The human voice, perfect as it may be in tone, is yet always very deficient in compass, as is obvious from the fact that the bass voice, the barytone, the contralto, and the soprano have all different registers, and are all required to produce a complete vocal harmony.  If we could make organ-pipes with movable, self-regulating lips, with self-shortening and self-lengthening tubes, so that each tube should command the two or three octaves of the human voice, a very limited number of them would be required.  But as each tube has but a single note, we understand why we have those immense clusters of hollow columns.  As we wish to produce different effects, sometimes using the pure flute-sounds, at other times preferring the nasal thrill of the reed-instruments, we see why some of the tubes have simple mouths and others are furnished with vibratory tongues.  And, lastly, we can easily understand that the great interior spaces of the organ must of themselves furnish those resonant surfaces which we saw provided for, on a small scale, in the nasal passages,—­the sounding-board of the human larynx.

* * * * *

The great organ of the Music Hall is a choir of nearly six thousand vocal throats.  Its largest windpipes are thirty-two feet in length, and a man can crawl through them.  Its finest tubes are too small for a baby’s whistle.  Eighty-nine stops produce the various changes and combinations of which its immense orchestra is capable, from the purest solo of a singing nun to the loudest chorus in which all its groups of voices have their part in the full flow of its harmonies.  Like all instruments of its class, it contains several distinct systems of pipes, commonly spoken of as separate organs, and capable of being played alone or in connection with each other.  Four manuals, or hand key-boards, and two pedals, or foot key-boards, command these several systems,—­the solo organ, the choir organ, the swell organ, and the great organ, and the piano and forte pedal-organ.  Twelve pairs of bellows, which it is intended to move by water-power, derived from the Cochituate reservoirs, furnish the breath which pours itself forth in music.  Those beautiful effects, for which the organ is incomparable, the crescendo and diminuendo,—­the gradual rise of the sound from the lowest murmur to the loudest blast, and the dying fall by which it steals gently back into silence,—­the dissolving views, so to speak, of harmony,—­are not only provided for in the swell-organ, but may be obtained by special adjustments from the several systems of pipes and from the entire instrument.