Upon making up one of these ribs and testing it, it was found to possess remarkable stiffness, so much so that it was thought probable that it was as stiff in proportion to its size as the best thing that Nature had produced in the bird’s wing. A large quill from the wing of a harpy eagle was therefore stripped and the large end clamped in a special holder, and measurements were made of the deflection produced by weights at various distances from the clamp. As the main mid-rib of the wing of the aerodrome is placed approximately at the point of the center of pressure, the bending action on the cross-ribs may be assumed to act on a lever arm from the mid-rib towards the front, and from the mid-rib towards the rear in the cases of the pressure on the front and rear portions of the wing, respectively. In testing these cross-ribs, therefore, against the quill, the rib was clamped at the point where it crosses the mid-rib of the wing, and measurements were made of the deflection produced by weights placed at various distances from the point of clamping both front and rear. [Illustration: PL. 66 DETAILS OF RIBS AND FITTINGS FOR WINGS] [Illustration: PL. 67 CROSS SECTION OF RIBS] The quill on which the measurements were made was 19.5 inches long and had a gradual curve, the highest point of the curve being about the center of the length of the quill, and the depth of curvature being about 2 inches. When the butt of this quill was placed in the clamp the tip stood 17 cm. above the horizontal. The hollow spruce rib, when clamped at a point 5 feet from the tip (the point from which it tapers in both directions) had its tip 2.2 cm. above the horizontal, there being very little curve in that portion of the rib. The quill weighed 4 grammes when stripped and 18 inches of it projected from the [p201] clamp which held it during the tests. The rear portion of the spruce rib projected 5 feet from the clamp, being thus 3.3 times as long as the quill, and it weighed 120 grammes, the weight for the larger size having, therefore, increased slightly less than the cube of the length. The results of the tests of both the quill and the rib are given in the following table. The approximate cross-section of the quill at the point of clamping, the middle and the tip are shown in diagrams ‹A›, ‹B› and ‹C›, respectively, of Plate 67. The cross-sections of the rib at the corresponding points are shown in diagrams ‹D›, ‹E› and ‹F›. The cross-sections of the quill, enlarged five times, are shown in diagrams ‹A′›, ‹B′›, and ‹C′›. QUILL FROM THE REMIGES OF HARPY EAGLE. Weight, 4 grammes; length, 45 cm.; tip, 17 cm. above butt when the latter is horizontal.