Text: Organ stops, both flute andreed, having tubes twice the normal length but pierced with a small hole in the middle. Harmonic flute stops are of great purity and brilliancy, they are of 8 ft. or 4 ft. pitch. Harmonic piccolos are of 2 ft. pitch. Harmonic reed stops (tromba, tuba, trumpet, etc.) are generally on a high pressure of wind, one ofthe great advantages of all harmonic stops being that they will take a very strong pressure of wind without overblowing. The fact is, that the harmonic-tube, having two synchronous vibrating columns of air, partakes of the nature of a pipe already overblown to its first harmonic, the octave.

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Source: 125