Jan. 13, 1888.] 



SCIENTIFIC NEAVS. 



29 



show a peculiar torsion of the plane of the wing, the 

 existence of which would not be suspected from side- 

 views. 



This torsion occurs at the end of the down-stroke of the 

 wing at the moment when the joints begin to fold up 

 preparatory for the up-stroke. Hence there results a 

 screw-like aspect of the wing, recalling the form which 



ments of the two wings are perfectly synchronous, though 

 not equal in extent. We see lastly from these images 

 that the body of the bird turns, and leans in different 

 ways, so as to transfer its centre of gravity from one 

 side to the other, according to the requirements of the 

 equilibrium. 



Images taken in front show that the extremity of the 



Crested Heron. Transverse Flight, Ten Exposures a Second. 



M. Pettigrew regards as the essential cause of the pro- 

 pulsion of birds. We must observe that this shape is 

 produced only at the end of the downward stroke, at the 

 dead point of the action of the wing, mechanically speak- 

 ing, and at the moment when this has come to a rest, and 

 is about to be raised again by the resistance of the air. 

 These images further show a fact by no means fore- 



wing, the really active part of the organ, since it strikes 

 the air with most speed, presents at the moment of the 

 down-stroke changes of plane which are not shared by 

 the secondaries extending from the carpus to the 

 humerus. 



It is further useful to know the force which produces 

 the movements of the bird, and to measure the mechani- 



FiG. 2. Pelican. Transverse Descending Flight, Ten Exposures a Second. 



seen — that the acts of flight are not symmetrical. It 

 had been already rightly supposed that a bird which 

 wishes to alter the direction of its flight laterally exe- 

 cutes more extended movements on the side which is to 

 progress the more swiftly. That is to say, it gives more 

 amplitude to the movements of the right wing if it wishes 

 to turn to the left and inversely. 



It is scarcely necessary to say that photography entirely 

 overthrows the supposition that one of the wings of a bird 

 could beat more frequently than the other. The move- 



cal force which it expends in sustaining and transporting 

 itself in the air. Calculations founded on observations 

 show the following values for the forces which act during 

 the flight of the sea-gull. 



Vertical component . . . . 0.623 kilo. 



Horizontal ,, . . . . 0.898 



Total 1.53 1 kilo. 

 These forces are due to the down-stroke of the wings. 

 The up-stroke is passive, due to the pressure of the air 



