An instrument for drawing lines parallel to each other. The simplest parallel ruler is the common cylindrical ruler of the counting-house, represented at Fig. 1; it serves very well for common purposes, where great exactness is not required. Fig. 2 is the common parallel ruler, consisting of two flat rulers connected together by two small brass levers jointed at their extremities; the mode of using this instrument is too obvious and too well known to need explanation. An objection to the latter, of the bar moving circularly sideways, is obviated in the instrument represented at Fig. 3; the bars in these being crossed and connected at the point of intersection by a joint, and two of the ends sliding in grooves, as seen at b b, causes the rulers to move uniformly straight, or at right angles with their length. Fig. 4 exhibits another arrangement in use, whereby a similar rectilinear motion is produced by two pair of short bars, connected to an intermediate slip c, which, when the ruler is closed up, exactly fill up the cavity between the pieces on either side. Fig. 5 represents another parallel ruler of great convenience and utility.
It is usually made of black ebony, with slips of ivory at the edges, divided into inches and parts; a hole is cut through the ruler at e e, in which revolve two little brass wheels, projecting about an eighth of an inch from the under surface, and upon which the ruler is rolled, steadily held in the middle by the left hand, whilst the draughtsman draws the lines with his right. The brass wheels are fixed to a steel spindle, which turns in brass bearings at its extremities; between each bearing and brass wheel there is a little ivory cylinder, divided into equal parts, which revolves with the latter, and shows precisely the quantity of motion in the ruler, and thus enables the draughtsman to draw his lines at equal or any desired varying distances apart. The steel spindles in the more recently constructed rulers are cased over with ebony, which renders them more convenient as well as more durable.