Quadrant chiefly consists of streight Iron Bars, cross-ways joined together (as appears in the Figure) flat-ways and edge-ways, the Breadth of each of which is 2 Inches and 9 Tenths, and the Thickness \(\frac{1}{10}\) and \(\frac{3}{4}\) nearly; all those that are edge-ways being behind the others. They are so placed to strengthen them the more, and are besides farther strengthened by a great Number of short Iron Plates bent to a right Angle, and placed behind the Quadrant in the Angles made by the Bars, and riveted to them both. Behind at the Circumference of the Quadrant there is also a Bar placed edge-ways, bent circular, and fastened all along the middle of the flat Arch or Limb of the Quadrant, by a sufficient Number of little Iron Plates bent at right Angles.

The Arch or Limb of the Quadrant consists of two (every ways) equal quadrantal Arches laid upon one another; an Iron one behind, and a Brass one before; the Breadth of each being 3 Inches and 4. Tenths; and the common Part of their Breadths where they lie doubled one over the other and are riveted together, is 2 Inches and 2 Tenths; the Brass Limb being remoter from the Center, than the Iron one by 1 Inch and 2 Tenths. This Limb is reduced to a true Plane, by first placing the flat of the Quadrant very firm upon a Level or Horizontal Plane, and erecting perpendicularly over it’s Center an Iron Axis, to the Bottom of which Axis is fixed at right Angles, an Iron Arm, equal in Length to the Radius of the Quadrant, and to the other End of this Arm is fixed an Iron Scraper directly over the Brass Limb, and being firmly supported by the Arm and it’s Braces, was turned about the aforesaid Axis, ’till by scraping the Brass, the Surface of it was reduced to a perfect Plane; the Edge of the Scraper being exactly perpendicular to the Axis of it’s Motion.

Upon the Brass Limb, there are two Arches struck, one with a Radius of 96.85 Inches and the other with a Radius of 95.8 Inches, by a Beam-Compass, secured from bending by several Braces fastened to it; the inner Arch is divided into Degrees, and 12th Parts of a Degree; and the outward Arch into 96 equal Parts, each of which are subdivided into 16 equal Parts. These Divisions were made by bisecting every one of the equal Arches of 30 Degrees of the Quadrant, whereby the same became divided into 6 equal Parts, containing 15 Degrees apiece, and each of these 15 Degrees being trisected by trials, Arches of 5 Degrees were obtained; and the fifth Part of these was found by trials; and the Subdivisions of the Degrees into 12 Parts, or every 5 Minutes were made by Bisections and Trisections: the outward Quadrantal Arch was divided into 96 equal Parts by no other method than that of Bisection, ’till 60 Degrees of the Quadrant, or two thirds of it, became divided into 64, and the remaining third into 32 equal Parts, which make 96 in the whole; and every one of these were also divided into 16 equal Parts by continual Bisections. There is no occasion to direct skilful Workmen how they shall make these Divisions with the greatest accuracy; they know more ways than one how to do it of themselves. These two Sorts of Divisions are a check upon each other, being in effect two different Quadrants; and the Divisions in one being reduced into the Divisions of the other, by a Table made for that purpose, they are never found to differ above five or six Seconds in any place of the Limb, and when they do the Preference ought to be given to the bisected Divisions, as being determined by a simpler Operation. One of the 96 equal Parts of the Quadrant is \(\frac{15}{16}\). of a Degree, or 56\(\frac{1}{4}\) Minutes, and one of the 16 equal Subdivisions of every one of those, will be 3\(\frac{33}{64}\) Minutes.

To avoid the trouble of subdividing the Quadrantal Arch into smaller parts, the Telescope belonging to it, and moving about it’s Centre, carries a small Brass Nonius Plate which slides upon the Limb.