538 



THE POPULAR SCIENCE MONTHLY. 



the positions of the wings of the gull in the course of a complete oscil- 

 lation : 



Fig. 11. 



■<■/ 



/^3 jj" % v 



Showing the more or less Perpendicular Direction op the Stroke op the Wing in the 



Flight op the Gull. 



When the down-stroke is completed the bird has been raised, but 

 is lowered again when the wings have attained their maximum eleva- 

 tion. Thus it is seen how directly gravity aids in flight. The body 

 is the weight ; the wings are long levers attached to it at one end ; 

 the air is the fulcrum. Fig. 12 shows the undulatory track of a flying- 

 bird : 



Fig. 12. 



Bird at a ; down-stroke of the wing, b, lifts the bird to c— the track of the bird being in direc- 

 tion of ihe arrows. 



The instant the descent of the wing begins, the body moves up- 

 ward and forward ; but it is shown by the author that some forward 

 motion results also from the up-stroke. Certain it is that the upward 

 movement must not counteract the other. There is no provision for 



Fig. 13. 



Left Wing op the Albatross. — d, e, /, Anterior or thick margin of pinion ; 5, a, c, Posterior or 

 thin margin, composed of the primary (&), secondary (a), and tertiary (c) feathers. 



waste of energy. The form of the upper surface of the wing is con- 

 vex, the under surface being concave. The value of this will be ap- 

 parent, as the Duke of Argyle suggests, if we attempt to move the 



