


It has been argued that there is no proof that, weight for weight, a man is comparatively weaker than a bird, and that, inasmuch as he can raise his weight in walking up a stairway, he should be able to raise it by acting upon the air with a suitable apparatus, The weak point about this argument is not only that the weight and bulk of such an apparatus become a surcharge on the muscular power of the man, as would be, for instance, the case were an artificial pair of wings applied to an ostrich, but that among the birds themselves the power to rise vertically unaided does not exist for the larger species. These have to resort to various artifices, such as running against the wind or dropping from a perch, in order to gain that initial velocity which enables their surfaces to derive support from the air, and this probably furnishes a good reason why no flying birds exceed some 50 lbs. in weight; for small animals must possess more energy in proportion to their size than large ones.
Assuming that the speed of contraction in the muscles of two similar birds of different sizes is the same, it is evident that the work done per unit of time will be in ratio to the sectional area, or as the square of the dimensions, while the weight to be moved will vary as the cube of the dimensions; hence the rate of increase between the energy and the weight will be:
square root(Energy) varies as cubed root(weight)

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