Trigonometrical Survey. 509 
The method of finding the value of a revolution of the mi- 
crometer head in parts of a degree, &c. was as follows : 
A rod, 14 or 16 feet long, was placed horizontally about 
three quarters of a mile off, and the angle subtended by its 
ends measured with the instrument in the usual way : the rod 
was then set up perpendicular at the same place, and the cross 
wires in the telescope directed to one of its extremities : the 
telescope was then moved in the vertical plane, by means of the 
micrometer screw, till the cross wires coincided with the other 
extremity. In this manner, by counting the number of revo- 
lutions, &c. necessary to move the telescope from one position 
to the other, an angle was measured vertically with the mi- 
crometer screw, equal to the former horizontal angle. From 
repeated trials, the value of a revolution was found equal 
to 10' 27". 
This instrument, on account of its portable size, may very 
readily be taken to the tops of steeples, towers, & c. and is, 
therefore, extremely well adapted to the uses for which it was 
intended. 
art. n. Situations of the Stations on which Observations were 
made with the small circular Instrument , in the Summer of the 
Tear 1795. 
Folkstone Turnpike, the station made use of by General Roy 
in 1787. 
Hawkinge, about three quarters of a mile from Folkstone 
Turnpike. This station was chosen for the purpose of having 
a view of the Belvidere in Waldershare Park, which cannot be 
seen from the station of 1787. 
3 U 2 
