186 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. Li 



of the wind relative to the machine, the height of the aerodrome as shown by 

 a specially sensitive aneroid barometer, and the angle of the machine with the 

 horizontal plane of the earth. The construction of this instrument was under- 

 taken by a noted firm of instrument makers, but after many mouths of delay, 

 during which it was several times delivered as being complete, only to be re- 

 turned for further work, it was finally condemned as unsatisfactory, and it was 

 decided not to encumber the machine with such a delicate apparatus, which, 

 even if perfectly made, could not be depended on to work properly wheii mounted 

 on the aerodrome frame, in which there was a constant, though minute, tremor 

 due to the high speed and power of the engine. 



The completed frame, which is perhaps best shown in Plates 49, 50 and 51, 

 ami Plate 60, Figs. 1, '2 and 3, in spite of its size gave an appearance of grace 

 and strength which is inadequately represented in the photographs. In making 

 the designs for the large aerodrome no data were available for use in cal- 

 culating the strains that would come on the different parts of the frame while 

 in the air, and the size and thickness of the tubes and the strength of the guy- 

 wires were consequently determined almost entirely by " rule of thumb," backed 

 by experience with the models. Although the dimensions, shape, and arrange- 

 ment of most of the auxiliary parts of the machine were considerably changed 

 during the course of construction in accordance with the indications of the ex- 

 haustive series of shop tests, the fundamental features of the construction were 

 practically unaltered, but the changes in the guy-wire system and in the fittings 

 by which they were attached, made the frame as a whole several times as 

 strong as it was originally, and it was felt that the direction of further improve- 

 ments in it would be shown only by actual test of it in flight where any weak- 

 noses would be certain to manifest themselves. 



It may be wed to remark here that even with the data which were later ob- 

 tained, judgment based on experience proved after all to be the safest guide for 

 proportioning the strength of the various parts. It can be assumed that a live 

 stress will produce a strain ten times as great as that due to a static stress on 

 the part when the machine is stationary. For greater safety, it would be still 

 better to assume a strain twenty times as great. If one is building bridges, 

 houses, and similar structures, where weight is not a prime consideration, it 

 would he criminal negligence to fail to provide a sufficient " factor of safety," 

 or what in many instances may be more properly termed a " factor of igno- 

 rance," while at the present time the insistence mi large factors of safety in 

 machines intended to fly would so enormously increase the weight that, before 

 one-half the necessary parts were provided, the weight would be many times 

 what could possibly he supported in the air. Later.no doubt, as experience is 

 gained in properly handling the machine in the air, increased strength entail- 



