December 14, 1893] 



NA rURE 



157 



EXPERIMENTS ON FLYING. 

 T F we imagine the linear dimensions of a bird increased 

 -^ n times, its weight will be increased ;r' times. On 

 the other hand, the work necessary to keep it flying will, 

 as Helmholtz has shown, increase n~ times. ^ Now, we 

 can assume that the power, that is to say, the amount of 



work that can be done in the unit of time, increases in 

 proportion to the weight, or even less. Helmholtz, there- 

 fore, concluded that large dimensions are a disadvantage, 

 and that there is a limit beyond which the power will 

 become inadequate to the increased weight. This limit, in 

 his opinion, is already attained in the largest birds, whose 

 bodies appear to be constructed with 

 the utmost economy in weight, and whose 

 constitution and food seem adapted 

 to furnish the highest power. And he 

 therefore thought it improbable that man 

 would ever be able to fly by his own power. 

 To these discouraging observations, 

 however, some objections may be raised. 

 First, the work necessary to keep a bird 

 llying horizontally depends largely on its 

 horizontal velocity. It decreases with 

 increasing velocity up to a certain limit, 

 when, on account of the friction, too much 

 work must be spent on the horizontal 

 component of the movement. The air 

 will carry a body moving horizontally 

 better than a stationary one, for the same 

 reason that thin ice will sometimes carry 

 a skater, but break under his dead 

 weight. The moving skater is carried 

 as if he rested on long skates that spread 

 his pressure over a large area. The work 

 which is expended in flying horizontally 

 with a sufficiently high velocity may, in 

 spite of Helmholtz's observations, be 

 quite within the reach of human power. 

 The difficulty, then, would only be to start 

 and to arrive at this velocity, and this difficulty might 

 be met by special contrivances. The size of a flyer 

 might therefore be increased many times without 

 losing the possibility of quick horizontal flight, though 

 birds must be able to do without such contrivances for 

 starting and arriving at the necessary velocity. 



I Helmholtz, Gesaiiunelte Abhandlungen, bd. i, p. 165. 



NO. 1259, VOL. 49] 



A second objection is that we see many birds— and 

 especially the large birds— when soaring, evidently doing 

 an extremely small amount of work, or none at all, but 

 nevertheless moving rapidly, and even rising to great 

 heights. It seems certain 'that the wind must do the 

 work for them. The experiments of O. Lilienthal have 

 shown how this is effected He has 

 made diagrams of the direction of the 

 wind blowing over a plain, and has 

 found it to be on the average three 

 degrees upwards.^ His idea is that the 

 lower regions of the air are retarded by 

 friction against the earth, and that it is 

 therefore heaped up. Of course the 

 rising air or an equal amount would 

 have to come down again somewhere, 

 and this might take place in calm weather. 

 But however this may be, the wind in 

 some way or other does the necessary 

 work for soaring birds. With a bird of 

 linear dimensions increased n times, 

 this work, it is true, would only increase 

 in proportion to the surface of the wings, 

 that is, proportional to ?/-, while the weight 

 increases proportional to ;/^. But for man 

 there would be no difficulty in constructing 

 the wing surface much larger in com- 

 parison than that of a bird. 



The principal difficulty would lie in 

 the management of the apparatus, 

 in keeping the surface in the right 

 position according to the variations 

 of the wind, and according to the 

 direction that one intends to follow. Perhaps it 

 is not greater than the difficulty a skater meets with 

 in keeping his balance while moving in the direc- 

 tion he pleases ; but the consequences of a wrong move- 

 ment are worse. O. Lilienthal seems to me to have 

 taken a step in the right direction by trying to learn 



soaring.^ The accompanying illustrations, which are 

 reproductions of instantaneous photographs taken in 

 Steglitz, near Berlin, show the way he slides down a 



10. Lilienthal, " Der Vogelfliig," p. 115; see also No. 55 of P^o- 

 jiietheiis, p. 37- 



* See his article in Nos. 204 and 205 of Prometheus, from which the 

 illustrations are taken. 



