the bird to sink or submerge itself, it ought to have in its power to contract itself to such a degree that it would only displace a volume of water less than its own weight, let us say 1| pound ; but I am per- suaded that this is not possible, as the contraction which would expel a cubic inch of air from its body* would not of itself be sufficient to cause such an effect.
Mr. Slaney speaks also of the use air-cells may be of, to birds of prey ; I am fully persuaded that they are greatly conducive to their buoyancy, but I do not believe that they have anything to do (in the manner that gentleman intimates) with the sudden swoops these birds make, which are I think produced by a sudden and powerful move- ment, giving the impulse (obliquely downwards) and followed by a momentary semi-expanded state of the wings (serving as a parachute to prevent their too rapid fall); to this must be added the motions of the legs balancing the centre of gravity, and the expanding, closing, and turning of the sail in various degrees, regulating and directing the flight.
The air-cells of birds of prey are a beautiful provision of nature, as in augmenting their buoyancy, it impedes greatly the extraordinary velocity with which solid bodies are known to fall through the atmos- phere (by the laws of gravitation), and which would cause an eagle pouncing suddenly on its prey from a height of from between 2000 and 3000 feet to be dashed to pieces, without its having power to save itself, if it were deprived of these cells.
I finish here this dissertation, which I fear has been too long, and return to my birds, concluding the migratory ones <rf the present group by
c. Water-birds, which at the time of their migration through Bel- gium are to be found indiscriminately along our coast, and in the in- terior, on rivers, lakes, ponds, marshes, &c.
Ringed Plover, Charadrius Hiaticula. Not uncommon. It runs very fast on the sands of the sea-shore ; taking but short flights when driven up. Its note is a shrill whistle. Seen in small flocks or in couples, and never nestles here, though it does so commonly on the banks of the Moselle.
I do not suppose a moorhen can expel more than that quantity of air at each
expiration, as man when breathing only throws out about twenty times that sum.
