VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 789 



which place but few observations were made, the weather being either tempestuous 

 or hazy, during the greatest part of the time they were at that station : these were 

 however made under favourable circumstances, in other respects, and are there- 

 fore likely to afford accurate results. As in the case of Rippin Tor, with respect 

 to Black Down, so Hensbarrow, in Cornwall, was selected as the spot for con- 

 necting St. Agnes Beacon with the station on Butterton; for these latter are not 

 visible from each other, the high land about St. Austle, on the northern part of 

 which is situated Hens or Hengist barrow, being higher and intermediate. The 

 staff to which the lights and star were referred, was placed on a hill called Hem- 

 merdon Ball, a secondary station in the series of 1795. On the 1st of May, the 

 party proceeded to St. Agnes Beacon, at which place the observations were com- 

 pleted on the 8th. After these directions of meridians were determined, the 

 party proceeded with the survey, and for that purpose from St. Agnes Beacon 

 repaired to Trevose Head, a promontory on the northern coast of Cornwall. Hence 

 they continued the survey through Cornwall, Devon, and the other counties to the 

 eastwards in succession ; the account of which, and the list of the angles taken at 

 the several stations, concludes the account of the operations of the year 1797* 



The object first attained the next year, 1798, consisted in a trigonometrical survey 

 of the counties adjacent to the northern and southern shores of the Thames. The 

 stations in Kent, besides that of Wrotham, where a former survey ended, were 

 Gravesend, Gad's Hill, and the Isle of Sheppey ; those in Essex were Hadleigh, 

 South End, and Prittlewell. Observations made from these places afforded data for 

 the proposed survey. The new base on Sedgemoor was next measured. This 

 measurement was begun in July, and finished in August ; in the course of which, 

 very little interruption arose from any inclemency of weather. It is unnecessary to 

 enter minutely into a description of the difficulties which arose from the frequent in- 

 tervention of ditches ; let it suffice to observe that, possessed of a 50- feet chain in 

 addition, and similar to the 100 feet one, these were rendered less material than 

 they would otherwise have been. King's Sedgernoor being sufficiently level, the 

 base was measured horizontally ; an advantageous circumstance ; and the process 

 was quite similar to that used in measuring the former bases, that have been already 

 described. After the conclusion of this operation, the party proceeded to select 

 such stations in the neighbourhood of the base, as might afford means of connect- 

 ing it with the triangles carried on in the preceding year. The two chosen for this 

 purpose, were Dundon Beacon, and a spot near the village of Moor Lynch. The 

 station at Ash Beacon was visited subsequent to these just spoken of, and afterwards 

 that on the Mendip Hills, for the purpose of taking the angle between Moor Lynch 

 and Dundon Beacon. The operations of 1798 then terminated with a diligent 

 search after some spot in Cornwall ; for a base of only 2 or 3 miles in length : this 

 search however was fruitless. 



On commencing the account of the operations in 1799> *t is observed, that 

 were the length of a degree of the meridian, in these latitudes, accurately knownj 



