pipe is sounded near the base of the instrument. Thence the sound waves are conducted through the two vertical tubes, one of which is capable of being lengthened, like a trombone. They then reunite and are conducted by a 
FIG. 9single tube to a "manometric capsule," which impresses the resulting vibrations on a gas jet, the trembling of the jet being rendered visible in a revolving mirror.
When the two branch tubes are of equal length, the waves reach the flame in the same phase, causing it to
FIG. 10 vibrate, as shown by the character of the image in the revolving mirror, Fig. 9; while, if one of the branches be made half a wave[1] longer than the other, the disturbance disappears, and the image appears as shown in Fig. 10.
A very simple and instructive experiment may be made
- ↑ The length required will depend on the tone of the organ pipe. For middle C (256 vibrations per second) the double length required is two feet.
