QUADRANT, from quadrans, the fourth part of a circle, or 90°, is the name given to an instru ment for measuring angles not exceeding 90°, though it may be fitted up so as to measure greater angles.
In our article ASTRONOMY, we have explained the principle of the astronomical quadrant, and have also described Bird's tiliar quadrant. alr. Troughton's as tronomical quadrant, and the method of adjusting and using the astronomical quadrant.
We shall now, therefore, proceed to describe various other quadrants for astronomical and nautical pur poses.
1. Description of Graham's Mural Quadrant in the Royal Observatory at Greenwich.
This instrument which has become so celebrated in the history of astronomy, was made by Mr. George Graham, and was presented to the observatory of Greenwich by George I. for the use of Dr. Halley, who made an immense number of observations with it on the moon.
With the exception of the circular limb, the quadrant is chiefly composed of straight iron bars, joined toge ther, as in Plate CCCCLXXVI Fig. 1. The breadth of every bar is two inches and nine-tenths, and its thickness 1 i tenth. The lines in Fig, 1. represent the disposition of all the flat bars, or those in the plane of the quadrant, and those in Fig. 2. the perpendicular bars, or those at right angles to the former, and placed behind the flat ones. The whole fabric is farther strength ened by a great number of short iron plates, or pieces of the same iron bars, bent to a right angle, and placed behind the quadrant in the angles made by the flat and perpendicular bars, and rivetted to them both. Their number, and the places where they are rivetted, arc shown in Fig. 2. by the small parallelograms adjoining to the lines, and in order to make more room for the rivets, the edge of each perpendicular bar does not divide the breadth of the flat bar in the needle, but in the ratio of two to one ; and the little plates are rivctted on the broader side. The black thickening of the lines at their crossing in Fig. 2. represents small iron plates bent at right angles, and rivetted in the angles made by the intersections of the perpendicular bars. At the circum ference of the quadrant there is also a perpendicular bar ben( circular, and fastened all along the middle of the breadth of the limb or flat arch of the quadrant, by a sufficient number of the above mentioned little plates.
The limb of the quadrant consists of two similar quadrantal arches, one of iron, and the other of brass, laid over it. The breadth of each is thijee inches four tenths, and the common part of their breadths, where they lie doubled over one 'another, and are rivetted to gether is two inches and two-tenths, the brass limb being an inch and two-tenths more remote front the centre than the iron one. In order to reduce the limb to a true plane, the quadrant a b d o, Fig S. was placed firmly on a level plane with its brass limb upwards. To a vertical axis I in, pointing to the centre o, was fixed an iron arm in n, carrying an iron scraper p, which, when turned round the axis scraped the brass to a perlect plane, the edge of the scraper being correctly perpen dicular to the axis of motion.
Two arches were struck upon a brass limb, one with a radius of 96.85 inches, and the other with a radius of 95.8 inches.
The outward arch is divided into ninety-six equal parts each, which are again divided into sixteen equal parts, and the inner arch is divided into degrees and twelfth parts of a degree.* The beam of the compass, by which these arches were struck, was prevented from bending by several braces, and when an arch was struck, 60 degrees of it was determined by placing one point of the compass at e, and making a stroke with the other at b. The arch a b was then bisected in c, by drawing two small arches upon a and b as centres, and with such a radius, as to cross the arch a c b in two points, as near together as possible without touching. The small interval between these points was then bisected at c by the estimation of the eye, aided by a magnifier. After this, the linen ids a c or c b was taken with the beam compass, and trans ferred from b to d, which determined the length of the quadrantal arch a d. Each of the three arches being bisected in the same manner, the quadrant was divided into six equal parts, containing fifteen degrees each, and every one of these was divided into three equal parts, in the following manner. To avoid making any superfluous points in the quadrantal arch, with its ra dius unaltered, but upon any other centre there was struck a faint arch, upon which the chord of fifteen degrees, already found, was transferred from the qua drantal arch, and the third part of fifteen degrees, being determined by trials on the faint arch, was transferred back again upon the quadrantal arch, which was thus divided into eighteen equal parts% containing five de grees each, and the filth part of these was found by trials as before, in dividing a separate arch, drawn up on a new centre, for this purpose only, the subdivi sions of the degrees into twelve equal parts were made by bisections and trisections as before, so that the whole quadrant was thus divided without using any superflu ous point The outward arch of the quadrant was divided by bisections into ninety-six equal parts, till sixty degrees or two thirds of the quadrant became divided into sixty four, and the other third into thirty-two equal parts, making in the whole ninety-six. Every one of these was likewise divided into sixteen equal parts by con tinued bisections. These two sorts of divisions form a check upon each other, being in reality two different quadrants ; and the divisions of the one being reduced to those of the other by a table. (See ASTRONOMY.) They never differed above five or six seconds in any part of the limb, the preference being always given to the bisected divisions, as having been determined by a simple operation.