Page:Popular Science Monthly Volume 44.djvu/770

752 subject, in the light of these experiments, has led me to some important modifications of my previous views. What, then, if any, are the fallacies in the above argument?

1. We have spoken of two functions in the wing, viz., as propeller and aëroplane. These two are of necessity united in the bird. The bird's wing was not made at once—a special organ for a special purpose—but by modification of limbs. But there was only one pair of limbs to spare for the purpose of flying, and therefore one organ, the wing, was used for both functions. Now, there is probably a disadvantage in this; for short wings with great rapidity and sweep of the stroke are probably the best propellers, while long wings are undoubtedly the best aëroplanes. Thus in long-winged birds, like the condor, the ability to rise quickly and fly rapidly is sacrificed to easy, graceful, long-continued circling and sailing; while short-winged birds, like the turkey and bustard, rise more easily and fly more rapidly, but never remain long on the wing. There seems little doubt that the limit of weight in rising is higher in short-winged birds; and the great flightless birds were of this kind before they passed beyond the limit of flight and their wings became rudimentary. Now, it is needless to say that in artificial flying these two functions may and will be separated. The propeller will be used wholly for lifting and onward progress. I suppose, therefore, that the limit of weight may be raised higher than I have placed it. But if this were all, I can not think that it could be pushed much, if at all, beyond one hundred pounds. But this is not all.

2. We have said that the animal body using fats and starch as fuel, and getting force through the mechanism of nerve and muscle, is more economical—i. e., will generate more force and do more work with the same weight of fuel and machine than any artificial contrivance yet devised or likely to be devised. This is certainly true; but there is another important element here concerned, viz., the intensity of the force—i. e., the amount of force developed and work done in a given time. This depends on the rate of combustion of the fuel. Now, there is a strict limit to the rate of combustion of fuel, and therefore of development of force, in the animal body. This limit may doubtless be greatly overpassed in an artificial machine. But be it remembered that this entails greater weight of fuel and of all parts of the machine