The relation between the number of segments on the plate and the pitch of the note, can be ascertained by using a circular plate clamped in the centre. “If the finger on the plate and the fiddle-bow are one-eighth of the circumference apart, the fundamental note will be produced. If one-sixteenth apart, the higher octave will be heard.” Sensitive flames will detect air vibrations, and flames can also be made to sing. Sensitive flames were discovered by Mr. Barrett, who noticed the effect a shrill note had upon a gas flame from a tapering jet. The flame was a very long one (fourteen inches), and when the sound was produced it shortened at once, while the upper part expanded like a fan; the same effects, in a less marked degree, were observable when the shrill sound was prolonged from a distance of forty feet. Professor Tyndall was immediately interested in this discovery, and in January 1867 he lectured upon it at the Royal Institution. If any one wish to try the experiment, a piece of glass tubing should be obtained, and let the mouth be tapered down to a small orifice one-sixteenth of an inch in diameter. Then when the highest pressure is on for the evening, light the gas and sound a shrill whistle. The flame will sink down and spread out. The illuminating power may thus be increased, and many experiments may be made. For instance, if a person be in the room and try to read, he will probably not be able to do so at a little distance; but if his friend whistle to the gas it will so expand itself as to enable him to read, so long as the whistle lasts. A very ingenious burglar-detector was made upon the principle of the sensitive flame, which expands at a noise and heats a welded plate of gold, silver, and platinum. The plate swerves aside, the metals being unequally affected by heat, and as it is connected with a battery, rings a bell by electricity. A small high flame has been made sensitive to the chinking of coin, or even to the ticking of a watch. We will now give some explanation, derived partly from Professor Tyndall, of the cause of sensitive flames. A sensitive flame is one just on the point of “roaring,” and about to change its aspect. “It stands,” says Tyndall, “on the edge of a precipice. The proper sound pushes it over.... We bring it to the verge of falling, and the sonorous pulses precipitate what was already imminent.” The flame is in a state of vibration, so sounds being vibrations, practically increase the pressure; and the flame acknowledges the pressure thus invisibly applied by air waves.