The instruments usually had recourse to for this purpose [p373] are, celestial globes, planispheres, and atlases, but none of these afford such ready and certain means of finding or identifying particular fixed stars, as at first might be expected from them.
The Globe possesses the great advantage of being easily rectified to the place of observation and adjusted to the exact hour of the night. It likewise exhibits all the stars in their proper situations of altitude and azimuth;—but the constellations being delineated on a convex surface, and viewed from without, whilst the heavens appear to us a concave viewed from within, the groups of stars are seen reversed, a circumstance which, occasions no small degree of perplexity to a learner.
Planispheres and atlases exhibit the constellations as they appear to the eye when on the meridian, but in a position very different from that which they assume when removed far from it. In short, except the pleiades and a few remarkable groups, it is difficult to recognise a constellation in every position, without great practice and continued observation.
The Equatorial furnishes the best and readiest means of discovering or identifying any particular star, but the great price of this instrument, and the complicated nature of its adjustments, render it unfit for learners.
The instrument which we are about to describe, is in its principle the same as the Equatorial, though not pretending to any thing like the same degree of accuracy; but it has this advantage over it,—its adjustments are more simple and obvious, consequently, better adapted to the capacity of learners; and it can be afforded at a very moderate expense, the price not exceeding that of a common globe.
A, B, C, D, is the stand of the instrument, composed of three triangular pieces of wood glewed together, so that the plane of the upper piece, D B C, makes with that of the lower piece, A B C, an angle equal to the co-latitude of the place it is intended for.
On the upper piece, D B C, is described a circle, E F G, the circumference of which is divided into twenty-four hours, and every hour into twelve parts, equal to five minutes each.
From the centre of this circle, and perpendicular to the [p374] plane on which it is described, rises a pillar, the top of which appears at H above the cone I, which can be made to revolve upon it as an axis.—On the surface of the cone is delineated the principal stars visible in England, and the lower edge is divided into 365 parts, representing the days of the year.
On the top of the pillar is fitted a segment of a circle, K, of [p375] about 140°: viz. 90° of north, and 40° of south declination, which may be made to revolve upon the pillar as an axis independently of the cone. To this part is attached the scale, L, divided into degrees of north and south declination, corresponding to those on the semicircle,—and so contrived as in every situation to touch lightly the surface of the cone.