What is required, however, is something analogous to an organ pipe which produces a continuous sound.
Ellis used this indication to have an organ pipe made which with one-sixteenth diameter and a wind-pressure of 34 in., at one-fourth Schlick's length, gave f' 301.6, from which he derived a just major third of a' 377, which would compare very well with an old Greek a'.
In the organ pipe - as in the common whistle - a thin sheet of air is forced through a narrow slit at the bottom of the embouchure and impinges against the top edge, which is made very p c. sharp. The disturbance made at the commencement of the blowing will no doubt set the air in the pipe vibrating in its own natural period, just as any irregular air disturbance will set a suspended body swinging in its natural period, but we are to consider how the vibration is maintained when once set going.
The whole process is exactly analogous to the operation by which a violin string or organ pipe creates an air or sound wave.