Disk No. 5 has three circles of 36, 36, and 72 holes. The air currents
are directed against the circles of holes through the movable tubes,
made so that they can be detached at pleasure. All these experiments
require great precision in the arrangement of these wind tubes. To make
sure that the tubes are simultaneously before two holes of the disk, it
is well to put little rods through the holes, reaching into the wind
tubes, and to remove them only when the tubes are firmly attached. The
experimenter should be careful also to place the two tubes exactly
at the same distance from the turning disk. It is clear that
notwithstanding all these precautions we never obtain perfect
interference, but only the weakening of notes that ought to disappear
entirely if all the arrangements were made with mathematical exactness,
and also if the ear could have absolutely the same position with regard
to impulses produced in opposite directions.


IV.

_Beats_.

Disk No. 6 has--

8 circles of holes to the number of 1, 2, 23, 24, 25, 47, 48, 49.

Circles 3 and 4, 4 and 5, 6 and 7, and 7 and 8 ought to produce as many
beats as circle 1 produces simple shocks; and circles 3 and 5, 6 and 8,
as many beats as circle 2 produces simple shocks; but we must content
ourselves in these experiments with a much less perfect result, for the
following reasons: The disk never being rigorously plane, alternately