§ 9. _Experiments on Flames from Circular Apertures_ A long flame may be shortened and a short one lengthened, according to circumstances, by sonorous vibrations. The flame shown in Fig. 125 is long, straight, and smoky; that in Fig. 126 is short, forked, and brilliant. On sounding the whistle, the long flame becomes short, forked, and brilliant, as in Fig. 127; while the forked flame becomes long and smoky, as in Fig. 128. As regards, therefore, their response to the sound of the whistle, one of these flames is the complement of the other. In Fig. 129 is represented another smoky flame which, when the whistle sounds, breaks up into the form shown in Fig. 130. When a brilliant sensitive flame illuminates an otherwise dark room, in which a suitable bell is caused to strike, a series of periodic quenchings of the light by the sound occurs. Every stroke of the bell is accompanied by a momentary darkening of the room. The foregoing experiments illustrate the lengthening and shortening of flames by sonorous vibrations. They may also produce _rotation_. From some of our homemade burners issue flat flames, about ten inches high, and three inches across at their widest part. When the whistle sounds, the plane of each flame turns ninety degrees round, and continues in its new position as long as the sound continues. [Illustration: FIG. 125.] [Illustration: FIG. 126.] [Illustration: FIG. 127.] [Illustration: FIG. 128.] [Illustration: FIG. 129.] [Illustration: FIG. 130.] A flame of admirable steadiness and brilliancy now burns before you. It issues from a single circular orifice in a common iron nipple. This burner, which requires great pressure to make its flame flare, has been specially chosen for the purpose of enabling you to observe, with distinctness, the gradual change from apathy to sensitiveness. The flame, now 4 inches high, is quite indifferent to sound. On increasing the pressure its height becomes 6 inches; but it is still indifferent. When its length is 12 inches, a barely perceptible quiver responds to the whistle. When 16 or 17 inches high, it jumps briskly the moment an anvil is tapped or the whistle sounded. When the flame is 20 inches long you observe a quivering at intervals, which announces that it is near roaring. A slight increase of pressure causes it to roar, and shorten at the same time to 8 inches.