The Observatory of his Grace the Duke of MARLBOROUGH, at Blenheim. The staff erected over the quadrant, was observed from White Horse Hill and Whiteham Hill. At the former station, the latter makes an angle of 36° 30′ 13′′,5, with the parallel to the meridian of Dunnose. The staff, therefore, bears from the parallel 25° 59' 29",75 NE.; consequently, its distance from the meridian of Dunnose is 36540 feet, and from the perpendicular 446458 feet. These respectively subtend 5′ 58",3, and 1° 13′21′′,4; therefore, the latitude of the Observatory is 51° 50′ 28′′,3, and its longitude 9' 39",9 from Dunnose: but 1° 11′ 36′′ is the longitude of that station; therefore, 1° 21′ 15′′,9, or 5′ 25′′,2 in time, is the longitude of the Observatory west from Greenwich. As the meridian of Dunnose passes at no great distance from that of Blenheim, I have deduced the latitude and longitude from the former, to avoid the errors which creep in, when computations are carried on from remote meridians. It may be worth while, however, to show that the extent of those errors would not be great, were the meridian of Dunnose neglected, and the Observatory at Blenheim referred to the meridian of Greenwich. The distance of White Horse Hill from the meridian of Greenwich is found to be 356050 feet, and from its perpendicular 39425 feet; the bearing of Nuffield, from the parallel at that station, being 89° 59' 27" SE. Blenheim will, therefore, be found to bear 26° 55′ 25′′ N E from the parallel at White Horse Hill; consequently, its distance from the meridian of Greenwich is 307224 feet, and from its perpendicular 135569 feet. These give the arcs 50' 12",4, and 22′ 16′′,1; from whence we get 51° 50′ 28′′,1 for the latitude, and 1° 21′ 16′′ for the longitude, of the Observatory west of Greenwich. Either of these determinations may be taken for the true result, but I shall prefer the first. Being favoured by his Grace with the latitude and longitude derived from astronomical observations, we have the following comparisons: The angle at the station on Shotover, between the Atlas on the top of the Observatory and the parallel to the meridian of Dunnose, is 79° 50′ 51′′,75 N W: therefore, its distance from the meridian is 14719 feet, and from the perpendicular 416985 feet. The figure representing Atlas is 33 feet due east of the Quadrant Room; consequently, no correction will be required in the computed latitude. The space 14719 feet subtends an arc 2' 24",3, and 416985 feet an arc of 1° 8' 30",8. These data, with the latitude and longitude of Dunnose, give 51° 45′ 38′′ for the latitude, and 1° 15′ 29′′,2 for the longitude, of the Observatory. As in the former case, with respect to Blenheim, so in the present instance, it is immaterial whether the calculations be carried on from the meridian of Greenwich or that of Dunnose, as differences of only o", 1 in both the latitude and longitude are found in the results. The latitude and longitude of this Observatory are given in the Requisite Tables; the first is 51° 45′ 38′′, and the last 1° 15′ 30′′, or 5mg in time. Doctor HORNSBY, however, has furnished me with what he conceives to be more accurate determinations; from which, and the above, we have the follow I conclude this article with expressing an opinion, that the coincidence between the computed and, no doubt, accurately observed longitude of this Observatory, affords strong reason for supposing, that the operations at Beachy Head and Dunnose, in 1794, for finding the length of a degree of a great circle perpendicular to the meridian on the earth's surface, were made with the required accuracy. SECTION THIRD. Trigonometrical Surveys of the Northern and Western Parts of Kent, the County of Essex, and Parts of the adjoining Counties, Suffolk and Hertford, executed in the Years 1798 and 1799. (See Plate XXXII.) It will be convenient to treat of the operations carried on in the north of Kent and Essex, before we speak of those executed in the western parts of the former county. In a former article I have observed, that from the old station at Wrotham, (General Roy's,) the view towards the north is obstructed, and also that it became necessary to select a new one: this station was found to be 205,5 feet from the other; the distance was accurately measured, and afterwards the angle taken at the old station, between the staff on Severndroog Tower, Shooters Hill, and the one newly chosen; this angle subtended 94° 19' 0",5. The distance from Severndroog Tower to the old station at Wrotham, is 79960 feet. But, it must be observed, this distance is not precisely the same as that given by General Roy, because an allowance is made for the error in the reduction of the bases, in the surveys of 1787 and 1788. With the distances 79960 feet and 205,5 feet, and the included angle, 94° 19′ 0′′,5, we find the distance of the Flag-staff on Severndroog Tower, from the new station = =79944 feet; with this distance, a part of the following triangles have their sides computed. ART. XXXVIII. Principal Triangles. The distances of Gadshill from Halstow, and from Halstow to the Isle of Sheppey, in the following triangle, viz. Halstow 128 34 28 Sheppey 18 18 3 Gadsbill give the distances between Gadshill and the station in the Isle of Sheppey 70687 and 70685 feet: the mean, 70686 feet, may be taken for the true distance. To find the distance between Langdon Hill and the spindle of the weather-cock on Rayleigh Steeple, we have the following quadrilateral. 360 0 0, which gives the distance from the centre of Rayleigh Steeple to the staff on Langdon Hill 44131 feet; but the point on the top of Rayleigh Tower, over which the instrument was placed, was just 7 feet farther from Langdon Hill than the spindle; therefore, 44131 + 7 = 44138 feet, is the distance between Langdon Hill and the station on the steeple.-The angles in the following triangles, Hadleigh 134° 11′ 55′′ Langdon Hill Langdon Hill 27 446 give the distance of the Spindle on Rayleigh Tower from Langdon Hill = 44131 Feet. Hadleigh = 15554 S From the preceding quadrilateral, the distance between the spindle on Rayleigh Tower and the station on Langdon Hill, was found 44131 feet, which is the same as the other determination. |