24 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOI.,. -7 



for a sustained effort, and, therefore, cannot be used for propelling aerodromes 

 intended for a flight that is to be prolonged beyond a few seconds. 1 



It may be desirable to present a tabular view of the theoretical energy of 

 available motors, which it will be noticed is a wholly different thing from the 

 results obtained in practice. Thus, we represent the weighl of rubber only, with- 

 out regard to the weight of the frame required to hold it. In the steam engine, 

 we consider the theoretical efficiency per pound of fuel, without regarding the 

 enormous waste of weight in water in such small engines as these, or the weight 

 of the engine itself. We treat the hot-water engine in like manner, and in re- 

 gard to carbonic acid and compressed air, we take no note of the weight of the 

 containing vessel, or of the cylinders and moving parts. In the same way we 

 have the theoretical potency of elect ricity in primary and storage batteries, 

 without counting the weight of the necessary electromotors; and of the inertia 

 engine without discussing that of the mechanism needed to transmit its power. 



Foot pounds of energy in one pound of 



Gasoline 15.625,280 



Al< <>liol 9,721,806 



Gunpowder 960,000 



Hot water, muter pressure of 100 atmospheres 383,712 



Air. under pressure of um atmospheres, isothermal expansion 120,584 



Liquid carbonic acid, a! temperature of 30° and pressure of LOO 



atmospheres 78.S00 



Electric battery; short lived, iliin walled; chromic arid and platinum. 75,000 



SI. '.d ring, S inches in diameter, at speed of 3000 turns per minute.. 19,000 



Storage battery 17,560 



Rubber, pulled 2, 



Rubber, twisted ;:uu 



It may he interesting to consider next, in even a roughly approximate way, 

 what may be expected from these various sources of energy in practice. 



Steam Engine 

 The steam engine on a small scale, and under the actual restrictions of the 

 model, must necessarily he extremelj wasteful of power. If we suppose it to 

 realize '_' per cent of the theoretical energy contained in the fuel, we shall lie as 

 suming more than was actually obtained. The energy of the find cannot he 

 obtained at all, of course, without boiler and engine, whose weight, for the pur- 

 pose of the following calculation, must be added to that of the fuel; and if we 

 suppose the weight of tip. boilers, engines and water, for a single minute's flight, 

 to be collectively ten pounds, we shall take an optimistic view of what may be 

 expected under ordinary conditions. We have in this view 1/500 of the theo- 



ngular fact connected with the stretching of rubber is that the extension is not only not 



13 proportional to the power producing it, bul that up to a certain limit it increases more 



lly than the power, and after this the relation becomes for a time more nearly constant, and 



liu Hie extension becomes less and [ess in proportion. 



her words, if a curve be constructed whose abscissae represent extensions, and ordinates the 



ling weights, it will show a reverse curvature, one portion being concave toward the axis 



of abscissae, the other convex. 



