The hen of all kinds of gallinaceous fowls, from the Bantam to the Cochin-China, sits for twenty-one days, at which time, on an average, the chickens break the shell; but if the eggs are new laid it will often lessen the time by five or six hours, while stale eggs will always be behind time. For the purpose of breaking the shell, the yet soft beak of the chicken is furnished, just above the point of the upper mandible, with a small, hard, horny scale, which, from the position of the head, as Mr. Yarrell observes, is brought in contact with the inner surface of the shell. This scale may be always seen on the beaks of newly-hatched chickens, but in the course of a short time peels off. It should not be removed. The peculiar sound, incorrectly called "tapping," so perceptible within the egg about the nineteenth day of incubation, which was universally believed to be produced by the bill of the chick striking against the shell in order to break it and effect its release, has been incontestably proved, by the late Dr. F. R. Horner, of Hull, in a paper read by him before the British Association for the Advancement of Science, to be a totally distinct sound, being nothing more than the natural respiratory sound in the lungs of the young chick, which first begins to breathe at that period. Of course there is also an occasional sound made by the tapping of the beak in endeavouring to break the shell. The time occupied in breaking the shell varies, according to the strength of the chick, from one to three hours usually, but extends sometimes to twenty-four, and even more. "I have seen," says Reaumur, "chicks continue at work for two days together; some work incessantly, while others take rest at intervals, according to their physical strength. Some, I have observed, begin to break the shell a great deal too soon; for, be it observed, they ought, before they make their exit, to have within them provision enough to serve for twenty-four hours without taking food, and for this purpose the unconsumed portion of the yolk enters through the navel. The chick, indeed, which comes out of the shell without taking up all the yolk is certain to droop and die in a few days after it is hatched. The assistance which I have occasionally tried to give to several of them, by way of completing their deliverance, has afforded me an opportunity of observing those which had begun to break their shells before this was accomplished; and I have opened many eggs much fractured, in each of which the chick had as yet much of the yolk not absorbed. Some chicks have greater obstacles to overcome than others, since all shells are not of an equal thickness nor of an equal consistence; and the same inequality takes place in the lining membrane, and offers still greater difficulty to the emergent chick. The shells of the eggs of birds of various species are of a thickness proportionate to the strength of the chick that is obliged to break through them.