of the Pelecauus Aquilus of LimtUtm. 1 1 



the human heart. The use of this muscle is evidently to dimi- 

 nish the cavity of the sac, and to expel the air which it contains 

 when the bird is on the wing. I could not, however, discover 

 by what aperture the air is admitted into, or expelled from it : 

 no connexion between it and the mouth or trachea could be 

 discovered either by the eye, the probe, or the blow-pipe, though 

 the trachea is distinctly seen running behind it through its whole 

 length. But as it becomes inflated as soon as the bird rises into 

 the air, and remains flaccid while it is on the ground, little doubt 

 can be entertained that it is a receptacle for air, and affords an 

 additional facility to its flight. 



In conclusion it may be remarked, that the bird which ap- 

 proaches nearest in its habits and general structure to the Fri- 

 gate-bird, is the Phaeton athcrcus, or Tropic-bird. Both are 

 seen at immense distances from the land, enveloped in clouds, 

 and scarcely perceptible to the eye. Both seize on their food 

 by darting through the air with inconceivable velocity. Neither, 

 when placed on the level ground, can raise itself into the air. 

 But the great peculiarities of the Frigate-bird are wanting in 

 the Tropic-bird. No air-bag is found under its throat : the 

 lower extremities, though very ill proportioned to the wings, are 

 less so than in the Frigate-bird ; and the feet being webbed, it 

 is able to swim, and to feed itself in that position : the plumage 

 is also defended against the action of the water : the structure of 

 the tail is less complicated, and the flight of the bird is chiefly 

 guided by two feathers in it of extraordinary length : nor do the 

 pectoral muscles bear so large a proportion to the size of the 

 bodv. 



[!. The 



