[18] A singular fact connected with the stretching of rubber is that the extension is not only not directly proportional to the power producing it, but that up to a certain limit it increases more rapidly than the power, and after this the relation becomes for a time more nearly constant, and after this again the extension becomes less and less in proportion.
In other words, if a curve be constructed whose abscissae represent extensions, and ordinates the corresponding weights, it will show a reverse curvature, one portion being concave toward the axis of abscissae, the other convex.
[19] The following table taken from “Experiments in Aerodynamics,” p. 107, gives the data for soaring of 30 × 4.8 inch planes, weight 500 grammes.
| Angle with horizon α. | Soaring speed V. | Work expended per minute. | Weight with planes of like form that 1 horse-power will drive through the air at velocity V. | |||
|---|---|---|---|---|---|---|
| Metres per second. | Feet per second. | Kilogram-metres. | Foot-pounds. | Kilogrammes. | Pounds. | |
| 45° | 11.2 | 36.7 | 336 | 2,434 | 6.8 | 15 |
| 30 | 10.6 | 34.8 | 175 | 1,268 | 13.0 | 29 |
| 15 | 11.2 | 36.7 | 86 | 623 | 26.5 | 58 |
| 10 | 12.4 | 40.7 | 65 | 474 | 34.8 | 77 |
| 5 | 15.2 | 49.8 | 41 | 297 | 55.5 | 122 |
| 2 | 20.0 | 65.6 | 24 | 174 | 95.0 | 209 |
The relations shown in the above table hold true only in case of planes supporting about 1.1 pounds to each square foot of sustaining area. For a different proportion of area to weight, other conditions would obtain.
[20] This pressure per unit of area varies with the area itself, but in a degree which is negligible for our immediate purpose.
[21] See “Internal Work of the Wind”; also Revue de L’Aeronautique, 3e Livraison, 1893.
[22] More recent experiments under my direction by Mr. Huffaker give similar results, but confirm my earlier and cruder observations that the curve, used alone, for small angles, is much more unstable than the plane.
[23] As stated in the Preface, Part III has not yet been prepared for publication.