75 2 THE POPULAR SCIENCE MONTHLY. 



subject, in the light of these experiments, has led me to some im- 

 portant 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 pro- 

 peller and aeroplane. 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 aeroplanes. 

 Thus in long- winged birds, like the condor, the ability to rise 

 quickly and fly rapidly is sacrificed to easy, graceful, long-con- 

 tinued 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 mus- 

 cle, 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 con- 

 cerned, viz., the intensity of the force i. e., the amount of force 

 developed and work done in a given time. This depends on tne 

 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 over- 

 passed in an artificial machine. But be it remembered that this 

 entails greater weight of fuel and of all parts of the machine en- 



* Among these possible fallacies or oversight of my previous article I have not thought 

 it worth while to mention the difference between the reciprocating motion of a wing and the 

 steady pressure of a screw propeller such as would probably be used in any artificial ma- 

 chine ; because I believe that, in the comparison, what is lost in the bird's wing in recovery 

 for another stroke, is gained in the application of the force in the direction of greates 

 efficiency. 



