SAFETY DEVICES IN BIEDS WINGS GRAHAM 



291 



nature of soaring ; they use a quick flap for a great part of their time 

 in the air. Bearing this, and the somewhat different construction 

 of their slots, in mind, we might do worse than try to find some other 

 advantage that they may derive from them. 



All small birds that are well equipped with slots possess com- 

 paratively short, square-tipped wings; just the opposite in shape to 

 those of the few that have no slots at all ; and the slots seem to vary 

 in number and development so strictly in accordance with the shape 

 of the wings that one might almost formulate a law governing the 

 matter. 



Compare the wings of the smaller birds among those shown in 

 Figure 28. The swallow's is the longest and thinnest (relatively) and 

 has no slots, though the tips of the first two flight feathers are per- 



__„i 



FiGDRB 26. — Upper and lower surfaces of 

 the left wing-tip of a song thrush 



FiGDRE 27. — Upper and 

 lower surfaces of the left 

 wing-tip of a swallow 



mitted by the friction areas just to separate for a distance of about 

 half an inch inwards from their points (fig. 27). Then comes the 

 pointed wing of the starling, with two very short slots, and of the 

 quail with about the same development. The latter's wings are com- 

 paratively long and narrow. The wryneck has exactly the same 

 length of slot (1.2 inches), but that is relatively a better equipment, 

 because its wing is 1.3 inches shorter than the quail's, yet of the same 

 breadth. The wheatear, with its much broader and squarer wing, has 

 three well-developed slots, as also have the square-tipped wings of the 

 goldfinch and the thrush. 



Here we seem to have an indication of the use of slots in the wings 

 of small birds. It has long been known that the ideal airplane wing, 

 from the point of view of " lift " alone, is one of infinite span, because 

 such a wing, if it existed, would have no tip over which the air could 

 escape sideways. Air, like other things we know, will avoid doing a 

 job of work if it possibly can. Some of the air underneath a wang, 

 instead of lifting a bird by allowing itself to be forced downwards by 

 149571—33 20 



