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purpose, however, is only effected by attaching the battens to the back of the board in the following manner : At the middle of the length of each batten which should be 1 in. less than the width of the board a stout well-fitting wood screw is firmly inserted into it, and made to penetrate the board for about J in., the head of the screw being made flush with the surface of the batten. On either side of this central screw two others, about 3J in. apart, are passed through oblong holes in the battens, and screwed into the body of the board until their heads are flush with the central one ; fitted in this way the board itself can expand or contract lengthwise or crosswise, while its surface is prevented from warping or bending. The working surface of the board or its front side should be perfectly smooth, but instead of being quite flat it should have a very slight camber, or rounding, breadthways, this latter feature in its con- struction being to prevent the possibility of a sheet of paper when stretched upon its surface having any vacuity beneath it. The four edges of the board need not form an exact rectangle, as much valuable time is often wasted in the attempt to produce such a board ; but it B 2 FIRST PRINCIPLES OF will answer every purpose of the draughtsman so long as the adjacent edges at the lower left-hand corner of it are at right angles to each other, or square. To produce really good work in the shape of a mechanical drawing, one perfectly straight edge only is required on a drawing-board, and that the left one, which is always known as the working edge ; but for the convenience of being able to draw a long line across the board at right angles to its lower edge, this edge is made