510 ON THE POWERS OF CERTAIN WATER BIRDS. 



all fresh-water clivers. With the quickness of thought it disappears 

 beneath the surface, and that so as scarcely to leave a ripple on the 

 spot ; and when your anxious eyeii seek around for the bird, you are 

 astonished to find it many hundred yards distant, the head, perhaps, 

 merely above the water for a moment ; or you may chance to perceive 

 the bill alone, gently cutting the water, and producing a line of wake, 

 not observable beyond the distance of thirty yards from where you 

 are standing. With habits like these it easily eludes all your efforts 

 to obtain it." In speaking of the Purple Gallinule (Galli/iula Mar- 

 tinica)\\e says: — "It runs with great speed, and dives with equal 

 address — often moving off under water with nothing but the bill 

 above." Again, he snys of the Common Moorhen {G. Chloropus): — 

 "At all other times, when raised, they suffer their legs to dangle, 

 proceed slowly to a short distance and drop among the reeds ; or if 

 over water, they dive and hide, leaving nothing but the bill projecting 

 above the surface." One more and I have done. The Clapper Rail 

 {Ballus crepitans): — "It dives well, remains a considerable time under 

 water, and in this manner dexterously eludes its pursuers. When 

 hard pressed it often sinks just below the surface, keeping the bill 

 above in order to breathe, and in this position, if not detected, re- 

 mains for a considerable time ; if perceived and approached, it in- 

 stantly dives, and uses its wings to accelerate its progress, but rises 

 as soon as it comes to a place of safety." These extracts all show a 

 wonderful power of control over their specific gravity in birds, which 

 are, except the first, only moderately aquatic in their habits and 

 conformation, and which one would hardly expect to exhibit the 

 highest developement of diving powers. 



Before entering on the mode in which the bird accomplishes this 

 state of submergence, it may be well to remark that birds, in addition 

 to the air contained in their lungs, are also furnished with large cavi- 

 ties called air cells, in every part of the body, where they do not in- 

 terfere with the organization of the part. These air cells communi- 

 cate with the lungs, but do not appear to be capable of being emptied 

 of the contained air at the will of the bird. The bones and barrels 

 of the feathers are also full of air, so that the bird is naturally a good 

 deal lighter than water, and cannot sink without some effort on its 

 -own part. 



I will now proceed to consider the main subject of inquiry, namely, 

 bow the bird probably may be supposed to alter its buoyancy to such 



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