To the Editor of the Scientific American:

Your correspondent on this subject in the issue of April 14 cites an array of facts from which it would seem the proper conclusions should be inferred. I think the whole difficulty arises from a confusion of terms, and by this I mean a want of care to explain the unknown strictly in terms of the known; and I think underlying this error is a misconception as to what an animal is, and what animal strength is, only of course with reference to this particular discussion, i.e., in so far only as they may be considered physical organisms having no reference to the intellectual or moral development, all of which lies beyond the sphere of our discussion.

Purely with reference to the development of physical strength, which alone is under consideration, any animal organism whatsoever must be considered simply in the light of a machine.

A compound machine having two parts, first an arrangement of levers and points of application of power, all of which is purely mechanical, together with an arrangement of parts, designed, first, to convert fuel or food into heat, and, secondly, to transform heat into force, which is purely a chemical change in the first instance, and a transformation of energy in the second. So much for the animal - man or beast - as a machine physically considered.

What then is animal strength considered in the same light? The animal is not creative. It can make nothing - it can only transform. Does it create any strength or force? No. The strength it puts forth or exerts is merely the outcome of this transformation, which it is the office of the machine to perform.

What do we find transformed? Simply the energy, or potential, contained in the fuel or food we put into the machine. Its exact equivalent we find transformed to another form of energy, known as animal strength, which is simply heat within the system available for the working of its mechanical parts. How, then, is this energy which exists in the shape of animal strength used and distributed? This is the question the answer of which underlies this whole discussion as a principle. It is distributed to the different parts of the machine in proportion to the relative amount of physical work that nature has made it the office of any particular part to perform.

Let us see how it is with the bird machine. In course of flight he is called upon to remain in the air, which means that should he cease to make an effort to do this, i.e., should he cease to expend energy in doing it, he would fall during the first second of time after ceasing to make the effort some sixteen feet toward the center of the earth. But he remains in the air for hours and days at a time. What is he, then, doing every second of that time? He is overcoming the force of gravitation, which is incessantly pulling him down. That is, every second he is doing an amount of work equal to his weight - say 10 lb. multiplied by 16 - say 160 lb. approximately; all this by beating the air with his wings. Now let us institute a slight comparison - and the work shall be performed by a man, who climbs a mountain 10,000 feet high in 10 hours. The man weighs 150 lb.; he climbs 10,000 feet; 1,500,000 foot pounds is, then, the work done. He does it in 10 hours, or 36,000 seconds, which gives an amount of work of only 42 foot pounds per second performed by his muscles of locomotion.

At the end of the ten hours the man is exhausted, while the bird delights in further flight. To what is this difference of condition due? It is due simply to the difference in the machine; but this, you say, is not explaining the unknown in terms of the known. Let us see, then, if we cannot do this. In the two accounts of work done as above cited in the case of the man and the bird, an amount of energy, i.e., heat of the system, has been expended just proportional to the work done.

Now while the bird has expended more energy in this particular work of locomotion than has the man, we find the bird machine has done little else; he has consumed but little of his available heat force in exercising his brain or the other functions of his system, or in preserving the temperature of the body, and but little of his animal heat, which is his strength, has been radiated into space. In short, we find the bird machine so devised by nature that a very large proportion of the available energy of the system can be used in working those parts contrived for locomotion, and resist the force of gravity, or, what is the same thing, nature has placed a greater relative portion of the whole furnace at the disposal of these parts than she has in man. The breast muscles of the bird are so constructed as to burn a far greater proportional amount of the fuel from which all energy is derived than do the muscles of the rest of the body combined.

Let us see how it is with the man who has climbed the mountain. In this machine we find affairs in a very different state. During his climbing he has been doing a vast amount of other work, both internal and external. His arms, his whole muscular system, in fact, has been vigorously at work, all drawing upon his total available energy. His brain has been in constant and unremitted action, as well as the other internal organs, which require a greater proportional amount of energy than they did in the bird. Besides this, he has been radiating his animal heat into space in a far greater amount. All these parts must be supplied; they cannot be neglected while the accumulated surplus is given to the machinery for locomotion or lifting. This then is what constitutes what I call the difference in the machine, which is purely one of organic development depending upon the functions nature has determined that the different organs shall perform. As for the pterodactyl quoted in the last article, I have only to remark that this discussion arose purely from a consideration of what was the best type of flying apparatus nature had given man to study, and I claim that this prehistoric bird of geology does not come within this class.

For if it is not fully established that this species had become extinct long before the appearance of man on the globe, it is at least certain that the man of that early day had not dreamt of flying and was presumably content if he could find other means to evade the pterodactyl's claw.

F.J.P., U.S. Army.