NATURAL HISTORY. 295 



" The wanderers of heaven 

 Each to his home retire, *ave those that love 

 To take their pastime in the troubled air, 

 Or skimming flutter round the dimply pool." THOMSON. 



certain density of the atmosphere, which is best suited to the 

 weight and action of the wings of birds t and which cannot be 

 found nearer the earth. 



Another reason also may be, that the upper regions to which 

 birds thus ascend are exempt from the momentary gusts and, 

 squalh to which the lower atmospheric strata are subject. 



948. Eagles are heavy birds, even for their powerful wings, and yet they are 

 high flyers, although their abodes are at great elevations in the mountains. 

 Wild geese and herons take the sky when they set out on long journeys. 

 Rooks may be seen to adjust the height of their daily excursions from the rookeries 

 to the distance at which the pasture upon which they feed lies ; and the swallow 

 tribe wheel about far more rapidly and gracefully when they fly high before rain, 

 than when they skim the pools in fine weather. When birds are in long and 

 swift flight, they acquire a momentum in proportion to their velocity, and the 

 difference between their specific gravity and that of the air. In consequence of this 

 momentum, they continue their progressive motion with much less effort, which is 

 of itself sufficient to explain why they prefer flying high. 



949. Why are birds lohich winter out of England supplied 

 with a cover of black down under their feathers ? 



Because black is a colour capable of retaining the greatest 

 amount of heat, and in this instance the object is, to keep in 

 the heat arising from the heart and the circulation of 

 the blood. 



950. It is likewise remarkable that this provision is not found in larger 

 birds for which there is also a reason. Small birds are much more exposed to the 

 cold than large ones, forasmuch as they present, in proportion to their bulk, a much 

 larger surface to the air. It is necessary, therefore, that small birds should be more 

 warmly clad than larger ones ; and this appears to be the expedient by which that 

 exigency is provided for. 



951. Why are the birds which are brought alive to England 

 from distent regions generally hard-billed? 



Becau i; j hard-billed birds subsist on seeds which are easily 

 carried ou board ship ; while the soft-billed birds, which are 

 supported by worms or insects, or as a substitute, fresh raw meat, 

 can obtain neither in long voyages. 



