In June, 1889, however, new rubber-driven models were made in which the wooden frames were replaced by tubes of light metal, which, however, were still too heavy, and these subsequently by tubes of paper covered with shellac, which proved to be the lightest and best material in proportion to its strength that had been found. The twisted rubber was carried within these tubes, which were made just strong enough to withstand the end-strain it produced. The front end of the rubber being made fast to an extremity of the tube, the other end was attached directly to the shaft of the propeller, which in the early models was still supplied with four blades. A detailed description of one of these early models, No. 26, shown in Plates 1 and 4, follows: In each of the two tubes of paper, stiffened with shellac, which form a part of the framing, is mounted a hank of twisted rubber, which connects with a propeller at the rear. There are two pairs of wings, superposed and inclined at an angle, the one above, the other below the frame. A light stem connected with the frame bears a triangular Pénaud tail and rudder. Length of model 105 cm. Spread of wings 83 " Width of upper wings 14 " Width of lower wings 19 " Diameter of propeller 29 " Area of upper wings 1134 sq. cm. Area of lower wings 1548 " Area of tail 144 " Weight of wings 51 grammes Weight of tail 7 " Weight of frame 38 " Weight of wheels 20 " Weight of rubber (.09 pound) 40 " Total weight 156 " No. of turns of rubber 100 Time of running down 8 seconds Horse-power from preceding data 0.001 HP The aerodromes made at this time were too heavy, as well as too large, to be easily launched by hand, and it was not until 1891 that the first one was constructed light enough to actually fly. This first flight was obtained from the north window of the dome of the Allegheny Observatory, on March 28, 1891, and imperfect as it was, served to show that the proper balancing of the aerodrome which would bring the center of gravity under the center of pressure, so as to give a horizontal flight, had yet to be obtained.