[13] One pound of twisted rubber appears, from my experiments, to be capable of momentarily yielding nearly 600 foot-pounds of energy, but this effect is attained only by twisting it too far. It will be safer to take at most 300 foot-pounds, and as the strain must be taken up by a tube or frame weighing at least as much as the rubber, we have approximately 0.0091 as the horse-power for one minute, or 0.091 horse-power for six seconds as the maximum effect, in continuous work, of a pound of ‹twisted› rubber strands. The longitudinal pull of the rubber is much greater, but it is difficult to employ it in this way for models, owing to the great relative weight of the tube or frame needed to bear the bending strain. In either form, rubber is far more effective for the weight than any steel spring (see later chapter on Available Motors). [14] The aerodrome is sustained by the upward pressure of the air, which must be replaceable by the resultant pressure at some particular point, designated by ‹CP›. [15] See Century Magazine, October, 1891. [16] Subsequent observations indicate that the maximum velocity of horizontal flight must have been about 10 metres per second. [17] Observers following de Lucy have long since called attention to the fact that as the scale of Nature’s flying things increases, the size of the sustaining surfaces diminishes relatively to the weight sustained. M. Harting (Aeronautical Society, 1870) has shown that the relation √area/∛weight is surprisingly constant when bats varying in weight as much as 250 times are the subject of the experiment, and later observations by Marey have not materially affected the statement. As to the muscular power which Nature has imparted with the greater or lesser weight, this varies, decreasing very rapidly as the weight increases. The same remark may be made apparently with at least approximate truth, with regard to the soaring bird, and the important inference is that if there be any analogy between the bird and the aerodrome, as the scale of the construction of the latter increases, it may be reasonably anticipated that the size of the sustaining surfaces will relatively diminish rather than increase. We may conveniently use M. Harting’s formula in the form ‹a› = ‹n›^2‹w›^{2/3} = ‹l›^2/‹m›^2 where ‹a› = area in sq. cm., ‹w› the weight in grammes, ‹l› the length of the wing in cm., ‹n› and ‹m› constants derived from observation. [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.