_Effects of interference produced by shocks in opposite directions_.
1. If we direct against a circle of holes two currents of air in
opposite directions, the note obtained with a single current is very
much weakened, if the two currents reach the holes simultaneously.
If the impulses are not isochronous, the intensity of the note is
increased.
2. If the two currents are directed against two circles of the same
number of holes, the effect is the same as for the two preceding cases.
3. If two currents of air are directed against two circles, one of which
has twice as many holes as the other, we obtain only the low note if
every shock of one is isochronous with every shock of the other.
We obtain the notes of both circles, one of which is the octave of the
other, if there is no isochronism between the shocks.
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.