142 Capt. F. W. Hutton on the Flight of Birds. 



the greater part of its support is derived from its forward pro- 

 gress, as I have already explained when describing the sailing 

 flight of the Albatros (Phil. Mag., Aug. 1869). This forward 

 progress is obtained by the backward stroke, or rowing motion, of 

 the primaries from the carpal joint, which, combined with the 

 downward movement, makes the feathers press on the air in a 

 downward and backward direction. While, therefore, the move- 

 ment of the main part of the wing from the shoulder is vertical, 

 the tips, by having also a horizontal movement, do not describe 

 an undulating line in the air, but a cycloidal curve, thus con- 

 firming the experiments of Prof. Marey. 



No twisting of the wing on its axis is perceptible by the eye ; 

 but such a movement pi'obably takes place ; for the anatomical 

 investigations of Dr. Pettigrew show that " during flexion the 

 anterior margin is slightly directed downwards, and in extension 

 decidedly directed upwards^' (/. c. p. 241). It is, I think, the 

 erroneous idea that flexion must occur during the up-stroke that 

 has led Dr. Pettigrew astray in his theory of the flight of birds. A 

 moment^s consideration will show that it is during the up-stroke, 

 which must tend to depress the bird, that the largest surface of 

 wing is required to take advantage of the progressive movement 

 obtained by the down-stroke, and so, by acting like a kite, 

 prevent the bird from falling. If the flexion of the wing during 

 the down-stroke be allowed, and this I have distinctly seen, the 

 descrepancy in the description of the movement of the wings by 

 Dr. Pettigrew and Prof. Marey, which I have already pointed 

 out, disappears. 



Dr. Pettigrew's experiments, Nos. 14 and 18, show that when 

 the primary feathers, or the tips of the wings, are much shortened, 

 flight is stopped, or much impeded in birds, but not in insects, 

 the reason of this difi'erence being that the latter cannot bend 

 their wings, but obtain forward progression in quite a diff'erent 

 manner to birds. Experiments Nos. 12 and 13 show that if the 

 primaries are left entire, or nearly so, flight is but little impaired, 

 although the greater part of the other feathers may be cut off. 

 This is because the primaries are the portion of the wing by 

 which progression is obtained, and it is progression that 

 principaay supports the bird; for without progression, either 



