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, gave for the longitude Two emersions of Jupiter's first satellite, calculated by ditto Transit of Venus, calculated by Mr. WITCHELL 20TM 30 ̊,6 21 14,5 (Doctor MASKELYNE 20 57,0 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 bę supposed, that of this difference, more than 2o 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 and of the Lizard Point. 5 11.46 the longitude} of 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 Six meridional observations of the Pole Star be- α realis, and Serpentis, the extreme results of which were 49° 57′ 54",7 and 49° 57′ 2′′,7, gave for the latitude Fifteen observations of a, ß, y Draconis, the extreme results of which were 49° 57′ 22′′,2 and 49° 57′ 2′′7, gave for the latitude The mean of which is 49° 57′ 35′′ 49 57 23,2 49 57 29 49 57 33 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, gave for the longitude Two emersions of Jupiter's first satellite, calculated by ditto Transit of Venus, calculated by Mr. WITCHELL 20" 30',6 21 14 5 (Doctor MASKELYNE 20 57,0 From the trigonometrical operations, we find the longitude in time to be 20TM 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 bę supposed, that of this difference, more than 2s 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′′S W, 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 the longitude}o the longitude of the Lizard Point. 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 which, combined with the distances of the stations from the meridian of St. Agnes, give 246801feet, 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 the Flagstaff of the Fort in the Island of St. [ Pertinney Mary, from Pertinney 65° 32' 30 S W 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 143597 feet from the perpendicular of 140876 St. Agnes. From the same page, and the data furnished in this work, we also find the bearing of St. Agnes Light-Sennen House from St. Buryan 69 5 56 SW; which gives 265865 feet, for the distance from the meridian, and 265879] 1491211 149128 feet, for the distance from the perpendicular of 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 the latitude of Flagstaff, ditto St. Agnes Light-House* Day-mark in St. Martin's 49° 58′ 2′′,9 Windmill, St. Mary's 1° 2′43′′,1 from the meri-6 16 38,7 6 17 57 4 25 11,9 6 19 23 4 25 21,5 • In the Requisite Tables, published by order of the Board of Longitude, the lati tude 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 6o 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′′ (1m 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 4o,5 in time from that found by the survey. |