Trigonometrical Survey. 461 
was observed again. This precaution was taken to obviate the 
errors which might arise, from the arms of the instrument being 
out of the parallel with the plane of the circle, owing to any 
imperfections in the position of the Ys, on which the transit 
rested. It was, however, seldom found, that a greater diffe- 
rence subsisted between the readings of the opposite micro- 
scopes, than what might be supposed to be the consequence 
of a shake in the centre, or errors in division. A mean of the 
readings was always taken. It must be also mentioned, that 
out of twenty, three and four inch white lights, which were 
fired at Beachy Head, only three of them were seen : but the 
angle between that place and the staff on Brading Down was 
considered, from the near agreement in the observations, to be 
determined with the necessary accuracy. 
After the business was finished at Dunnose, the instrument 
was taken to Chanctonbury Ring, and Ditchling Beacon ; and 
from the latter place to Beachy Head, in order to observe the 
direction of the meridian ; but after placing a staff upon the high 
ground above Jevington, we were obliged to defer the attempt, 
as it was found, that owing to the effects of heat, the air was 
not sufficiently steady for the staff to be seen distinctly, when 
the star came to its greatest elongation in the day time, if the 
sun shone out. We therefore left Beachy Head, and proceeded 
to the following stations, viz. Fairlight Down, Brightling, 
Crowborough Beacon, and Botley Hill ; from which latter 
place we returned in June to Beachy Head, and observed the 
direction of the meridian. 
From this station, the party went to Dean Hill, and thence 
to Salisbury Plain, for the purpose of fixing on the extremities 
of the new base. This being done, the instrument was taken 
MDCCXCV. g O 
