226 Dr Hankin, The Problem of Soaring Flight 



in oiiding flight and always glided at a faster rate than the locusts. 

 This was the case whether they were travelhng horizontally or 

 o'hding upwards at an angle of 10 or more degrees with the horizon. 

 Such upward ghding was often a continuation of a horizontal 

 course. It was not due to any momentum obtained by a preceding 

 gUde in a downward direction (X). 



How, it may be asked, could turbulent motion be present of 

 sufficient force to propel the cheels and yet to have no visible effect 

 on the course of the locusts? If, as is the fact, in virtue of its larger 

 size, the cheel is a more efficient flying machine than the locust, one 

 would expect the latter to be the more readily deflected or influenced 

 by turbulence. Had dragon-flies been present, animals smaller than 

 locusts, there is no doubt that they would have been seen flying 

 faster than the latter. 



(b) A dragon-fly whose flight is mostly by flapping (Rhyothemis 

 variegata) is common near Calcutta. A soaring dragon-fly may 

 sometimes be seen gliding to and fro in a group of these flapping 

 dragon-flies. Its speed is distinctly faster than that of the latter 

 (IV, p. 388). What form of pre-existing air movement can be imagined 

 that propels the soaring dragon-fly and yet has no effect on the 

 flapping dragon-fly though the latter is of hghter weight and 

 loading? 



(c) Cheels have been seen soaring in air containing small 

 masses of discrete cloud material which were so numerous that the 

 movement of any one cubic foot of air relatively to that of adjacent 

 cubic feet could be seen if it existed. No such relative movement 

 was visible. It is difficult to see how such an observation is con- 

 sistent with the idea that turbulent movement is the cause of 

 soaring flight (IV, p. 104). 



For soaring flight to be possible under such conditions a strong 

 glare from the sun shining through cloud is necessary. Glare due 

 to light reflected from a cloud seems unable to furnish the energy 

 necessary for soaring flight (IV, pp. 102 and 105, II, p. 24). 



{d) Vultures have been seen gliding in air that was so full of 

 aerial seeds that it looked like a snowstorm. These floating seeds 

 were in slow equable movement that showed no turbulent motion 

 such as one might expect to be necessary to propel a bird weighing 

 ten or twelve pounds (IV, p. 102). 



(e) Small feathers have often been seen floating in the air in 

 the midst of soaring birds. These feathers in their course showed only 

 the smallest deviations from a straight Hne. Sometimes a slow 

 partial rotation of a feather may be seen but any evidence of 

 energetic turbulent movement is conspicuously absent (IV, p. 57). 



On one occasion a floating feather was seen to pass directly 

 under the wing of a cheel. It was instantly shot out sideways to a 

 distance of several metres from the bird, thus yielding evidence of 



