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
w = 300 g foot-pounds
which for four units of extension corresponds very closely to the 1300 foot-pounds which Pénaud 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, rubber weighs enormously more than a steam engine, besides being less reliable [p024] 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.[18]
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 weight of rubber only, without 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 regard 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 electricity 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 |
| Alcohol | 9,721,806 |
| Gunpowder | 960,000 |
| Hot water, under pressure of 100 atmospheres | 383,712 |
| Air, under pressure of 100 atmospheres, isothermal expansion | 120,584 |
| Liquid carbonic acid, at temperature of 30° and pressure of 100 atmospheres | 78,800 |
| Electric battery; short-lived, thin walled; chromic acid and platinum | 75,000 |
| Steel ring, 8 inches in diameter, at speed of 3000 turns per minute | 19,000 |
| Storage battery | 17,560 |
| Rubber, pulled | 2,000 |
| Rubber, twisted | 300 |
It may be interesting to consider next, in even a roughly approximate way, what may be expected from these various sources of energy in practice.