VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 651 



northern part of Russia, and in the south of France, where the methods we 

 have taken to measure this degree would also be applicable. 



Having given the length of a degree of what may be considered a gi'eat circle 

 on the earth's surface, as deduced from the observations made at Beachy Head 

 and Dunnose, and drawn such conclusions as appear to arise from it ; we shall 

 close this section with observing, that as the preserving of the points marking 

 these stations has been considered of great consequence, his Grace the Duke of 

 Richmond ordered an iron gun to be inserted in the ground at each of those 

 places, which was done in the autumn of 17Q4. By these points being ren- 

 dered permanent, the truth of this part of the operation can be examined, by 

 re-observing the directions of the meridians ; and that this may be done with 

 the least trouble, we have preserved the points, where the staffs were erected on 

 Brading Down and the Hill above Jevington, by inserting large stones in the 

 ground, having a small hole in each of them, for the purpose of denoting the 

 exact points over which the centres of the staffs were placed ; therefore the an- 

 gles which we have given, being the directions of the meridians with respect to 

 those points, can be examined without the trouble of firing lights at Beachy 

 Head and Dunnose. There is however another method of determining whether 

 61 182 fathoms be nearly the length of a degree of a great circle on the earth's 

 surface ; which is by observing the directions of the meridians at Shooter's Hill 

 and Nettlebed, whose distance is already determined, being 242731 feet nearly. 

 The points marking these stations are not likely to be soon removed, and can be 

 found without difficulty. 



The account then treats of the distances of the stations from the meridians of 

 Greenwich, Beachy Head, and Dunnose ; and also from the perpendiculars to 

 those meridians. In operations of this kind, the usual method of obtaining the 

 distances of the stations from a first meridian, and from a perpendicular to that 

 meridian, is by drawing parallels to those lines through the several stations, and 

 then proceeding in a manner similar to that of working a traverse, after the 

 bearings of the stations, with respect to those parallels, have been deduced from 

 the angles of the triangles. This mode of computation might be considered as 

 accurate, if the surface of the earth to the whole extent of the triangles was 

 reduced to a flat : and it will not produce very erroneous results, if the series of 

 triangles be in a north and south, or an east and west direction nearly, provided 

 they are on, or near the meridian, or its perpendicular ; but if the triangles be 

 considerably extended, and in all directions, the bearings of the same stations, 

 if they may be so termed, must evidently differ, and that sometimes considerably, 

 when obtained from different triangles. To avoid, in a great measure, the errors 

 which might affect the conclusions derived from the present triangles, if all those 

 distances were determined from the meridian of Greenwich only, we have con- 



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