348 



any but the very feeblest vibration. Admitting, however, that 

 these extremely faint vibrations of the air may contribute some- 

 what to the result, it can hai'dly be doubted that the main influence 

 by which the movement of the jet produces the effect in question 

 is by causing so rapid a mixture of the adjoining air with the gas 

 that the latter, before being inflamed, is brought into a condition 

 to produce those small explosions which by their quick succession 

 give origin to the sound. The effect of motion in bringing about 

 this explosive condition of the flame is well exemplified by the 

 following experiment : — 



(2.) Fastening a jet pipe some ten inches in length into the 

 end of the flexible tube through which the gas is supplied and 

 holding it erect by a point a little below the insertion so that we 

 can readily cause it to vibrate, we ignite the gas and adjust it to 

 form a slender flame about an inch long. If now the flame be 

 moved from side to side at a moderately rapid rate, it will assume 

 according to a well-known visual law the appearance of a con- 

 tinuous arch of whitish light retaining at the extremities the 

 whole height of the stationary flame but growing narrower from 

 either side towards the middle. In these conditions the flame is 

 entirely noiseless. As we gradually increase the speed of the 

 vibrations the arch, at a certain stage, suddenly breaks in the 

 middle w^here a faint bluish flame takes the place of the usual 

 whitish light and at the same instant a sharp noise is heard, due 

 to the inflammation of the explosive mixture at this part of the 

 vibration. It is hardly necessary to say that as the vibration is 

 quickest at the midway point, coming to a pause at each end of 

 the arch, the gas becomes more largely mixed with air at the 

 middle than towards the extremities of the motion, and is, there- 

 foi'e, at this point, earliest reduced to the state of an explosive 

 mixture. 



As we increase the velocity of the vibrations, the sonorous part 

 of the arch extends towards the ends, until the path of the flame 

 presents the aspect of a narrow bluish band irregularly serrated 

 at top and flanked at the exti'emities by a tall flame of the usual 

 whitish color. As might be expected, when the jet is revolved rap- 

 idly in a circle, the white light entirely disappears, and the ring of 

 bluish flame which results gives forth a continuous but not dis- 

 tinctly musical sound. When made in a dark room these simple 



