October 15, 1914] 



NATURE 



^73 



-Outline of a cheel 

 curling. 



in flex-gliding at low speed. 



Fig. 3. — Outline of a cheel 

 with wings strongly 

 flexed, as occurs in fast 

 flex-gliding. 

 From " Animal Flight." 



from day to day, at which sailing flight com- 

 mences. Unfortunately, however, he gives very 

 few observations as to the time when it ceases, 

 and in this connection further records are needed. 

 It would be very much more conclusive if he had 

 plotted curves showing the number of sailing 

 birds of different kinds observed during different 

 hours of the day, in the same way that examiners 

 plot curves showing the number of candidates 

 who gain different percentages of marks. In any 

 case, he has fully estab- 

 lished the fact that "soar- 

 ability " commences at a 

 time when the sun's rays 

 are beginning to warm the 

 earth. Furthermore, the sail- 

 ing flight of these large birds 

 only occurs at a certain 

 height above the ground, 

 which is greater the heavier 

 Fig. 2.— Outline of a cheel with the bird. In the casc of 



wings slightly flexed, as seen • ,• a- i ^ ^i 



- - circlmg flight there is not 



much difficulty in attribut- 

 ing the effect to localised 

 upward convection currents. 

 Such currents would natur- 

 ally start at a definite in- 

 stant, when the equilibrium 

 of the atmosphere had 

 become unstable owing to 

 the heating and consequent 

 expansion of the lower layers. But Dr. Hankin 

 considers that this explanation is not able to 

 account for a certain form of sailing flight which 

 he describes as " flex gliding " {cf. Fig. 4). 



The peculiarity of flex gliding is that the bird 

 glides uniformly in a straight line without loss 

 of height, and that a number of vultures may 

 be seen simultaneously flex gliding in different 

 directions. If the energy required were supplied 

 by localised air currents, these would affect the 

 motion or the posi- 

 tions of the wings, 

 and Dr. Hankin finds 

 that this is not always 

 the case. 



But it must not be 

 forgotten that the ex- 

 pansion of the lower 

 layers of the atmo- 

 sphere caused by the 

 sun's heat necessarily 

 tends to lift the upper 

 layers as a whole, 

 so that, apart from 



all convection currents, there must be a general 

 upward movement which increases with the alti- 

 tude, even though there is not a breath of wind 

 at the surface of the earth. A bird gliding down- 

 wards with a relative velocity of which the verti- 

 cal component is equal to that of the ascending 

 . air would perform a horizontal flex glide. 



From a purely qualitative point of view this 

 explanation exactly fits the circumstances of the 

 case. The cause must necessarily exist under 



NO. 2346, VOL. 94] 



exactly the conditions under which the effects 

 have been observed. The only question is whether 

 the upward motion is sufficient. 



Without specifically considering this particular 

 kind of ugward motion, one of Dr. Hankin 's 

 main objections to any hypothesis of the kind 

 appears to be based on his observation that in 

 fast flex-gliding the quill feathers are bent for- 

 ward, and that in order to bend them a con- 

 siderable force, sometimes amounting to 150 

 grams on a single quill, is necessar>'. For this 

 reason Dr. Hankin asserts that " sun energy " 

 must be capable of producing a pressure on the 

 wings determined by this force. But he does 

 not adequately discuss the conditions of equili- 

 brium, which require that, for uniform motion, 

 the forward pressure on the wings must be 

 balanced by an equally large head resistance. It 

 is futile to try to explain the action on the wings 

 without accounting for the equal and opposite 

 reactions on the bird's body. 



The attempts which Dr. Hankin makes to deal 

 with these statical and dynamical considerations 

 cannot be regarded as altogether satisfactory. In 

 the preface he claims to have avoided the use of 

 technical terms. So, indeed, have many other 

 writers of books on flight, but the mistake is 

 when they substitute something far worse. This 

 book contains several instances of the misuse of 

 the principles and nomenclature of theoretical 

 mechanics. For example, on p. 20S the author 

 finds that there are four chief forces acting on a 

 bird commencing gliding, namely, the pull, the 

 drag, the lift, and the weight, and he says : 

 "The pull consists of the momentum of the 

 bird. This acts in a horizontal direction at the 

 centre of gravity." On the next page he makes 

 the drag and this momentum form a couple 

 "that tends to rotate the bird upwards round its 

 transverse axis — in other words that tends to 

 maintain the angle of incidence." Such an un- 



FiG. 4. — Chrel circling and then flex-gliding up-wind at slow rate. At the time there was thin cloud and no fast 

 flex-gliding had occurred. A few minutes later as cloud got thinner, fast flex- glidirg of cheels began. Track 

 mark ed at i sec. internals. Wind at the time not strong enough to move leaves. Fiom " Aniiuil Flight." 



balanced couple would increase, not maintain, the 

 angle of incidence, as he himself shows elsewhere. 



Not contented with confusing momentum and 

 force, the author says, on p. 206, "Thus we 

 have arrived at the conclusion that in flex gliding 

 the sun energv stored in the air acts as a constant 

 force." 



It is also a pity that the known term "in- 

 stability " is used to denote rotations which he 

 evidently considers are often due to atmospheric 



