to escape,--remaining under water, however, if need
be, an almost incredible time, and swimming underneath it to great
distances. Here we have, if we would only think of it, the same
question as that about the water-ouzel, how it _keeps down_; and
we must now note a few general points about diving birds altogether.

It is easy to understand how the properly so-called divers can plunge
with impetus to great depths, or keep themselves at the bottom by
continued strokes of the webbed feet; but neither how the ouzel walks
at the bottom, if it be specifically lighter than the water, nor how a
bird can swim horizontally under the surface; at least it is not enough
explained that the action must be always that of oblique diving, the
bird regulating the stroke according to the upward pressure of the
water at different depths.

109. But there are many other points needing elucidation. It is said
(and beautifully insisted on, by Michelet,) that great spaces in the
bones of birds that pass most of their lives in flight are filled with
air: presumably the bones of the divers are made comparatively solid,
or it is even conceivable--if conceptions or suppositions were of any
use,--that the deep divers may take in water, to help themselves to
sink. The enormous depths at which they have been caught, according to
report, cannot be reached by any mere effort of strength, if the body
remained as buoyant as it evidently is on the surface. The strength of
the wing must, however, be enormous, for the great northern diver is
described as swimming under water "as it were with the velocity of an
arrow in the air" (Yarrell, vol. iii., page 431); or to keep to more
measured fact, Sir William Jardine says, "I have pursued this bird in a
Newhaven fishing-boat with four sturdy rowers, and notwithstanding it
was kept almost constantly under water by firing as soon as it
appeared, the boat could not succeed in making one yard upon it"
(_ibid._, p. 432).

110. But this is followed by the amazing statement of Mr. Robert Dunn,
p. 433, that in the act of diving it does not appear to make the least
exertion, but sinks gradually under the surface, without throwing
itself forward, the head being the last part that disappears. I am not
fond of the word 'impossible,' but I think I am safe in saying that
according to the laws of nature no buoyant body can sink merely by an
act of volition; and that it must pull itself down by some hitherto
unconceived actio