DP PETTIGREW ON THE PHYSIOLOGY OE WINGS. 399 



above, beneath, or on a level with the body ; or it may be directed forwards, 

 backwards, or at right angles to the body. In either case the body is balanced 

 mechanically and without effort. To prove this point, I made an artificial wing 

 and body, and united the one to the other by a universal joint. I found, as I had 

 anticipated, that place the wing in whatever position I chose, whether above, 

 beneath, or on a level with the body, or at either side of it, the body almost 

 instantly attained a position of rest. The body was, in fact, equally suspended 

 and balanced from all points. 



Rapidity of Wing Movements partly Accounted for. — Much surprise has 

 been expressed at the enormous rapidity with which some wings are made 



Fig. 51.* 



to vibrate. The wing of the insect is, as a rule, very long and narrow. As a 

 consequence, a comparatively slow and very limited movement at the root 

 confers great range and immense speed at the tip, the speed of each portion 

 of the wing increasing as the root of the wing is receded from. This is 

 explained on a principle well understood in mechanics, viz., that when a rod 

 hinged at one end is made to move in a circle, the tip or free end of the rod 

 describes a much wider circle in a given time than a portion of the rod nearer the 



* In this diagram I have represented the wing hy a straight rigid rod. The natural wing, how" 

 ever, is curved, flexible, and elastic. It likewise moves in curves, the curves heing most marked towards 

 the end of the down and up strokes, as shown at m, n, o, p. The curves, which are double figure 

 of 8 curves, are obliterated towards the middle of the strokes (r, a). This remark holds true of all 

 natural wings, and of all artificial wings properly constructed. The curves and the reversal thereof are 

 necessary to give continuity of motion to the wing during its vibrations, and what is not less important, 

 to enable the wing alternately to seize and dismiss the air. 



VOL. XXVI. PART. II. 5 L 



