NEW LIGHTS ON THE PROBLEM OF FLYING. 745 



impermeable yet flexible plane all this has been often insisted on 

 by writers on design in Nature. But there are two points not so 

 often noticed which especially concern us here. 



1. Of the two vanes of each feather, the hinder one is much 

 the broader. This, together with the manner of overlap, causes 



FIG. 1. LONGITUDINAL SECTION OF THE WING PLANE AND CBOSS-SECTION OF THREE FEATH- 

 ERS, a, shaft ; , ', vane. 



the feathers to rotate and close up into an impervious plane in 

 the downstroke, and to open and allow the air to pass freely 

 through in the upstroke, as shown in the figure (Fig. 1). This 

 structure and arrangement produce the greatest possible effective- 

 ness of the downstroke and the least possible loss in recovery for 

 another stroke. 



2. The plane of the wing is supported not along the middle, 

 but along the extreme anterior border, as shown in Fig. 2, which 



FIG. 2. DIAGRAMMATIC CROSS-SECTION OF BIRD'S WING, a, wing bones ; J, plane. 



is a diagrammatic cross-section of the wing. The effect of the 

 down stroke is to tip up the wing behind, as shown in Fig. 3. The 

 whole force of the stroke, a b, is resolved into two components 

 one, a c, sustaining, and the other, 6 c, propelling onward. In 

 easy flight, therefore, only downward flapping is necessary, al- 



FIG. 3. DIAGRAMMATIC CROSS-SECTION OF BIRD'S WING DURING DOWNSTROKE. o, 5, whole 

 force ; a, c, part sustaining ; i, c, part propelling. 



though in rapid flight doubtless the stroke is also a little back- 

 ward. 



The same admirable adaptation is carried out in every part of 

 the bird. The whole bird is an exquisitely constructed flying 

 machine. The smallness of the head, the feet, and the viscera, 



