IS2 



SCIENCE-GOSSIP. 



SAILING FLIGHT OF BIRDS. 



By G. H. Bryan-, Sc.D., F.R.S. 



tT is not often that papers read before the 

 •'■ Mathematical and Physical Section of the 

 British Association possess a biological interest, 

 but the flight of birds is a subject of importance alike 

 to physicists, to biologists, and to engineers, who 

 hope sooner or later to reproduce this form of 

 locomotion bj' mechanical means, and accordingly 

 •we have invited Dr. Bryan to furnish us ^\-ith an 

 abstract of a paper read by him last September, 

 which he has kindly sent us, with additional 

 notes. — Ed. 



Sailing Flight of Birds. 



That birds are, under certain circumstances, 

 capable of supporting themselves indefinitely in the 

 air without expending energy by flapping their 

 wings is a matter of common observation. To 

 account for this apparent realization of " perpetual 

 motion," various theories have been proposed, 

 among which the most important are the three 

 which suppose the seat of available energy to lie in : 



(i) Upward air-currents (^Mr. ISIaxim). 



(2) Variations of the wind-velocity at different 

 heights above the ground (Lord Rayleigh). 



(3) Variations of the wind-velocity from one 

 instant to another, the wind habitually blovsdng in 

 gusts separated by lulls (Professor Langley and 

 others). 



Before proceeding further, another source of 

 energy may be mentioned, namely, the presence of 

 vortices, i.e. miniature whirlwinds or cyclones in 

 the atmosphere. Even on a perfecth- calm day 

 one of these little vortices may sometimes be seen 

 travelling across a road, carrj-ing up a funnel- 

 shaped cloud of dust. According to mathematical 

 theorj', a vortex always consists of the same 

 particles of fluid ; and, even under the modified 

 conditions which occur in nature, our experience 

 of cyclones tells us that such vortices are remark- 

 able for their persistency, and their motions are so 

 regular that it would be easy for birds to take 

 advantage of them. This would account for the 

 fact that birds so often congregate in a certain spot 

 when in sailing flight. 



Against the third hypothesis has been objected : 



(i.) That to take advantage of every puff of wind 

 in such a way as to be lifted by it would be a 

 difficult feat of aerial gymnastics, whereas birds 

 appear to circle in the air without requiring to 

 exercise any particular alertness or agility. 



(ii.) That the variations in wind-velocity are not 

 sufficient to sustain the weight of a bird in the air. 



In answer to the first objection, it is to be ob- 

 served that if the bird's centre of mass is' slightly 



helo'ii' the wing-surface — especially if the wings are 

 sHghtly curv-ed upwards, as described by Mr. S. E. 

 Peel in a letter to Nature for August 6th, 1896 — the 

 action would be purely automatic. We may illus- 

 trate this point perhaps better by considering the 

 parallel effect in the seeds of many composite plants 

 (such as the common ' dandelion '), which are sup- 

 ported in the air by a parachute placed at some 

 distance above them. If a sudden gust of ^vind 

 blows upon such a seed, the parachute is set in 

 motion more rapidly than the seed, causing the 

 whole to heel over so as to receive the wind on the 

 under surface of the parachute, and this Ufts the 

 seed. When the wind subsides, the greater inertia 

 of the seed carries it on in firont of the parachute, 

 causing the latter to again present its under side to 

 the air, which again lifts the seed. The more the 

 seed is blov.-n about, the more it rises in the air. 



This action v.-ould take place automatically in the 

 same way in anj' body whose supporting parachute, 

 aeroplane, or wing-surface was slightly ahove its 

 centre of mass. The height of the supporting 

 surface should not be too great, other-wise the 

 body would heel over too much, and would make 

 so great an angle -with the horizon that the lift 

 would be considerably reduced. The effect evi- 

 dently depends on the inertia of the body, and the 

 lift could therefore be increased b}' increasing the 

 body's mass. But this v.'ould also increase the 

 weight of the body in the same proportion, so that 

 no advantage would be gained. The difficult}- is 

 overcome in the case of the sailing bird by the 

 increased buoyancy which it is able to obtain from 

 the air in consequence of the horizontal speed at 

 v.hich it travels, and herein, to my mind, lies the 

 answer to the second objection. Professor Langlej-(ij 

 has found (i) that a horizontal plane under the 

 action of gra-^-ity falls to the ground more slowly if 

 it is travelling through the air vvith horizontal 

 velocity than it would do if allowed to fall verti- 

 cally, and (2) that the horse-power required to 

 support a body in horizontal flight by means of an 

 aeroplane is less for high than for low speeds. 

 Hence it readily follows that the bird's forvvard 

 motion causes it to fall through a smaller height 

 between successive gusts of wind than it would do 

 if it were at rest, and that when a side wind strikes 

 the bird {i.e. a wind at right angles to the bird's 

 course), the lift is considerably increased in con- 

 sequence of the bird's forward Velocity. 



According to this theory, the sailing bird derives 

 its energy firom fluctuations in the wind-velocity, 

 v.hich causes it to strike the bird at right ajigles to 

 (^) " Experiments on Aerodynamics." 



