to the South and the other to the North, it might be better. It will be sufficient if the transit instrument be brought within a quarter of a minute of Azimuth of the true meridian, and the error of its position may be ascertained by the equal altitudes of the Sun, and the transit of the circumpolar stars, and allowed for afterwards in calculation.
V. Observe the transits of the sun and moon as often as visible, and of the principal fixed stars of the first and second magnitude, lying between the tropical circles, and which are dayly observed at the Royal Observatory at Greenwich. Observe also frequently, the transits of the polar Star and other circumpolar stars above and below the pole. The last mentioned Observations, serve to prove or correct your meridian; the former compared with the Observations made at Greenwich, about the same time, will serve to determine the longitude of the place. For this purpose, the Occultation of [sign] Gemini by the moon which happens on May 9, if visible at the north cape, will also conduce . But you should particularly with a view to this purpose, attend to the eclipse of the Sun, which happens a few hours after the transit, and observe the beginning and end very caefully, with the magnifying power 100, moreover, measure the distance of the cusps, and the greatest quantity of the obscuration, or digitseclipsed, with Dollond's micrometer, by several observations, about the middle of the eclipse.
*N.B. With the long eye-piece and long focus little speculum, the reflecting telescope magnifies 50 times; with the short eye-piece and long focus little speculum 75 times; with the long eye-piece, and short focus little speculum 100 times; and with the short eye-piece and short focus little speculum 150 times. * In order to catch the beginning of the eclipse of the Sun better, compute at what distance from the Zenith of the sun's limb, measured along the circumference, the Moon will make the first impression, ['and whether to the right or left' crossed out] For other observations tending to settle longitude of the place; see page 62 of my printed instructions.
VI. To determine the latitude of the place, observe meridian altitudes of the Sun, both to the North and to the South; and also of Venus; and of such of the fixed stars as you can see with the Telescope of the Quadrant; and set down the height of the barometer and also of the two thermometers, one within and the other placed without doors, in the shade, at the time.
VII. Observe frequently the rising and setting of both limbs of the Sun, if seen in the horizon of the Sea, when they happen. Take also a series of altitudes of the Sun and Venus, from the horizon to the meridian, in order to ascertain the astronomical refractions; and note the time of the rising and setting of Venus, and of the bright fixed stanrs, using a Telescope. And ascertain the elevation of your Observatory above the Sea in feet. And note the height of the barometer and Thermometer at all these times. And observe the depression of the horizon of the sea, by your Quadrant, noting the azimuth, upon which you observe.
VIII. In observing the transit of Venus over the Sun on June 3rd be very careful to catch the first least visible impression of Venus on the Sun's limb, or the first external contact; in order to which, be looking out for it 10 or 15 minutes before the time calculaed in my instructions page 47. The point of the sun's circumference, where the first external contact may be expected, will be 23 degrees 0' to the east or left hand of the sun's zenith point at the north cape. Observe the contacts of the limbs of the sun and Venus, with the magnifying power 100; viz with the long eye-piece and short focus, or smaller llittle speculum. Set down all the circumstances attending your observations, and particularly in what manner you observe the contacts, but especially the internal contacts, whether by the coincidence of the limbs of the sun and Venus, or by the breaking in of the Sun's light behind the hinder limb of Venus, at the first internal contact; or by the disappearance of the light between the limbs at the 2nd internal contact, take notice also whether any crescent is to be seen about that part of Venus's body, which is not yet entered upon the Sun at the beginning of the transit. Note also, while Venus appears wholly upon the Sun, whether any thin faint dilated ring is to be seen surrounding the circumference of Venus, or any other signs of an Atmosphere. After the first internal contact is over, put Dollond's micrometer on the telescope, using the longest focus little speculum, and the eye-glass with the moveable wires; and adjust the vernier scale which is on the side of the tube, to that division, which shews the proper adjustment of distinctnesse of the Telescope with its speculum eye-piece, and Micrometer on, as found by trials of previous days on the limb of the sun, and spots in the Sun. Then measure the diameter of Venus several times with (o) of the Vernier, placed alternately to the right and left of the beginning of the divisions of the Micrometer scale, which will at the same time give both the diameter of Venus and the correction of adjustment of the scale, see page 36 of my instrucitions: Measure also the horizontal diameter of the Sun, 4 or 5 times, which will serve to give the value of the Scale. Then proceed to take several measures of the difference of declination of the north limbs of the Sun and Venus, in the manner prescribed in my written account, which may be compared with the measures of the nearest distance of the centers that may be observed in the South Seas, or at Hudson's bay, for determining Venus's parallax in declination, whence the parallax of the Sun may be inferred. Toward the middle of the transit, take several measures of the nearest distance of the limbs of the Sun and Venus, in order to asscertain the nearest approach of Venus to the Sun's center: And in taking these measures just mentioned, use the long-focus little speculum, and longest eye-piece, and with the same make some observations of the horizontal diameter of the Sun; Also of Venus on both sides of the beginning of the scale.
April 3rd 1769. Nevil Maskelyne Astronomer Royal ' |