Take a common tuning-fork, and on one of its branches fasten with
sealing-wax a circular piece of card of the size of a small wafer, or
sufficient nearly to cover the aperture of a pipe, as the sliding of
the upper end of a flute with the mouth stopped: it may be tuned in
unison with the loaded tuning-fork by means of the movable stopper or
card, or the fork may be loaded till the unison is perfect. Then set
the fork in vibration by a blow on the unloaded branch, and hold the
card closely over the mouth of the pipe, as in the engraving, when a
note of surprising clearness and strength will be heard. Indeed a flute
may be made to “speak” perfectly well, by holding close to the opening
a vibrating tuning-fork, while the fingering proper to the note of the
fork is at the same time performed.