This admission of air into every part of the system serves not only to fan the vital flame, and rouse the energies of the bird to an extraordinary degree of tension; it likewise assists in giving buoyancy to its movements, bearing it upward, as the gas does a balloon; for it is evident that the air received into the body being raised to a temperature corresponding to the heat of its blood, the specific gravity of the bird is proportionately diminished, and it rises into the air almost without an effort, and even hovers in the sky with scarcely a perceptible movement of its wings. A knowledge of the mechanism of their mode of respiration will likewise enable us to explain another remarkable feature in the history of the feathered tribes, namely, their power of song. Who that has listened to the prolonged warblings of a linnet, the flood of melody poured forth from the little throat of the canary, the "lengthened sweetness long drawn out" which almost pains the enraptured ear as we listen to the song of the nightingale, but has wondered how such tiny birds can ever find sufficient breath for the utterance of such long-sustained, such interminable notes? What would our prima-donnas at the opera give for but the tithe of the capacity of these favoured little songsters? No human breast could ever hold sufficient breath for such performances. We now see, however, that the vocal organs of a bird are exactly adapted to the nature of their music. Their whole body is a bellows, as large in proportion to their size as the bellows of an organ is in relation to the pipes into which it has to pour the sound. The little bird is, in fact, a living harmonium--its singing apparatus is not situated at the top of its throat, but is implanted in the inferior termination of its windpipe; and just as the tongue of the harmonium is thrown into vibration by the issuing current of air caused by pressure upon the bellows, so are the vocal chords of the feathered songster rendered sonorous as the air passes over them. In proportion to the capacity of the bellows must be the duration of the note, and we have already seen that the air-cells of the bird are capable of furnishing a supply not easily exhausted. There is, however, this remarkable difference between the two instruments: the tongue of each key of the harmonium can give utterance but to one sound--one never-varying tone--while the corresponding part of the bird, rendered more or less tense by muscles provided for the purpose, contains within itself a whole gamut, and there is not a note in the scale that is not instantly at the command of the inimitable little musician. In the perching birds, among which are found by far the most accomplished singers, five pairs of muscles are connected with this exquisitely-contrived apparatus, and are so disposed as to influence both the diameter and the length of the air-passages. In the parrots three pairs are met with; some of the swimming birds have two, while