Dr HanJcin, The Problem of Soaring Flight 225 



6. Soaring flight is not due to the effect of ascending currents. 



(a) If isolated clouds are passing over the sky soaring dragon- 

 flies are apt to collect in the neighbourhood of a convenient 

 ascending current and glide into it whenever the sun is observed. 

 They glide out of it and keep out of its range as soon as sunshine 

 returns. The behaviour of dragon-flies is such as to suggest that 

 ascending currents are in some way inimical to their soaring flight 

 (XII). 



(b) Inland birds similarly avoid ascending currents so long as 

 the sun is shining during the day time. They use them when the 

 sun is obscured and also both in the early morning and late in the 

 afternoon when the air is not soarable. Under a cloudy sky in 

 disturbed weather some winds are soarable and some are un- 

 soarable. In the latter case only, do birds make use of ascending 

 currents (IV, pp. 20 and 283). 



7. Convection currents and soarability. 



Ample evidence exists that convection currents in the air caused 

 by the heat of the sun, whether at ground level (IV, p. 263) or at 

 a height (IV, p. 23) have nothing to do with soarability. 



8. The theory that turbulent motion in the air ynay be the source of the 



energy of soaring flight. 



In favour of this idea the following facts may be noticed: 

 (a) Atmospheric turbulence, in fine weather, decreases to- 

 wards sunset. At this time soarability for inland birds and dragon- 

 flies comes to an end at ground level. But soarabiUty at this level 

 may persist after sunset in the presence of stormy winds when 

 turbulence must obviously continue (IV, pp. 80, 281 and 375), In 

 the case of flying-fishes a loss of soarability has been observed 

 shortly after sunset in the absence of appreciable wind (XI). 



(6) The stronger the wind the more turbulence is likely to be 

 present. The speeds attained by vultures in horizontal soaring flight 

 have been measured and found to be greater the stronger the wind. 

 In winds of 10 to 20 metres per second the mean air speed of the 

 vulture has been found to be about three times the speed the same 

 bird reaches in calm after sunset when in flapping flight. As above 

 stated, both cheels and dragon-flies often show an increase of speed 

 when struck by a gust (IV, pp. 250 and 377, XII). 



9. Difficulties in accepting turbulencies whose effects can be seen as an 



explanation. 



(a) Cheels have been observed catching locusts. Each locust 

 flew in a straight line whether it was flapping, as was usually the 

 case, or gliding downwards as sometimes occurred. The cheels were 



