PKOors OF oega:xising mind 313 



also grow obliquely outwards towards the tip of the barb. 

 Laterally they touch each other with smooth, glossy surfaces, 

 which are almost air-tight, yet allow whatever slight motions 

 that may be required during use, while remaining interlocked 

 with the barbules of the adjoining barb in the manner just de- 

 scribed. They are the essential elements of the feather, on which 

 its value both for flight and as a protective clothing depends. 

 Even in the smallest wing- feathers they are probably a liundred 

 thousand in number, since in the long wing-feather of a crane 

 the number is stated by Dr. Hans Gadow to be more than a 

 million. 



What are termed the " contour-feathers " are those that 

 clothe the whole body and limbs of a bird with a garment of 

 extreme lightness which is almost comj^letely impen-ious to 

 either cold or heat. These feathers vary greatly in shape on 

 different parts of the body, sometimes forming a dense velvety 

 covering, as on the head and neck of many species, or de- 

 veloped into endless variety of ornament. They fit and overlap 

 each other so perfectly, and entangle so much air between 

 them, that rarely do birds suffer from cold, except when un- 

 able to obtain any shelter from violent storms or blizzards. Yet, 

 as everv sino'le feather is movable and erectile, the whole bodv 

 can be freely exposed to the air in times of oppressive heat, or 

 to dry the feathers rapidly after bathing or after unusually 

 heavy rain. 



A great deal has been written on the mechanics of a bird's 

 flight, as dependent on the form and curvature of the feathers 

 and of the entire wing, the powerful muscular arrangements, 

 and especially the perfection of the adjustment by which dur- 

 ing the rapid do^vn-stroke the combined feathers constitute a 

 perfectly air-tight, exceedingly strong, yet highly elastic in- 

 strument for flight ; while the moment the upward motion be- 

 gins the feathers all turn upon their axes so that the air passes 

 between them with hardly any resistance, and when they again 

 begin the down-stroke close up nutomatically as air-tight as 

 before. Thus the effective down-strokes follow each other so 



