PROGKESSION IN OK THROUGH THE AIR. 



119 



bat, where the stroke is delivered more vertically than in the 

 insect, the hasis of support is increased by the tip of the wing 

 folding inwards and backwards in a more or less horizontal 

 direction at the end of the down stroke ; and outwards and 

 forwards at the end of the up stroke. This is accompanied 

 by the rotation of the outer portion of the wing upon the 

 wrist as a centre, the tip of the wing, because of the ever 

 varying position of the wrist, describing an ellipse. In in- 

 sects whose wings are broad and large (butterfly), and which 

 are driven at a comparatively low speed, the balancing power 

 is diminished. In insects whose wings, on the contrary, are 

 long and narrow (blow-fly), and which are driven at a high 

 speed, the balancing power is increased. It is the same with 

 short and long winged birds, so that the function of balancing 

 is in some measure due to the form of the wing, and the 

 speed with which it is driven ; the long wing and the wing 

 vibrated with great energy increasing the capacity for balanc- 

 ing. When the body is light and the wings very ample 

 (butterfly and heron), the reaction elicited by the ascent 

 and descent of the wing displaces the body to a marked 

 extent. When, on the other hand, the wings are small 

 and the body large, the reaction produced by the vibration 

 of the wing is scarcely perceptible. Apart, however, from 

 the shape and dimensions of the wing, and the rapidity 

 with which it is urged, it must never be overlooked that all 

 wings (as has been pointed out) are attached to the bodies 

 of the animals bearing them by some form of universal 

 joint, and in such a manner that the bodies, whatever the 

 position of the wings, are accurately balanced, and swim 

 about in a m.ore or less horizontal position, like a compass set 

 upon gimbals. To such an extent is this true, that the posi- 

 tion of the wing is a matter of indiff*erence. Thus the pinion 

 may be 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 eff'ort. To prove this point I made an artificial 

 wing and body, and united the one to the other by a uni- 

 versal joint. I found, as I had anticipated, that in whatever 

 position the wing was placed, whether above, beneath, or on 



