ANTIMETER, or REFLECTING SECTOR, an instrument invented by Mr William Garrard, for the purpose of measuring angles, particularly small ones, with a greater degree of accuracy than can be done by Hadley's quadrant or by the sextant.
The frame of this instrument is similar to that of Hadley's quadrant, having two radii, a limb, and braces; but with this difference, that the further radius is produced upwards of four inches beyond the centre of motion of the index; and the great speculum, or what is called the index-glass in Hadley's quadrant, being placed there, is called the upper centre. In this instrument there is no provision for the back observation.
The
Antimeter. The horizon-glass is like that in Hadley's quadrant; there are two sight vanes, to suit two different situations of the large speculum or object glass: these vanes are adapted to receive a small telescope. On the centre of the index, where the index-glass of Hadley's quadrant is fixed, is a brass or bell-metal semicircle, two inches in diameter, and one-eighth of an inch thick: this semicircle is screwed fast to the index, in such a manner that the axis of the index is a tangent to it. On the upper centre are two circular brass plates, which revolve concentrically, either together or separately. The under plate has a lever, or part perpendicular to the plane of the instrument, projecting downwards, a little beyond the lower centre: this lever is acted upon by the semicircular plate at the lower centre, to which it is always kept close by a spring on the other side. In the upper of the above mentioned circular plates are two circular perforations or slits, through one of which a screw takes into the head of the instrument, and thro' the other a screw takes into the lower moveable plate. The large speculum is fastened to the upper plate; and by the above mentioned screws the position of this glass may be altered. A circular plate is fixed to the lower centre by three pillars: in its centre is a nut to admit a screw, by which the plate carrying the large speculum may be fastened here occasionally.
The scale on the limb is divided into 45 equal parts or degrees, and not into half degrees as is the case in Hadley's quadrant, by reason of the double reflection. These divisions are numbered in a retrograde order; zero being at the extremity of the further radius. Although the limb contains 45 degrees, yet the greatest angle which can be measured, the large speculum remaining fixed to the circular plate, is ; the distance between the two centres being four inches, and the radius of the semicircle one inch. Agreeable to these dimensions, the inventor has given a table exhibiting the value of each primary division on the limb; he hath also given a more ample table, adapted to a distance between the centres of three times the radius of the semicircle, which he says hath been found the most convenient in practice. If an angle greater than is wanted, it may be measured by the method of anticipation, as the inventor calls it, which is as follows: Let the screw which fastens the two circular plates on the upper centre be made fast, and loosen the screw which fastens the upper circular plate to the instrument: Now adjust the glasses by the usual method; bring forward the index to any given division on the limb, and make it fast; also fasten the screw which was before loose, and loosen the other screw; then bring the index to zero, and proceed as before.
The inventor gives the following directions for adjusting and using the instrument.
The first thing to be attended to is, to set the horizon-glass perpendicular to the plane of the instrument, which is performed as follows: Hold the instrument with its plane perpendicular to the horizon, and look over backwards into the glass and beyond it. If the limb of the instrument appears in a right line with its reflection, the glass is upright; but if it does not appear so, loosen or tighten the little screw on the foot of the glass until it be adjusted: Then with the instrument, as in taking an altitude, look through the sight vane or telescope at some distant object, with the index fixed
in any intended situation; the two screws at the upper centre being loose, turn the glass about till the same object appears nearly in the same part of the horizon-glass: Next hold it in a horizontal position, and adjust the object-glass or large speculum with the screws which are behind and before, on the foot of it, till the object and its reflection are seen in the same horizontal line. Lastly, with the instrument upright, turn the tangent-screw belonging to the horizon-glass at the back of the instrument, until there be a perfect coincidence of the object and its reflection that way, and the adjustments are complete.