There are a good many conventionalities adopted which have no correspondences in fact, with the object of saving the draughtsman's time; or else, as in the case of superposition of plans and sections, to show in one view what would otherwise require two drawings. Among the convenient conventionalities are the indications of toothed wheels by their pitch lines only, of screws by parallel lines and by diagonal shade lines; and of rivets, bolts and studs by their centres only. The adoption of this practice never leads to error. In the preliminary preparation of drawings in pencil no distinction is made between full or unbroken lines, and dotted or centre lines, and the actual outlines of the objects. These differences are made when the inking-in is being done. Indian or Chinese ink is used, because it does not run when colours are applied. There are conventional colours used to indicate different materials. But colouring is not adopted so much as formerly, because of the practice of making sun prints instead of the more expensive tracings for the multiplication of drawings. When tracings are coloured the colour is applied on the back instead of on the side where the ink lines are drawn. The economical importance of the printing department of the drawing office cannot be overestimated. Before its introduction drawings could only be reproduced by laborious tracing on paper or cloth, the first being flimsy, the second especially liable to absorb grease from the hands of the workmen. By the sun copying processes (see SUN COPYING) any number of prints can be taken from a single tracing. But even the fickle sun is being displaced by electricity, so that prints can be made by night as well as day, on cloudy days as well as on bright ones. Twenty minutes of bright sunshine is required for a print, but the electric light produces the same result within five minutes. Prints are blue, white or brown. The advantage of white is that they can be coloured. But the majority are blue (white lines on blue ground). All can be had on stout, thin or medium paper. An innovation in drawing-office equipment is that of vertical boards, displacing horizontal or sloping ones. They have the advantage that the draughtsman is able to avoid a bending posture at his work. The objection on the ground that the tee-square must be held up constantly with one hand is overcome by supporting and balancing it with cords and weights. (J. G. H.)