The circle is divided on the silver ring shown in the plan, into 720 parts, each of which answers to a degree ; as this in strument measures double the angle shown upon the arc, the same as the Ilad ley's quadrant, these are subdivided into three, each of which will be twenty mi nutes. The verniers include fifty-nine of these divisions, and are divided into sixty, the coincidences of these will subdivide each original division of the circle into sixty parts, each equal to twenty seconds. The arm on which the vernier, ll, is fixed, has a clamp at its end to fasten it to the circle ; and a fine screw, x, to move it slowly a small quantity after it is clamped.
We shall now describe the manner of making an observation by this instrument, of the angle between two objects, nearly in the same horizontal plane; we suppose all the adjustments of the instrument to be perfect ; the observer first holds the instrument in his right hand by the han dle screwed to the lower handle of the tube G, he looks through the telescope, k, and unsilvered part of the horizon glass, , lass h and directs it to one of the ob jects which will be in the dotted line, he then turns the index and index glass, b, by its arm D (which must he undamp ed) until the other object in the line, y b, is reflected from b to h, and from h, by the silvered part of the glass, into the line in which is placed the obsener's eye ; he then clamps the arm, D, and gently turns the screws, x, backwards or forwards, until the reflected image of the object in the line y, and the other object seen through the telescope, both exactly co ver one another. The observation is now
half made, and the observer reads off and writes down the degrees, minutes and se conds, of each vernier, he then inverts the instrument, holding it by the handle, in, and directs the telescope to the object, in the line y b, and brings the reflected image of the object, in the line k g, into view, by turning round the index and in dex glass the same as before ; the obser vation is then read off and registered.— To determine the angle measured, a mean of the products of both observations must be taken; this is the angle between the lines, y b and k g. A small micro scope, M, in the plan, is used to examine the verniers, and it can be applied to either verniers as required.
The dark glasses, op, are only wanted in observing the sun or moon.
It is evident that by inverting the in. strument, as we have described, the index error is of no consequence, as it will be always more in one observation and less in the other,