224 Dr Hanhin, The Problem of Soaring Flight 



they attain less speed than other fishes near them that are not 

 using the adjustment (XI). 



If their flight was due to undiscovered wing movements why 

 should they not be able to check it by decreasing these movements? 



(b) Dragon-flies can check speed by hanging down the abdomen 

 and hind legs (IX and XII). Those kinds of soaring dragon-flies 

 that habitually soar in a group over a restricted area commonly use 

 this air brake, when in continued flight, between about 11 a.m. and 

 3 p.m., if there is strong sunshine. If small clouds pass over the 

 sun the brake is taken out of use to be applied again when the sun 

 is clear. This happens even when the clouds are too thin to throw 

 a shadow or to cause any apprfeciable decrease in the intensity of 

 the sunshine. After the sun comes out there is a latent period of 

 about 23 seconds before the brake is reappHed (XII). These facts 

 give a striking proof that, in fine weather, the energy for their flight 

 is derived from sunshine and are quite inconsistent with the idea 

 that their soaring is due to undiscovered wing movements. 



(c) Among birds the albatross furnishes a proof that soaring 

 flight is not due to undiscovered wing movements. Observers are 

 agreed that this bird cannot soar in a calm when near sea level (VII). 

 If its soaring flight was due to wing movements, why, it may be 

 asked, should it be unable to execute these movements in the 

 absence of wind? 



5. Soaring flight is not due to the effect of lateral gusts of wind. 



If soaring animals habitually carried their wings inchned so that 

 the wing-tips were at a higher level than the body, then it is con- 

 ceivable that soaring flight might be due to the eflect of lateral 

 gusts which, striking the underside of the wings from one side or the 

 other, would give a succession of lifting impulses and hence keep 

 the bird aloft. 



This idea is negatived by the fact that in high speed flight the 

 wings are placed in the flat position. When this is the case lateral 

 gusts can have no lifting eflect. Flying-fishes sometimes place their 

 wings dihedrally-down, i.e. the wing-tips are at a lower level than 

 the bases of the wings. This disposition is probably that used in 

 flight at highest speed (XI). If lateral gusts existed and were 

 operative their only eflect in this case would be to drive the fish 

 under water. 



Though wind is favourable to the high speed flight of dragon- 

 flies, the clearest proof exists that, in calm air in the early morning, 

 when soarability is developing under the influence of sunshine, the 

 coming of the lightest draught of wind causes soaring to be replaced 

 by flapping (XII). This observation is entirely inconsistent with 

 the idea that soaring flight is due to the eflect of lateral pulsations. 



