222 Dr Hankin, The Problem of Soaring Flight 



THE PROBLEM OP SOARING FLIGHT. 



By E. H. Hankin, M.A., Sc.D. " 



This paper contains a short summary of existing evidence as to ; 

 the nature of soaring flight. 



1. Comparison of soaring flight in birds, dragon-flies and flying-fishes. \ 



Birds, dragon-flies, and flying-fishes can exhibit two kinds of { 

 soaring flight — slow and fast — characterised by different wing 

 dispositions. 



As a rule, both with birds and flying-fishes (XI)*, slow soaring 

 flight is carried out with wings dihedrally-up {i.e. with wing-tips 

 at a higher level than the body). In fast soaring flight the wings 

 are either flat or occasionally in the case of flying-fishes dihedrally- 

 down. In dragon-flies the wings are strongly dihedrally-up in slow 

 flight and either flat or less dihedrally-up in fast flight (XII). 



In each of the three classes of soaring animals slow soaring flight 

 is dependent, as a rule, on the presence of sunshine and fast flight 

 is always dependent on the presence of wind (IV, pp. 52, 98, 251, 

 299). 



With birds and flying-fishes lateral instability occurs more often 

 late in the afternoon, when the air is becoming unsuited for soaring 

 flight, than at other times of the day (IV, p. 294 and XI). A rare 

 form of lateral instability in which the oscillations are just too rapid 

 to count has been observed in each of the three classes of flying 

 animals. 



The speeds attained in soaring flight seem to be remarkably 

 similar in the three classes. The slow speed flight is between 5 and 

 10 metres per second. The high speed flight has been estimated for 

 dragon-flies at above rather than below 15 metres per second (XII). 

 For flying-fishes, in strong winds, it may exceed 20 metres per 

 second (XI). The air speed of vultures has been measured and 

 found to reach a mean speed of 20 metres per second in winds of 

 medium strength (XI). 



In view of the above resemblances between the soaring flight 

 of birds, flying-fishes and dragon-flies there seems to be no room 

 for doubt that we are dealing with the same phenomenon in each 

 case. 



2. The regularity of soaring flight. 



A remarkable, though familiar, phenomenon is shown by a flock 

 of cranes in soaring flight. The birds may be seen to keep their 

 distances from each other with marvellous exactitude and this not ' 

 only when they are ghding in a straight fine but also when they are 



* Roman numerals in brackets refer to the appended list of my publications.. 



