Roadmaker's and Drainers Sector and Plumb Rule. (fig. 74.) This instrument consists simply of a piece of board, 1.25 in. thick, the upper end pierced for the sight in the direction of the dotted line a b; the hole fitted for the eye at a, the cross wire being at b. A saw draught is passed from c to d, from which the plummet (e) is suspended. f g, an are of a circle described about the centre d, and divided into ten degrees, commencing from the perpendicular indicated by the plumb line, each degree having subdivisions of ten minutes; h, a hole cut through the board to allow free motion to the plummet; i. l, two legs, which are stretched out in setting the instrument, the narrowed end or foot of the board being stuck into the ground. The inclination, whether ascent or descent, of the surface surveyed is marked along the line of each degree, and the whole, with the subdivisions, is given in a table pasted on the board, and sold along with it. The mode of using this instrument is as follows. Set the instrument as upright and steady as possible, by striking its foot fast in the ground, with its head longitudinally in the direction in which you intend taking the inclination. Let a person mark the height of the instrument upon a staff, and take his station within reach of the eye, where you wish to know the difference of level; and exhibit the mark on the staff to the person at the instrument. The head of the instrument must then be inclined to the mark, by the eye looking along or through its sight; and when the wire crosses the mark, observe upon what line the plummet has settled, and you will find the inclination marked in degrees and minutes, which are given in feet, from 10 minutes, which is a rise of 1 in 343.8, to 10 degrees, which is a rise of 1 in 5.5, the scale being in 10-minute divisions. This instrument is a combination of all the instruments used for plumbing, levelling, and giving inclinations; is simple in its construction, easily adjusted, and can be used by any person; entirely superseding the use of costly instruments and long calculations. Invented by Mr. Archer, Road-Contractor, Auchterarder."
