VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 631 



means of the screws; and as it was found to be exceedingly difRcult to turn them 

 equally, and by a quantity which was just sufficient, an application was made to 

 • Mr. Ramsden to apply something to the axis, which might enable us to adjust 

 the circle with greater ease and accuracy. Accordingly, on the party arriving in 

 town, the instrument was taken to his house, and left there for the winter, 

 during which he made the desired alteration. 



The progress made in the survey during the last season, determined the extent 

 of the business for this year, 1793: and it was then imagined, that with good 

 weather, we might be enabled to join the triangles to the eastward with those of 

 General Roy, and also observe the remaining angles in the series, having first 

 made the necessary observations at Dunnose and Beachy Head for obtaining the 

 directions of the meridian. It had also been foreseen, that it would soon become 

 necessary to select some spot for the measurement of a new base, not only to 

 verify the triangles remote from Hounslovv Heath, but also to determine the 

 sides of those which might be afterwards projected for the survey of the west of 

 England. The situation which we had looked forward to, as being the only one 

 which would afford a base line of sufficient extent, was Sedgemoor in Somerset- 

 shire, not having then imagined that any place could be found fit for the purpose 

 to the eastward of that situation. 



By maturely deliberating on the steps to be taken for this necessary business, 

 it soon appeared that Sedgemoor, from its remoteness, would not suit for a base 

 which was intended to be applied as a test to the sides of the great triangles, 

 which were now constituted. Inquiry was therefore made for a spot which 

 might be less exceptionable; and as information was obtained that Longham 

 Common, near Poole, in Dorsetshire, was likely to afford such a base, we ex- 

 amined it in the January of this year; but not finding it fit for the purpose, we 

 proceeded to Salisbury Plain, where we found that a base line of nearly 7 miles 

 might be measured without much difficulty between Beacon Hill, near Ames- 

 bury, and the Castle of Old Sarum. With respect to the nature of the ground, 

 as any observations concerning it will be introduced with more advantage when 

 we treat of the particulars of the measurement, it will be only necessary to ob- 

 serve, that prior to determining on the possibility of measuring it with the 

 necessary accuracy, we considered of the errors which would be likely to creep 

 in from the many hypotenuses which the base would consist of, and from other 

 circumstances which the ground from its inequality might be supposed to produce. 



As the principal object of this year's business was, to determine the directions 

 of the meridians, the party left London for the Isle of Wight early in the month 

 of March, that they might arrive at Dunnose in proper time for making the 

 requisite observations. The instrument however was first taken to Motteston 

 Down, for the purpose of intersecting many places whose bearings had been last 



