50 
grounds — rising with difficulty and alighting with a splash. 
Compare these with another class of ocean feeders, the gulls and 
terns, which take their food generally on the wing ; nothing can he 
more gi-aceful and rapid than the flight of these birds — the 
beautiful little terns stopping in their rapid flight in a moment, 
and dashing headlong into the sea to secure their prey. 
The equilibrium of the bird is maintained by the relative position 
of the wings and the heaviest parts of the body— the wings being 
placed above, and close to, the centre of gravity, the heavy body 
of the bird serving as ballast to keep it always in one position, and 
as a fly-wheel to store up and regulate the momentum acquired by 
the action of the wings. 
Having, by repeated flappings of the wings, raised itself into the 
air, and acquired a certain degree of momentum, many birds 
possess the power of “ sailing,” or continuing their course without 
motion of the wing, for a considerable length of time. The most 
wonderful example of this mode of flight is the albatross. “I 
have watched them by the hour,” says Major Holland,* “ in the 
strong trade winds off the Cape of Good Hope, when the ship has 
been reeling and staggering before the driving gale, making thirteen 
knots, with no sail but a close-reefed fore and main topsail, shoot 
by her like a swift arrow, passing about a mile a-head and then 
just turning on one side, without a flap of the wing, they rose some 
200 feet, and swooped past again, against a breeze which the ane- 
mometer showed was travelling at twenty-six to thirty miles an hour.” 
Captain Hutton, in the article before quoted,t says, “never have 
I seen anything to equal the ease and grace of this bird [the 
albatross] as he sweeps past, often within a few yards, every part 
of his body perfectly motionless, except the head and eye, which 
turn slowly and seem to take notice of every thhig. I have some- 
times watched narrowly one of these birds sailing and wheeling 
about in all directions, for more than an hour, without seeing the 
slightest movement of the wings. This is, however, longer than 
usual.” In order to perform this seemingly difficult feat of 
“ sailing,” the bird first acquires, by means of its wings, a certain 
degree of momentum, it then, by combining the pressure of the 
air against its wings, with the force of gravity, according to the 
laws of mechanics, as we have described before, is enabled to sail 
* In a letter to the writer. + “ Ibis,” 1865, p. 294. 
