It will be found a good plan to use a heavier piece back of the front edge or at the top of the curve. In building your plane, follow the former directions of laying a stick on the board to give you the height of the curve. The shorter cross ribs may then be fastened by glueing to the longer ribs. By using a light lath or strip for the cross ribs, it will be possible to make them sufficiently rigid merely by glueing without the trouble of nailing. A skeleton frame of this kind has the advantage of being very elastic. In covering the frame, draw the cloth tightly across the upper side of the frame and touch with glue at regular intervals along the ribs. When dry, trim away the cloth between the points of the ribs and the open ends. The rear edge may be held in position merely by the shorter cross ribs. Trim the cloth along the edge. In such a plane it is well to stiffen the cloth covering by painting with shellac or varnish. This also lends a semi-transparent effect which improves the general appearance of the plane. By cutting away the side and end pieces of the frame such a plane three feet in width may be made to weigh less than one ounce. Since it is very important that the covering of the planes may be perfectly smooth, it will be well to experiment with several different methods of attaching the cloth or silk or paper. By covering with paper, a taut surface like a drumhead may be had. Use a rice or fiber paper and moisten the sheets by laying them between damp cloths, as was explained in detail in a previous volume. In drying, the paper contracts and tightens. In covering a frame with cloth, the angle of the two sides may be altered by stretching the covering over the raised ribs on one side and drawing it tightly from edge to edge on the reverse side. If you have difficulty in making your surface smooth, try lacing it to the sides. You will need a strong hem at the edge. By using a thread, you will be able to pull the cloth taut much the same as tent flaps are tightened. The proper curve for a flexed plane is still a matter of dispute. The highest part of the curve should come well forward, while the rear surface is drawn straight. A good camber may be plotted very simply. Draw a rectangle with a length sixteen times its height. Now mark off a point on the upper side one-fourth of the way from the left-hand corner and draw diagonal lines from this point to the two lower corners. Next round off the broad angle formed by the two lines and you will have a good curve to imitate in flexing your planes. CHAPTER VI SCIENTIFIC PROPELLER BUILDING