The “tail-rudder,” shown in Plate 17, comprised a vertical and horizontal surface of silk intersecting in a central rod or axis, having a length of 115 cm. (3.8 feet). The framing was of spruce and consisted of two sets of four arms, each radiating from the central rod, the hexagonal outline of the surfaces being formed of piano wire, over which the silk was drawn and sewed. The area of each surface was about 0.6 square metres (6.45 square feet), and the total weight was 371 grammes (0.8 pounds). A flat steel spring inserted in the forward end between the rudder and the midrod gave it a certain desirable degree of elasticity in a vertical direction. The rudder was held in place by a pin passing through the midrod, and was so set as to coincide with the line of direct flight, its purpose, as already explained, being to guide the aerodrome, but to take no part in its sustention. In balancing Aerodrome No. 5 on May 6, the wings were so adjusted that in accordance with the notation given above, p. 15: ‹CP›_{fw} = 1575 ‹CP›_{rw} = 1415.5; and as the wings were of equal size, from what has preceded in the present 3‹CP›_{fw} + 2‹CP›_{rw} ‹CP›_1 = ------------------------------  = 1501.2. 5 The center of gravity was located at 1497, so that there should have been a very slight tendency on the part of the aerodrome to rise, as was actually the case. The formula was perhaps not quite so accurate as the prolonged flight of the aerodrome would seem to indicate, as it takes no account of the thrust of the propellers, which in action tended to elevate the aerodrome in front while their resistance would tend to depress it when they had ceased to revolve, which consideration accounts for the action of the aerodrome on May 6, as described in Chapter IX. The formula may, however, be regarded as approximately correct. In the final successful trial with No. 6 on November 28, 1896, the wings used were similar in general construction and manner of guying to those of No. 5 on May 6, but, as shown in the photograph (Plate 29A, Chapter X), the front rib at its outer extremity was bent to a quadrant to connect with the midrib, this construction being somewhat stronger than that adopted in the wings of No. 5. The curvature was but one-eighteenth of the width of the wing instead of one-twelfth as in No. 5. The front and rear pairs were similar and equal and had a combined area of 5 square metres (54 sq. ft.), and a weight of 2154 grammes [p091] (4.74 lbs.). The flying weight of the aerodrome was 12,120 grammes (26.7 lbs.), the sustaining surface thus amounting to slightly more than 2 square feet to the pound. The position of the wings, in accordance with the notation adopted, was ‹CP›_{fw} = 1563.2, ‹CP›_{rw} = 1374. Since the wings were equal in size, 3‹CP›_{fw} + 2‹CP›_{rw} ‹CP›_1 = -----------------------------  = 1487.5. 5