292 Captain Sabine’s comparison of barometrical and 
a second measurement, with the same precautions as on 
the first occasion, the difference did not amount to more than 
an inch and half. The extremities of the base, being abreast 
of two projecting points of land, one on the main shore, and 
the other on a small rocky island, offsets were made at right 
angles to the base, each of 3 8 feet, and the spots carefully 
marked, as containing between them the distance originally 
measured, with the additional advantage of a firm foundation 
at the extremities for future operations. This base is the 
line marked A B in the annexed plan, Plate XIV, 
A polished copper cone borne at the end of a staff was 
securely fixed at the summit of the hill ; the apex of the cone 
was proposed as the height to be measured, and was 44 
inches above the highest pinnacle of the hill. 
The base had been chosen for convenience in measure- 
ment, although its direction was not the most suitable for 
obtaining the horizontal distance of the cone nor indeed was 
the cone itself visible from B ; a third station, C, was there- 
fore selected across the bay, close to the waters’ edge, and 
the distance A C obtained from the base AB, and the angles 
at A, B, and C. AC thus became a base for the determina- 
tion of the horizontal distance of the cone, enabling its height 
to be ascertained from its zenith distance observed at A 
and C. 
The instrument used for the horizontal and vertical 
angles was a repeating circle upon the recently improved 
English construction ; concerning which, as it regards the 
vertical angles, it will be sufficient to notice, that of six 
observations of the meridian zenith distance of the sun to 
obtain the latitude of the Observatory, whereof four were on 
