The figure bears the date of 1810, the year in which the circle it was drawn from was really made ; but 1792 is the date of its first production by the artist, whose name it bears.
Beginning. our description at the bottom, the part BB represents a strong tripod, the feet screws of which, A,A, with milled heads downwards, support the whole, and enable it to stand upon a table, window-scat, or stone pillar, as circumstances may require ; its ow n packing box is also fitted up for a pedestal, and serves in cases where a better cannot be provided. Immediately above the tripod, and in close contact with it, is the azimuth circle CC ; ceiltered upon a vertical axis, which is strong ly united with the tripod at its centre. Tnis click is connected with the tripod, not only by the axis round which it. turns, but also at three places near its border, but in such a way as to admit of a motion of three or four degrees only, wholly governed by a slow-moving screw, the head of which is seen below to the right. Be low, and on the left, appears the eye-end of a telescope which is furnished with the means of vertical and ho rizontal motion. It is fixed to the circle, (not the tripod,) and partakes of the same motion with it, when acted on by the slow-moving screw. Round the vertical axis al ready mentioned, on the collide socket ll, revolves the azimuth index, near to the circle, but yet without touch ing it. This index is, like the circle, an entire plate, deep for the sake of strength, and hollow for the sake of lightness ; the edge terminates in a half contratc border, as appears at E. The index may be clamped fast to the circle by a screw, the head of which is shewn at a, when another, represented at b, furnishes a gradual slow mo tion Two micrometer microscopes FF, for subdividing and reading the divisions of the circle, are well secured to the index by three pillars each. Their positions are on directly opposite points of the index with which they revolve.
At right angles with the microscopes are erected upon The index two stout pillars GG, the use of which is chi:.f ly to sustain the weight of the upper part of the instru ment, and to render steady the ends of the transit axis. These pillars are connected by a piece T, with the socket of the azimuth axis near its upper end, by which means the pillars and socket are made mutually to support each other.
The principal part of the weight of the upper works is made to bear upon the tops of the pillars, and is sus tained by a friction-wheel at the centre of each, as at g. These wheels are urged upwards by a spiral spring, the force of which is nearly equivalent to the incumbent pressure. On receiving the weight, the wheels retire
w ithin the pillar, and allow the different parts to find their proper bearings.
For the purpose of supporting a hot izontal axis in ches long, which is nearly twice the distance of the pil lars, a strung piece, as at e, projects outwards horizontal ly from each, and to these are fixed the adjustable angles, in which the ends of the axis rest. as shewn at d. The horizontal axis II, thus supported, forms the central part or the altitude circle 1111. This circle is double, each part being w ell connected with the axis, and as firmly united to the other, by short pillars placed perpendicu larly between them. The divisions of this circle, as the one below, are degrees subdivided to every live minutes. and like them are read by micrometer-microscopes, which again subdivide to every single second. The microscopes are attached to the extremities of two bent conical tubes NX, which are screwed fast to one of the great pillars : the microscopic part is denoted by SS, and the scales, which exhibit the subdivisions, are represented at h, h. The circle is acted on by a quick and slow moving appa ratus K, which is made fast to the opposite pillar, as show n at G. It is so formed as to oppose a stout resistance in the direction of the plane of the circle, but in the line of the axis it is extremely flexible. The telescope ZZ passes through the central part of the horizontal axis, to which it is fastened ; it is also united to the double circle, between the parts of which it is situated. A lantern, not represented in the Figure, applies to one of the large pillars, opposite to the end of the axis, on the reverse side, from which light is directed through the axis, perforated for that purpose, and falling upon a plane covered with gold-leaf, is from thence reflected to the eye, for the purpose of illuminating the field of view of the telescope. A level, as shewn at X, hangs upon two pivots fixed to the crooked supports of the microscopes, and occupies a position between the pillar and divided side of the circle : one of its pivots is ad justable by the finger-screw r. The level has sliding ivory scales, which indicate to a second the position of the parts with which it is connected. A similar level, not represented in the Figure, applies occasionally to the pivots of the horizontal axis, upon which it is made to rest when in use, having been previously passed through between two of the radii of the doubleframed circle. This has also sliding divided scales, and, like the other, indicates single seconds.