To the declination circle K is attached the alidade M, which may be set to any degree required, and serves as an index to direct the eye of the observer to any object, which may be viewed along the edge of it, or through the small holes in the sights O P.
Having described the several parts of the instrument, it only remains to shew the use of it; which will be best explained by means of a few problems.
PROBLEM 1. To adjust the instrument.
The instrument being made for the place of observation, need only to be placed on a perfectly horizontal stand, and with the line joining the hours XII and XII on the circle, in the direction of the meridian: the former of these adjustments may be verified by means of a small level applied to the stand at N, in the directions B C and A N successively. If found incorrect in either position, let a piece of card be put under that foot (A, B, or C) from which the bubble is found to recede, and let this operation be repeated until the bubble rests in the middle, in both positions.
The instrument may be brought into the meridian by the assistance of a magnetic needle fixed to the frame at N, or on the opposite side, or more correctly by means of the sun, provided the time be exactly known, thus:—
Set the index M to the sun’s declination, turn the circle K round its axis, till the scale L points to the hour and minute on the circle E F G. Then if the instrument be correctly placed, the sun will be seen through the sights O, P, or what is the same thing, the light admitted at the hole O will fall on the hole P. If not, the instrument must be turned about till this effect is produced.
The instrument being once carefully adjusted to the meridian on any immoveable stand, such as the sill of a window, [p376] the top of a post, &c., lines may be drawn on the stand in the direction of the sides A B, A C, or B C, by means of which it may at any time be replaced with little trouble.
PROBLEM 2. The instrument being correctly placed and levelled, the next operation will be to adjust the conical projection to the day and hour of observation.
Turn the cone round till the day of the month on the circle, at the bottom of it, coincides with the hour and minute on the circle E F G.
Example.—To adjust the cone for the 15th January, at twenty minutes past nine at night. Turn the cone till the 15th January on the circle attached to it coincides with IX h. 20 m. P. M. on the circle E F G.