DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 21 i> 



the wing in them is made to oscillate. This is of the utmost importance in the science of 

 aerostation, as showing that flight may he attained by a heavy powerful animal with 

 comparatively short wings, as well as hy a lighter one witli enormously enlarged wings. 

 While, therefore, there is apparently no correspondence between the area of the wing and 

 the animal to be raised, there is, unless in the case of sailing birds*, an unvan ing relation 

 as to the weight and number of oscillations ; so that the problem of flight would seem to 

 resolve itself into one of weight, power, velocity, and small surface^, remits buoyancy, 

 debility, diminished speed, and extensive surfaces — weight in either case being a sine 

 qua non. In order to utilize the air as a means of transit, the body in motion, whether 

 it moves in virtue of the life it possesses, or because of a force superadded, must be heavier 

 than it. It must tread and rise upon the air as a swimmer upon the water, or as a kite 

 upon the wind. This is necessary for the simple reason that the body must be active, 

 the air passive. It must act against gravity, and elevate and carry itself forward at the 

 expense of the air and of the force which resides in it, whatever that may be. Hit were 

 otherwise — if it were rescued from the law of gravity on the one hand, and bereft of inde- 

 pendent movement on the other, it would float about uncontrolled and uncontrollable, as 



happens in the ordinary gas-balloon. 



The difference between an insect or a bird and a balloon, here insisted upon, was, I have 

 learned since writing the above, likewise adverted to by His Grace the Duke of Argyle 



in his very able and eloquent article in ' Good Words,' entitled " The Reign of Law "f. 



Wing-area variable and in excess.— That no fixed relation exists between the area of 

 the wings and the size and weight of the body is evident on comparing the dimensions of 

 the wings and bodies of the several orders of insects, bat s, and birds. I f such comparison 

 be made, it will be found that the pinions in some instances diminish while the bodies 

 increase, and the reverse. No practical good can therefore accrue to aerostation from 

 elaborate measurements of the wings and trunks of any flying thing; neither can any 



rule be laid down as to the extent of surface required for sustaining a given weight in 

 the air. The statements here advanced are borne out by the fact that the wings of 

 insects, bats, and birds may be materially reduced without impair! n - their powers of 

 flight. This is notably the case in the common Blow-fly, as the following rperiments 

 instituted by me in August 1866 will conclusively show. In the clear .unny mornings of 

 that month I caught a number of the insects in question, on the leavee of a vine-tree, 

 in such a manner as not to injure them in the least. I operated upon their pinions 

 with sharp-pointed scissors, with the following results :— 



Experiment 1. Detached posterior or thin half of either wing in its long axis. Flight 



perfect. 



la birds which skim, sail, or glide, the pinion is greatly elongated or ribbon-shaped (Plate XV. fig. 64), and the 

 weight of the body is made to operate upon the inclined planes formed by the wings, in such a manner that the bird 

 whe°n it has once got fairly under weigh, is in a measure self-supporting. Tbis is especially the a* when it is pro- 

 ceeding against a slight breeze-the wind and the inclined planes resulting from the upward mchnat.on of the w.ngs 

 reacting upon each°other, with this very remarkable result, that the mass of the bird moves steadily forwards in a 



more or less horizontal direction. . , ■ 



f « Good Words ' for February 186.5. This paper, I am glad to find, has been reprinted m a separate and extended 

 form with numerous illustrations, and should be consulted by all interested in the subject of Aeronautics. 



