The frames are now to be covered. There is a special balloon cloth made which is best for the purpose, but if that cannot be procured, strong cambric muslin will answer. Thirty yards of goods 1 yard wide will be required for the planes and the rudder. From the piece cut off 7 lengths for each plane, 4 feet 6 inches long. These are to be sewed together, selvage to selvage, so as to make a sheet about 19 feet 6 inches long and 4 feet 6 inches wide. As this is to be tacked to the frame, the edges must be double-hemmed to make them strong enough to resist tearing out at the tacks. Half an inch is first folded down all around; the fold is then turned back on the goods 2½ inches and sewed. This hem is then folded back 1 inch upon itself, and again stitched. Strips 3 inches wide and a little over 4 feet long are folded “three-double” into a width of 1 inch, and sewed along both edges to the large sheet exactly over where the ribs come. These are to strengthen the fabric where the ribs press against it. Sixteen-ounce tacks are used, being driven through a felt washer the size of a gun wad at intervals of four inches. If felt is not readily obtainable, common felt gun wads will do. The tacking is best begun at the middle of the frame, having folded the cloth there to get the centre. Then stretch smoothly out to the four corners and tack at each. It may then be necessary to loosen the two centre tacks and place them over again, to get rid of wrinkles. The next tacks to drive are at the ends of the struts; then half-way between; and so on until all are in, and the sheet is taut and smooth. For a finer finish, brass round-head upholsterer’s nails may be used. The rudder, so-called, is rather a tail, for it is not movable and does not steer the glider. It does steady the machine, however, and is very important in preserving the equilibrium when in flight. It is formed of two small planes intersecting each other at right angles and covered on both sides with the cloth, the sections covering the vertical part being cut along the centre and hemmed on to the upper and lower faces of the horizontal part. The frame for the vertical part is fastened to the two rudder bars which stretch out toward the rear, one from the upper plane, and the other from the lower. The whole construction is steadied by guys of the piano wire. [Illustration: Lilienthal in his double-deck glider. It proved unmanageable and fell, causing his death. The hill is an artificial one built for his own use in experimenting.] All wooden parts should be smoothed off with sandpaper, and given a coat of shellac varnish.