Lord Rayleigh's Acoustical Observations. 153 



particular wave-length equal to about three diameters; and 

 laboratory experiments with steadily maintained tones had 

 made me familiar with the phenomenon of sounds apparently 

 transferring themselves from one ear to the other when the 

 head is moved ; but I thought it desirable to try a few experi- 

 ments in the open air especially directed to the examination of 

 this point. 



The source of sound was a lamp-chimney and hydrogen- 

 flame, as described above, of pitch e' flat, so that the quarter 

 wave-length was about eleven inches. The apparatus was 

 placed at distances varying from about 18 to 50 feet in front 

 of a tolerably flat wall ; and the observer, with one ear stopped, 

 investigated the positions of the silences, holding the middle 

 plane of his head parallel to the wall. Although the positions 

 of the silences were not very well defined, presumably in con- 

 sequence of unequal amplitudes of the superposed vibrations, 

 the most inattentive observer could not fail to notice that 

 it was necessary to move the head considerably in order to 

 pass from a place where the sound was a minimum for one 

 ear to a place where it was a minimum for the other ear. We 

 may therefore conclude that Savart's statement is not generally 

 true, and that the views which he has founded upon it have 

 no sufficient claim upon our acceptance. 



When the median plane of the head is perpendicular to the 

 wall, the silences are observed at distances which are odd multi- 

 ples of the quarter wave-length, agreeably with theory. 



Sensitive Flames. 



The beautiful phenomenon of sensitive flames is now 

 familiar to students of acoustics ; but its rationale is by no 

 means understood. An important contribution to the facts, 

 from which some day a theory will doubtless arise, is con- 

 tained in the observation of Prof. Tyndall as to the " seat of 

 sensitiveness." My present purpose is to bring forward 

 another fact which also will probably be found important. It 

 may be thus stated. Under the action of stationary sonorous 

 waves a flame is excited at loops and not at nodes. 



The source of sound was a little contrivance on the principle 

 of the bird-call, blown from a well-regulated bellows. Pro- 

 bably a very high organ-pipe or whistle might be employed ; 

 but it is necessary to use a nearly pure tone ; and the pitch 

 must be high, or the flame will be not affected sufficiently. 

 At a distance of a few feet the sound was reflected perpendi- 

 cularly from a large board. The flame itself was that called by 

 Tyndall the vowel flame, issuing from a pin-hole steatite burner 

 fed from a gas-holder with gas at high pressure (9 or 10 inches). 



