April 27, 1899] 



NA TURE 



From the fact of its being necessary to use monochro- 

 matic light, and also from the limited range owing to the 

 short distance between the spectra, it would appear that 

 the use of the new spectroscope will of necessity be some- 

 what confined to special branches of research. For the 

 investigation of the Zeeman effect, and the resolution of 

 hitherto undivided multiple lines, it should prove of great 

 service, the relatively bright spectrum obtained rendering 

 it possible to examine the fainter lines which have 

 perforce been neglected up to the present. 



Naturally, the first essay in constructing such an 

 entirely new piece of optical work was fraught with many 

 difficulties ; but, encouraged by his success, Mr. Hilger 

 expresses himself as quite prepared to undertake the 

 building up of echelons of much higher power than the 

 one he has just completed. Ch.\rles P. Butler. 



FLIGHT OF BIRDS. 

 " TTHE way of a bird in the air" has been for ages a 

 -*- perplexing one ; and, until recently, its mechanical 

 explanation has been too much left to persons whom the 

 more sober members of the body scientific were apt to 

 class as "cranks," to use an Americanism. If confirm- 

 ation of such judgment were necessary, it was afforded 

 by a report, made to the Institute of P" ranee about 1830 

 by Navier, who was no " crank," member and reporter of 

 a commission of eminent scientific men, in which the 

 subject was discussed. Navier drew a distinction, since 

 generally accepted, between hovering, in which the bird 

 remains stationary in the air, as a hawk or kite "win- 

 nowing," on the one hand, and progressive flight, ac- 

 companied by flapping of the wings, in which the bird 

 moves forward rapidly, as a carrier pigeon or swallow on 

 a journey, on the other. A third mode of flight, called 

 soaring, has, of late, attracted much attention. It con- 

 sists in the bird maintaining forward motion, straight or 

 circling, sometimes for minutes together, with very occa- 

 sional and slight flaps of the wing ; in some cases with 

 none at all. In these three kinds of flight, observations 

 easily made by any one show that soaring appears to 

 require the least exertion, though not many birds can 

 manage it well ; progressive flapping flight is common, 

 and many birds can keep it up for hours ; while only a 

 few can manage hovering, which appears to involve much 

 exertion, and, as the Duke of Argyll expresses it, seems 

 to be the most difficult feat of wingmanship a bird can 

 execute. Navier, however, concluded hovering to be far 

 easier than progressive flight, requiring only about one- 

 twentieth of the exertion of this latter ; soaring he does 

 not appear to have considered. His results, moreover, 

 made very heavy demands on the animal, considered as 

 a machine. .According to him, a bird must be able to 

 give out energy at, or exceeding, the rate of 1000 foot 

 pounds per second per pound of its own weight, whereas 

 no ordinary land animal whose work is directly measur- 

 able gives out continuously for hours more than about 

 half a footpound per second per pound of its own weight. 

 This excessively disproportionate activity of birds, in the 

 proportion of over two thousand to one, as compared 

 with other animals, seems sufficient to invalidate the 

 basis of calculation ; but the argument is further 

 strengthened on finding that a carrier pigeon, for in- 

 stance, does not contain enough combustible matter to 

 maintain its own flight, and live, for anything like an 

 hour, if Navier be right. A thousand foot pounds is 

 rather more than one British thermal unit, so that one 

 heat unit, if wholly converted into useful mechanical 

 power, would carry a pound weight for somewhat less 

 than four-fifths of a second. The bird's feathers and 

 bones must be preserved, and water or blood are not 

 combustibles, so that not more than four ounces in the 

 pound of its weight can be reckoned as available com- 

 bustible matter. If this be as good fuel as paraffin oil 

 NO. 1539, VOL. 59] 



(and some birds are oily), each pound of the bird's body 

 can supply about 5000 heat units, able to carry it for 

 4000 seconds or thereabouts, or less than an hour and 

 ten minutes. .At the end of this time the bird would be 

 a wet feathered skeleton with a few ashes inside, rattling 

 among the bones, perhaps. On the other hand, Langley's 

 experiments, which were very carefully made, and agree 

 in all material points with others of the same nature, con- 

 clusively show that what is often called an aeroplane — 

 that is, a plane slightly inclined to the direction of its 

 motion in air — can, at reasonable velocities, be made to 

 soar while carrying a load of one pound, by an expend- 

 iture of from 5 to 10 foot pounds of work per second. 

 He also shows that the power required to sustain a given 

 weight varies almost exactly inversely as the velocity of 

 flight of the aeroplane ; while Navier, in his calculations, 

 made it vary directly as the cube of the velocity. On all 

 accounts, then, it appears highly probable that the flight 

 of birds is effected, in ordinary cases, by a process much 

 more nearly allied to that of the soaring of Langley's 

 planes than that investigated by Navier. The wings 

 must be taken to act principally as aiiroplanes, and the 

 flapping action be used to enable the bird to supply the 

 comparatively small power required for the propulsion of 

 soaring aeroplanes. In confirmation of this view, which 

 the present writer takes to be that now recognised as 

 affording the best explanation of birds' flight, he has 

 shown, in a paper recently read before the Royal Society, 

 that a machine consisting of a vertical cylinder and 

 piston, with a nearly horizontal aeroplane attached to the 

 rod, could, by the reciprocation of the piston, sustain and 

 propel itself in horizontal flight, with an expenditure of 

 energy little exceeding, perhaps falling below, that found 

 requisite in Langley's experiments. Incases comparable 

 to those of birds, rapidity of reciprocation aftects the 

 result but slightly, agreeing with the known fact that 

 some birds able to fly long distances flap slowly, while 

 others flap quickly ; whereas this would be improbable if 

 any considerable intrinsic difference in economy of labour 

 existed between slow and quick flapping. .As to hover- 

 ing, the conditions are too far removed from those of ex- 

 periments on soaring to allow reliance to be placed on 

 numerical results ; there does not appear any absolute 

 necessity for a higher rate of energy-expenditure than vci 

 ordinary flight, but the conditions of economical working, 

 particularly as to a sufficiently quick flapping, seem to be 

 more difficult of attainment. In prolonged soaring flight 

 it appears likely, from theoretical investigations by Lord 

 Kelvin, as well as consistent with observation, that local 

 air currents may be largely concerned, as a skilful bird 

 might, by gliding in and out among currents having 

 different velocities, appropriate for his own use and 

 support enough of the extra energy of the quicker cur- 

 rents, to keep himself going continuously, without any 

 exertion, beyond that involved in steering, on his own 

 part. 



With respect to the very interesting, though probably 

 not very practical, question of man's mechanical flight, 

 Langley's and Maxim's experiments show it to be at least 

 within the range of possibility ; but, if the conclusions of 

 the paper above alluded to be correct, the most eco- 

 nomical method of effecting it would involve using the 

 supporting aeroplanes as birds use wings — that is, as pro- 

 pellers and aeroplanes combined in one. Purely con- 

 structive difficulties interfere seriously with designs for 

 movable wings of any considerable size, especially if their 

 motion is a reciprocating one, while there is another 

 whole tribe of difficulties connected with the balancing 

 and steering, very possibly greater than those which 

 beset the beginner on a bicycle ; so that, on the whole, 

 we can scarcely feel as if we were within any very measur- 

 able distance of emulating the feats of Daedalus, without 

 almost certainly sufiTering the fate of his less fortunate 

 son. Maurice F. FitzGerald. 



