(By sounding only two circles, 3 and 4, or 4 and 5, we make the same experiment with two circles as disk No. 2 enabled us to make with circle 8 alone; also, by sounding circle 3 alone, we obtain the note corresponding to 12 holes; then pulling out button 4, the notes corresponding to 12 and 36 holes are heard suddenly and very strongly; but as soon as circle 5 is sounded also, the note of 12 disappears completely, and we have left only that corresponding to 36 holes.) III. _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. IV. _Beats_. Disk No. 6 has-- 8 circles of holes to the number of 1, 2, 23, 24, 25, 47, 48, 49.