12 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



tion. Even in the first model, the difficulty of launching the machine or giving it 

 the necessary preliminary impulse was disclosed — a difficulty which may perhaps 

 not appear serious to the reader, but which in fact required years of experiment 

 to remove. 



By dune, 1887 two other models, embodying various changes that had sug- 

 gested themselves, had been constructed. Kadi of these had a single propeller 

 (one an l8£-inch propeller with eight adjustable blades, the oilier a 24-inch pro- 

 peller with four adjustable blades) and was sustained by two pairs of curved 

 wings 4 feet 7 inches long. It is, however, unnecessary to dwell further on these 

 details, since these models also proved altogether too heavy in relation to their 

 power, ami neither of them ever made an actual flight. 



At this period my time became so fully occupied with the experiments in 

 aerodynamics (which are not here in question) that during the next two years 

 little additional was done in making direct investigations in flight. 



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 Iightesl and best material in proportion to its strength that had 

 Keen 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 at- 

 tached 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 pari 

 of the framing, is mounted a hank of twisted rubber, which connects with a pro- 

 peller 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 Penaud tail and rudder. 



Length of model 105 cm. 



Spread of wings s:', " 



Width of upper wings 14 " 



\\ iiiih of lower wiims 19 " 



Diameter of propeller 29 " 



Area of upper wings 1134 sq. em. 



Area of lower wings 154S " 



Area of tail 114 " 



Wright of wings ;>1 grammes 



Weight of tail 7 " 



Weight of frame 38 " 



Weight of wheels 20 " 



Weight of rubber (.09 pound) 40 " 



Total weight 130 " 



No. of turns of rubber 100 



Time of running down 8 seconds 



Horsepower from preceding data 0.001 III' 



