May r, 1891.J 



SCIENCE. 



245 



LETTERS TO THE EDITOR. 



•#* Correapondents are requested to be ae brief as possible. The writer^s name 

 is in all cases required as proof of good faith. 



The editor will be glad to publish any queries consonant with the character 

 of the journal. 



On request, twenty copies of the number containing his communication will 

 be furnished free to any correspondent. 



Flying-Machines . 



Fkom the age of mythology to the present time man lias at- 

 tempted to unravel the mysteries of flight, and to imitate the bird 

 in its easy conquest of the ocean above us. The study of this 

 question has been left to cranks or semi-intelligent dabblers in 

 science. One of the latest instances was that of Mr. Lancaster, 

 who was treated rather coolly at Buffalo at the meeting of the 

 American Association in 1886. An offer of a hundred dollars was 

 made for the display of a model that would meet his claims, but 

 it is needless to add that the money did not change hands. Only 

 last week, however, the usual rule was broken, as Professor 

 Langley, who has a world-wide reputation as an eminent scientist, 

 entered the lists as a champion of the idea tjhat a flying-machine 

 is practicable. We have been somewhat disappointed, however, 

 on looking carefully into his scheme, and very much fear that he 

 has only succeeded in more perfectly proving the impracticability 

 of a direct imitation of the bird. 



Professor Langley illustrates bis views by drawing a picture of 

 a man walking upon a series of cakes of ice, each one of which 

 is so small that he would sink if he does not pass very quickly 

 from one to the next. It is plain that if the man is given no 

 assistance except a violent up-and-down movement of the arms, 

 in imitation of a bird's wings, he would go down if he stood still ; 

 but suppose he had a pole resting on the bottom, it is easy to see 

 that by exerting a slight pressure upon the pole he would be sus- 

 tained by the cake of ice. We may well believe that the exertion 

 required to support a part of one's weight in this manner would 

 be very much less than that required to pass quickly from cake to 

 cake. The same reasoning may be applied to a heavy bird stand- 

 ing upon ice : it may run from cake to cake with wings closed, or 

 it may stand still and gently support a part of its weight by a use 

 of its wings. In the latter case the exertion required would be 

 much less than in the former. This idea of adaptability would 

 seem to lie at the bottom of this whole subject. 



If we had a balloon weighing two hundred pounds, and inflated, 

 it would rise till it reached an equilibrium at two thousand feet, 

 say. The exertion required to move it a limited distance in any 

 direction, down or up, or sidewise, would be exactly the same. 

 If, now, we empty the gas, we have changed all the conditions of 

 flotation ; and the covering, if compacted, at once falls with great 

 speed to the earth. To keep up this ball of cloth by a blast of air 

 would require the expenditure of a great deal of energy; and in 

 like manner, if we undertook to transport it horizontally by a 

 blast of air, and keep it from falling, it would require still more 

 force: in fact, it is evident that a horizontal blast could not keep 

 the body from falling, no matter what its force. On the other 

 hand, we may support the ball by a cord, and then we can move 

 it in any direction a short distance horizontally with the very 

 slightest exertion. 



Suppose the cloth of the balloon, instead of being compacted, 

 could be stretched in a plane surface. The velocity of its fall 

 would be much diminished; but to keep up a blast of air from 

 outside to support this plane, or to move it horizontally, would 

 require the expenditure of much more energy than before. Let 

 us change the condition and apply the force directly to the plane, 

 inclining it at the same time with the horizontal. It is evident 

 that with an angle of 45° the resistance from the air would be 

 large as compared with the skin-friction; but if the angle is made 

 very small, say one degree, the total resistance at a much higher 

 velocity would be the same as before. It would seem, however, 

 that a plane under these conditions could be balanced only with 

 the greatest diflSculty; and, as Professor Langley has said, the 

 steering and propelling apparatus have yet to be devised. It is 

 easy to see that, after all, these three points are really the essen- 

 tials ; and if it can be shown that a plane, which is so very differ- 

 ent from the bird in its form and adaptation to the air, is really 



essential to a solution of the problem, then we may say that it has 

 been conclusively proved that a flying-machine pure and simple- 

 cannot be constructed. We may hope to vie with the bird, hut 

 we can never go beyond it in its general form, adaptability, and 

 mode of action in flight. 



Professor Langley thinks we can go fast much more easily than we 

 can go slow. It is evident, however, that a bird does not support 

 itself by going fast, for we have examples of its soaring and re- 

 maining stationary for quite a long time. It would seem, also, that 

 the practical solution of the problem would be rendered much more 

 difficult at great velocities. As a matter of fact, it would be much 

 easier to go slow than fast; for the propeller, ballast, and other 

 parts would have to be increased in such a ratio as the velocity in- 

 creased, that the resistance of the air would become enormous, 

 amounting, as it does, to forty pounds per square foot at a hundred 

 miles per hour. 



Professor Le Conte of San Francisco, in a recent number of the 

 Popular Science Monthly, has summarized the arguments against 

 flying-machines, and his position certainly seems impregnable. 

 These arguments may be briefly paraphrased. 



1. We can never construct a mode of utilizing fuel or a source 

 of energy which shall equal the bird. 



3 We can never build a machine which shall have such perfect 

 adaptation to flight in all its parts as the bird has. 



3. There is a limit of weight, probably fifty pounds, beyond 

 which a bird cannot fly. Obviously a self-raising, self-supporting,, 

 and self-propelling flying-machine to carry a man is impossible. 



H. A. Hazen. 

 Washington, D.C., April S5. 



Protection from Lightning. 



I RECEIVED an invitation from you some time ago to criticise 

 your theory of lightning, and since then I have been rolling the 

 idea about in my mind to look at the lightning longitudinally, 

 transversely, and askance. It was so novel that I did not quite 

 get the idea at first reading, and it was so different from my al- 

 ready partly well defined views that I had to think about it, 

 which accounts for my delay in replying. Some of your argu- 

 ments are very strong; say, the observations of the stroke upon 

 the steeple, etc. , supposing that to be well authenticated. I don't 

 believe I am well prepared to deny but you may have the solu- 

 tion, and I should be glad to know that you had. 



Now, does not your theory imply that the first step in the trans- 

 ferrence of electric energy from an electxified cloud is to produce 

 a stress in the ether between the cloud and another adjacent body^ 

 say the nearest, either cloud or earth ; that the energy is therefore 

 in the ether until the discharge takes place, and the discharge is 

 the unloading the ether in a direction at right angles with the 

 direction of the stress ? The electricity, therefore, is not trans- 

 ferred from cloud to earth or from earth to cloud, but is only a 

 kind of static collapse. Perhaps this does not quite represent 

 your idea. A. E. Dolbeae. 



College Hill, Mass., April 19. 



BOOK-REVIEWS. 



Outlines of Physiological Psychology. By George Tridibull 

 Ladd. New York, Scribner. 

 Professor Ladd's larger work, "The Elements of Physiological 

 Psychology," is so well known to all students of this topic that 

 this abridgment of the larger work hardly calls for extended 

 notice. The scope of the work and the manner of treatment are 

 essentially similar to those of the "Elements," and its handier 

 form will undoubtedly make it a welcome vohime to a large circle 

 of students. It is distinctly the only work in English that pays 

 due attention to the experimental work of foreign psychologists; 

 and American readers, no matter what their points of agreement 

 or disagreement with Professor Ladd's views may be, should be 

 distinctly grateful for this useful service. One cannot repress the 

 wish, however, that, while so much pains and ability were being 

 exercised in compiling the volume, a little better pei-spective of 

 view, a little more lucid and attractive form of statement, ha(S 



