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and as we assume throughout this work that the student has little, if any, previous knowledge of the subject, it is especially necessary that he should know what constitutes a good serviceable instrument, as the possession of inferior ones will be a constant source of annoyance to him. Fig. 7 7. In giving the characteristics of a good instrument, it is of the first importance to understand the use to which it is applied. With draughtsmen, a pair of compasses and a pair of dividers serve two very different purposes, and are therefore differently constructed, but their names and uses are often misunderstood. " Compasses " are never used for dividing, nor are "dividers" applicable to compass- work. Beginners should therefore note that the former are specially intended for putting in circular lines in pencil or ink, and that the proper and only use of the latter is the division or measuring-off of lines and spaces. These separate and distinct purposes give at once a clue to their proper form and construction. They are both instruments with two movable legs, joined together by a forked end, and secured by a pin and washer, as shown in Figs. 7 and 8 at A, A. The compasses, however, being used to draw circular lines, or lines described about a point everywhere equi- distant from it, should have jointed legs, one with a knee-joint at B, and the other with a socket, as at C, to enable it to be easily removed and replaced by the ink- or pencil-points D, E, Fig. 10, when required. The purpose of the knee-joints shown at B in the compasses, and b b in the pen and pencil points, is to enable the lower parts attached to them to be adjusted perpendicular to the surface of the paper, in order to obtain a truly circular line, and to allow both nibs of the