44° 9′46′′ 98 1 30 Western Light-house 37 48 44; which gives 81342 feet, for the distance between the station Karnbonellis and the Light-house. This distance is said, in the Philosophical Transactions for 1797, p. 501, to be 81348 feet, which differs only 6 feet from the above determination; but it is probable the distance first given is most correct, as the two light-houses appearing nearly in the same line at Karnminnis, was the means of preventing us from clearly distinguishing the apex of either, and it was principally on this account that we preferred the observation made at Pertinney. The agreement however proves, that no inconsistency can be found to obtain with respect to the data before given, for settling the situation of this important headland. Karnminnis Karnbonellis and In the Philosophical Transactions for 1797, page 502, it is mentioned, that the distance from the spot where the late Mr. BRADLEY made his observations, to the place where his meridian mark was fixed, was 800 feet. But there appears to be some inconsistency in this particular; as Mr. BRADLEY'S own words, in an extract of a letter now before me, are, it was just 480 feet. Adding to this, 24 feet, the distance between the place of the meridian mark and the line joining the centre of the lighthouses, we get the distance of the point O, or the place of the Observatory, (see Phil. Trans. 1797, p. 502,) from the line joining the light-houses W, E, = 504 feet; a space corresponding to 5′′ of latitude, nearly; therefore, from the trigonometrical operations, we get, 49° 57' 44" for the latitude 5 11 4,8 for the longitude of Mr. BRADLEY'S station. S Mr. BRADLEY'S observations for finding the latitude, were made with a quadrant of one foot radius, the workmanship of Mr. BIRD; they were as follows. Nine meridional altitudes of the sun's limb, the extreme results of which were 49° 57′ 27′′,5 and 49° 57′ 44′′, gave for the latitude of the Observatory Six meridional observations of the Pole Star below the Pole, the extreme results of which were 49° 57′ 35′′ and 49° 57′ 20′′,4, gave for the latitude a Thirteen observations of Arcturus, & Coronæ Bo- 49° 57′ 35′′ 49 57 23,2 49 57 29 49 57 33 The mean of which is 49 57 30 According to the trigonometrical operations, the latitude is 49° 57′ 44′′; there is, therefore, a difference of 14" between the results; a quantity so large as justly to excite surprise, if it were not generally understood, that much dependance cannot be placed on observations made with an astronomical quadrant precisely similar to that made use of by Mr. BRADLEY. The extreme results in the above, differ so widely as to authorise the truth of the supposition on this occasion. The longitude of the Lizard was determined by the transit of Venus, Sun's eclipse, transit of the Moon, and two emersions of Jupiter's first satellite, as particularly set forth in the Preface to the Nautical Ephemeris of 1791. The conclusions were as follows. Four transits of the Moon, calculated by Mr. Wales, 21 14,5 (Doctor MASKELYNE 20 57,0 Transit of Venus, calculated by Mr. WITCHELL Mr. WALES 20 56,5 20 57,0 Sun's eclipse, calculated by Mr. WITCHELL Mr. SEJOUR Mr. EULER 20TM 30°,6 20 44,5 20 45,1 20 59,0 20 51,0 Mean of the whole 20 52,12 From the trigonometrical operations, we find the longitude in time to be 20" 44,3; there is, therefore, a difference of 7,82 between these different determinations: this is, probably, as near as we could have expected to find it; yet it can scarcely be supposed, that of this difference, more than 2' can be laid to the account of the survey. In the Philosophical Transactions for 1797, p. 502, it is observed, that angles were taken at the Lizard Light-house and Naval Signal-Staff, to determine the situation of the Point itself. This Point, marked P in the diagram, makes an angle of 2° 23′ 16′′ SW, with the parallel to the meridian of St. Agnes at the station on Karnbonellis, and is therefore 636,6 feet from that meridian, and 126394 feet from the perpendicular; therefore 49° 57′ 40′′,6 is the latitude of the Lizard Point. 5 11 46 the longitude } of and Scilly Islands. To determine the distances of the objects in these islands, from the stations near the Land's End, with sufficient accuracy, proper corrections were made for reducing the horizontal angles to those formed by the chords. On the present occasion, it will be right to use the horizontal, and not the chord angles; the distances from the meridians, and from their perpendiculars, being computed on the supposition of the earth's surface being a plane, which, within the limits of our fixed meridians, may be considered as true. The angles for finding the distances of these objects are given in the Philosophical Transactions for 1797, p. 503; from whence, and the data contained in this Work, we get the bearing of the Day-mark in the Island of St. Martin's from St. Buryan 75° 44′52′′ SW 71 14 22 SW 75 30 9 SW which, combined with the distances of the stations from the meridian of St. Agnes, give 246801 feet, for the distance of the Day-mark from the 246804 meridian of St. Agnes; 246821 and 122409 122410 feet, for the distance of it from the perpendicular. 122414 The mean of the first is 246809 feet, and the mean of the last 122411 feet; but the latter becomes 122419, because a line drawn from the Day-mark, perpendicular to the meridian of St. Agnes, cuts that meridian eight feet below the parallel. Again, we get the bearing of the Windmill Pertinney 65° 32′ 30′′ SW 66 53 5 SW from whence, after a similar correction with that just made, we find the distance of the Windmill 256304 feet from the the Flagstaff 260152 meridian, and From the same page, and the data furnished in this work, we also find the bearing of St. Agnes Light-[Sennen 68° 6' 54" S W House from St. Buryan 69 5 56 SW; which gives 265865 feet, for the distance from the meridian, and 265879J 149121 feet, for the distance from the perpendicular of St. Agnes. } the latitude of 143597 feet from the perpendicular of 140876 J St. Agnes. The mean of the first is 265872 feet, and the mean of the last, when corrected, 149133 feet. With the above data, and also the latitude and longitude of St. Agnes, we get and longitude west from St. Agnes. Day-mark in St. Martin's Day-mark 1° 2' 43',1 49°58′ 2′′,9 49 54 32,7 49 54 59,1 49 53 36,8 In Time. from the meri-61387 25 6° 14' 38,8 24 58,6 dian of Green-6 17 57 4 25 11,9 7 9 wich. 6 19 23 4 25 21,5 In the Requisite Tables, published by order of the Board of Longitude, the latitude of the Scilly Lights is said to be 49° 56′o", and longitude 6° 46′ o". The latitude, according to the survey, is 49° 53′ 36′′,8, and longitude 6° 19′ 23′′,4. An error of 2′ 23′′ in the latitude, may not perhaps be considered extraordinary; but how, in a maritime country, like our own, where chronometers are in such constant use, so great an error as 26′ 37′′ (1o 46' in time) in the longitude, should have remained undetected, excepting by one person, is surprising. J. HUDDART, Esq. visited the Scilly Isles, having with him a watch made by ARNOLD, and obtained his time at that spot in the island of St. Mary where the body of Sir CLOUDSLEY SHOVEL is said to have been thrown ashore, by means of equal altitudes of the Sun's limb; he then found, comparing his time with that shewn by the watch, that oh 25m 18 was the difference between the meridians of Greenwich and this spot in St. Mary's. Now St. Agnes Light-house is about 2' of a degree west of the place to which Mr. HUDDART alludes; therefore, 25′ 18′′ + 8" 25' 26" is the longitude of St. Agnes, through these means; which differs only 4,5 in time from that found by the survey. |
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