74 ON FLIGHT, 



Several writers have since then dwelt on the subject of flight. 

 Of these the careful essay of Miillenhoff (1) requii-es our especial 

 attention. 



Although Miillenoff dwells on his dissent from my views 

 extensively, he acknowledges the correctness of my deduction 

 referred to above, by stating that the correlation between the 

 rapidity of wing-flaps and weight of the body can be expressed by 

 asserting that the " centre of resistance " always moves with nearly 

 the same velocity. 



Among his numerous valuable statements, I will particularly 

 refer to his ingenious, and doubtlessly to a great extent correct 

 explanation of the mode in which eagles and vultures can rise by 

 describing circles in the air without the movement of their wings. 

 His explanation is the following : — He says the bird moves with 

 the wind, with his head turned to the direction to which the wind 

 blows downward, and moves in the opposite direction with his 

 head facing the wind ui^ward. The force which enables the bird 

 to rise higher when he is moving against the wind than he has 

 sunk when he was moving with the wind, is derived from the 

 feathers being raised from the body when the bird moves with the 

 wind, when the wind comes from behind, and that these feathers 

 lie close to the body, so that it presents a much smaller surface 

 when the bird moves against the wind. At the same time 

 MullenhoflP asserts that the bird moves with the wind, describing a 

 screw- line, following the surface of an inclined cylinder, in the 

 direction of the wind. I think that this explanation is a most 

 ingenious one, but I would at the same time like to submit, that 

 it appears highly probable that the bird moves its wings at the 

 same time round the axis of the forearm in such a manner as to 

 catch the wind on the lower surface which ever way it moves. 

 I have very often observed, through a telescope, eagles and hawks 

 rising in a slight breeze, and believe that these birds change 

 the position of wings and tail_ as they move round and round in 



(1) Midlenhoff. Die Grosse der Flugflachen Pfliigers Archiv fur die 

 Gesammte Physiologie. Band XXXV 



