If it be impossible to find clear spruce lumber 20 feet in length, the spars may be built up by splicing two 10-foot sticks together. For this purpose, the splicing stick should be as heavy as the single spar--1¼ inches wide, and ¾ inches thick--and at least 4 feet long, and be bolted fast to the spar with six ⅛ inch round-head carriage bolts with washers of large bearing surface (that is, a small hole to fit the bolt, and a large outer diameter) at both ends of the bolt, to prevent crushing the wood. A layer of liquid glue brushed between will help to make the joint firmer. [Illustration: Otto Lilienthal in his single-plane glider. The swinging forward of his feet tends to turn the glider toward the ground, and increase its speed.] Wherever a bolt is put in, a hole should be bored for it with a bit of such size that the bolt will fit snug in the hole without straining the grain of the wood. The corners of the finished spar are to be rounded off on a large curvature. The ends of the struts are to be cut down on a slight slant of about 1/16 inch in the 1¼ inches that it laps under the spar--with the idea of tipping the top of the spar forward so that the ribs will spring naturally from it into the proper curve. The ribs should be bent by steaming, and allowed to dry and set in a form, or between blocks nailed upon the floor to the line of the correct curve. They are then nailed to the frames, the front end first: 21 to the frame of the upper plane, and 20 to that of the lower plane, omitting one at the centre, where the arm pieces will be placed. Some builders tack the ribs lightly into place with small brads, and screw clamps formed from sheet brass or aluminum over them. Others use copper nails and clinch them over washers on the under side. Both methods are shown in the plans, but the clamps are recommended as giving greater stiffness, an essential feature. At the front edge of the frames the ribs are fastened flush, and being 4 feet long and the frame but 3 feet wide, they project over the rear about 1 foot. The arm pieces are bolted to the spars of the lower frame 6½ inches on each side of the centre, so as to allow a free space of 13 inches between them. This opening may be made wider to accommodate a stouter person. [Illustration: Plan and details of Glider. The upper plane has a rib at the centre instead of the two arm pieces.] The posts are then put into place and bolted to the struts and the spars, as shown, with ⅛inch bolts. The entire structure is then to be braced diagonally with No. 16 piano wire. The greatest care must be taken to have these diagonals pull just taut, so that they shall not warp the lines of the frames out of true. A crooked frame will not fly straight, and is a source of danger when making a landing.