Himatava Mounrarns, 201 
the latter by means of. micrometers, and is calculated..to. give elevations 
and depressions 'to two seconds. On.the horizontal circle the divisions are 
cut in brass, and are very fine, but:so close, that we were often puzzled to 
fix on the exact line of coincidence, for occasionally three lines on the 
yernier and limb appeared to the eye as equally coinciding: but in-such cases 
we take the mean, and when there is time, the observations are sometimes 
repeated on different parts of the limb—an instrument of twelve inches is 
ecco not large, but a much larger could not be carried in the moun- 
The weight is fifty pounds: with the two cases it. weighs 116 
ed and is carried in the hills on men’s backs... The, telescope was of 
twenty inches focal length, and had three eye pieces of the powers to thirty 
or forty nearly, and the wires, ten in. number, being five vertical and. 
five horizontal, were of fine spider's s web. "The advantages which cits 
cular instruments. , possess over quadrants or other portions of a. circle are 
too well known:to require much description.. They can be more accurately 
divided than. the latter, and are. capable of complete, reversion in every 
direction. - The mdex and collimation errors, are determined, on. the ob- 3 
served objects themselves, and when terrestrial angles, or the pole. star, are, 
taken, it. may be done before expansion cam have any. effect on, the 
Instrument. Whenever practicable, the circle was used on a, firm. pillar 
of brick or stone work erected for it, As to the adjustments, and levelling, 
they were always performed, - usual i in such instruments, by the ether 
level, but to make the altitude circle describe. a true vertical, J used 
the method of. bisecting the pole star, when: at its greatest elongation, 
first observing it by. direct vision, and immediately. afterwards its, image, 
with the faces of the circle, in both directions, and with the, telescope 
