48 THE BIRDS OF HELIGOLAND 



power of their instruments of flight almost exclusively to the 

 execution of their forward movements. This results partly from 

 the fact that by the filling of the air-sacs the volume of the bird is 

 enlarged, and its specific weight consequently diminished, but also 

 from the air taken in at any particular height being warmed by 

 the heat of the body, and considerably rarefied in consequence, so 

 that the contents of the air-sacs are always considerably lighter 

 than the air which occupies surrounding space. 



My own observations go to prove that the total volume of the 

 outer air-sacs when filled with air in itself already exceeds that of 

 the bird's body, and that if we add to this the volume of air con- 

 tained in the thoracic and abdominal cavities, and in the bones 

 and quills of the feathers, we may estimate the total volume of air 

 contained by the bird as easily exceeding that occupied by the 

 solid substance of its body. On the other hand, the temperature 

 of the atmospheric layers in question is always considerably below 

 the freezing point. Thus Glaisher at a height of 20,000 feet noted 

 a temperature of 25 C. (13 F. below zero) ; and since the internal 

 body-heat of birds is about 42 C. (107'6 F.), the difference of 

 temperature between the outer air and that of the air contained 

 in the air-sacs may reach 67 C. and above. 



More exact calculations, based on physical laws, have un- 

 doubtedly compelled us to recognise that this warm charge of air 

 in the air-sacs of birds is unable to facilitate their flight to any con- 

 siderable extent. Nevertheless, long-continued observations in 

 Nature have convinced me that birds must be endowed with a cer- 

 tain capacity for soaring or floating in the air which is independent 

 of the use of their external instruments of flight. Such a thought 

 at once suggests itself if we watch birds, like, for instance, the large 

 gulls, who soar about for hours long over the sea, be the weather 

 stormy or perfectly calm, at heights up to six hundred feet, advanc- 

 ing or turning about in whatever direction they please, without 

 executing the least movement with their wings. It is impossible to 

 conceive that excellent flyers like these should be able to maintain 

 these soaring movements for so long a time, and without all apparent 

 efforts, if they had not at their command other means besides the 

 mechanical aid of their wings. 



If, as I have had occasion to do for many years, one pays atten- 

 tion to the large numbers of Buzzards when they are about to 

 depart, one cannot fail to be convinced of the existence of some 

 such accessory means of locomotion. In one such instance, that of 

 the Common Buzzard, the birds soared over the island at an eleva- 

 tion of about two hundred feet. I intentionally confined my atten- 

 tion exclusively to one individual. Without any motion of its wings, 



