no . 3 [ANGLE'S MEMOIB o.N MECHANICAL FLIGHT 23 



The results of a large number of tests show that one pound of twisted rub 



ber can put forth from 450 to 500 or more fool-pounds of work, but at the cosl 

 of an overstrain, and thai a safe working factor can hardly be taken at higher 

 than 300 foot-pounds, if we are to avoid the " fatigue " of the rubber, which 

 otherwise becomes as marked as that of a human muscle. 



AVhile twisting is an exceedingly convenient form of application of the re- 

 silience of rubber to the turning of propelling wheels, the direct stretch is, as 

 has been remarked, much more efficient in foot-pounds of energy developed by 

 the same weight of rubber. It was found that rubber could not, without undue 

 " fatigue," be stretched to more than four and a half times its original length, 

 though experiments were made to determine the amount of work that a rubber 

 band, weighing one pound, was capable of doing, the stretching being carried 

 to seven times its original length. The results varied with the rubber used and 

 the conditions of temperature under which the experiments were tried, ranging 

 from 1543 foot-pounds to 2600 foot-pounds. The tests led to the conclusion that, 

 for average working, one pound of rubber so stretched, is capable of doing 2000 

 foot-pounds of work, but, owing to the weight of the supporting frame and of 

 the mechanism, this result can be obtained only under conditions impracticable 

 for a flying machine. In the more practicable twisted form it furnishes, as has 

 been said, less than a fifth of that amount. 



The conclusions reached from these experiments are: 



1. The length of the unstretched rubber remaining the same, the sustaining 

 power will be directly proportional to the weight of rubber; 



2. With a given weight of rubber, the end strain is inversely proportional 

 to the length of the unstretched rubber; 



3. With a given weight of rubber, the work done is constant, whatever the 

 form; hence if we let w; = the work in foot-pounds, r/ = the weight of the rubber 

 in pounds, and k = & constant taken at 2000 as given above, we have 



w=kg =2000 g foot-pounds. 

 This is for an extension of seven units of length, so that for a unit of extension 

 we would have approximately 



m;=300 g foot-pounds 

 which for four units of extension corresponds very closely to the 1300 foot-pounds 

 which Pcnaud claims to have obtained. 



4. The end strain varies with the cross section for a given unit of extension. 

 These results can lead to but one conclusion ; that for the development of the 



same amount of power when that amount shall be 1 horse-power or more, rub- 

 ber weighs enormously more than a steam engine, besides being less reliable 



