22 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. '_ I 



therefore, used on all of the earlier models, and sewed as the basis of calcula- 

 tions made to determine the amount of power that would be required to propel 

 aerodromes with other sources of energy. 



Some of the disadvantages inherent in the use of rubber are at once appar- 

 ent, such as the limited time during which its action is available, the small total 

 amount of power, and the variability in the amount of power put forth in a 

 unit of time between the moment of release and the exhaustion of the power. In 

 addition, serious, though less obvious difficulties, present themselves in practice. 



There are two ways in which rubber can be used; one by twisting a hank of 

 strands, and, while one end is held fast, allowing- the other to revolve; the other, 

 bj a direct longitudinal stretching of the rubber, one end being held fast and the 

 other attached to the moving parts of the mechanism. The former method was 

 adopted by Penaud, and was also used in all of my early constructions, but while 

 it is most convenient and simple in its (theoretical) application, it has, in addi- 

 tion to the above drawbacks, that of knotting or kinking, when wound too many 

 turns, in such a way as to cause friction on any containing tube not made im- 

 practicably large, and also that of unwinding so irregularly as to make the result 

 of one experiment useless for comparison with another. 



In 1895, some experiments were made in which the latter method was used, 

 hut this was found to involve an almost impracticable weight, because of the 

 frame (which must be strong enough to withstand the end pull of the rubber) 

 and the mechanism needed to convert the pull into a movement of rotation. 



As the power put forth in a unit of time varies, so there is a correspond- 

 ing variation according to the original tension to which the rubber is subjected. 

 Thus in some experiments made in 1889 with a six-bladed propeller 18.8 inches 

 in diameter, driven by a rubber spring 1.3 inches wide, 0.12 inch thick and 3 feet 

 long, doubled, and weighing 0.38 pound, the following results were obtained: 



Number of twists of rubber 50 75 100 



Time required to run .town 7 sec. 10 sec. 12 sec. 



Foot-pounds developed 37.5 63.0 124.6 



Foot-pounds developed per min 321.4 378.0 623.0 



Horse-power developed 0.0097 0.0115 0.01S9 



Thus we see that, with twice the number of turns, more than three times the 

 amount of work was done and almost twice the amount of power developed, giv- 

 ing ;is a maximum for this particular instance 328 foot-pounds per pound of 

 rubber. 



The usual method of employing the twisted rubber was to use a number of 

 tine strands formed into a bank looped at each end. One of these hanks, con- 

 sisting of 162 single or SI double strands of rubber, and weighing 73 grammes, 

 when given 51 turns developed 55 foot-pounds of work, which was put out in 4 



sec Is. This corresponds to 0.01 horse-power per minute for one pound of 



rubber. 



