Trigonometrical Survey. 467 
Hill in its way. In the observations of the year 1792, the 
angle at that station, between Rook's Hill and Dean Hill, is 
noted to be dubious. The reason which induced us to be of that 
opinion was, that the telescope, by some accident, was thought 
to have been moved after the observation of the light, and just 
at the time when the angle was about to be read off. As the 
season was then far advanced, and four lights had been fired, 
without our having seen more than one of them, it was deter- 
mined to leave the final observation of that angle till this year. 
Accordingly upon our arrival at Butser Hill this second time, 
a lamp was sent to each of the stations, and the angle repeat- 
edly taken, as given in the following article. The party from 
thence proceeded to Nine Barrow Down in the Island of Pur- 
beck. 
The reason of the business commencing so early in the sea- 
son, arose from the necessity of beginning the measurement of 
the base on Salisbury Plain, towards the latter end of June, 
that it might be finished before the year should be far advanced, 
when the cultivated ground a mile to the northward of Old 
Sarum would be ploughed. It was also necessary that the 
angles at Wingreen and Highclere should be observed. 
On account of the magnitude of the 24th and 27th triangles, 
the instrument was kept at the station in the Island of Purbeck 
till the angles between Dean Hill and the stations in the’ Isle of 
Wight were determined very accurately. It was, therefore, 
not till a month after the two first lights were fired, that as 
many observations were made as we deemed to be sufficient. 
As it will answer our purpose better, to give an account of 
the stations which were chosen this year, for the further pro- 
secution of the survey, in another part of this work ; it remains 
