6 The Bight Hon. Lord Bayleigh [Jan. 17, 



between either orifice and the flame, flaring ensues, the sound not 

 intercepted being free to produce its proper effect. 



The. analogy with Fresnel's bands would be most complete if we 

 kept the sources of sound at rest and caused the burner to move 

 transversely so as to occupy in succession places of maximum and 

 minimum effect. It is more convenient with our apparatus and comes 

 to the same thing, if we keep the burner fixed and move the sources 

 transversely, sliding the base-board without rotation. In this way 

 we may verify the formula, connecting the width of a band with the 

 wave-length and the other geometrical data of the experiment. 



The phase discrepancy necessary for interference may be intro- 

 duced, without disturbing the equality of distances, by inserting in 

 the path of one of the sounds a layer of gas having different acou- 

 stical properties from air. In the lecture carbonic acid was employed. 

 This gas is about half aB heavy again as air, so that the velocity of 

 sound is less in the proportion of 1 : 1'25. If I be the thickness of 

 the layer, the retardation is • 25 Z ; and if this be equal to the half 

 wave-length, the interj)Osition of the layer causes a transition from 

 complete agreement to complete opposition of phase. Two cells of 

 tin plate were employed, fitted with tubes above and below, and closed 

 with films of collodion. The films most convenient for this purpose 

 are those formed upon water by the evaporation of a few drops of a 

 solution of celluloid in pear-oil. These cells were placed one in the 

 path of each sound, and the distances of the cones adjusted to maxi- 

 mum flaring. The insertion of carbonic acid into one cell quieted the 

 flame, which flared again when the second cell was charged so as to 

 restore symmetry. Similar effects were j^i'oduced as the gas was 

 allowed to run out at the lower tubes, so as to be replaced by air 

 entering above.* 



Many vibrating bodies give rise to sounds which are powerful in 

 some directions but fail in others — a phenomenon that may be re- 

 garded as due to interference. The case of tuning forks (unmounted) 

 is well known. In the lecture a small and thick wine-glass was 

 vibrated, after the manner of a bell, with the aid of a violin bow. 

 When any one of the four vibrating segments was presented to the 

 flame, flaring ensued ; but the response failed when the glass was so 

 held at the same distance that its axis pointed to the flame. In this 

 position the effects of adjacent segm( nts neutralise one another and 

 the aggregate is zero. Another example, which, strangely enough, 

 does not appear to have been noticed, is afforded by the familiar open 

 organ pipe. The vibrations issuing from the two ends are in the 

 same phase as they start, so that if the two ends are equally distant 

 from the percipient, tlie effects conspire. If, however, the pipe be 

 pointed towards the percipient, there is a great falling off, inasmuch 

 as the length of the pipe approximates to the half-wave length of 



* III a still atmosphere the hot gases arising from lighted camlles may be 

 substituted for tlie layers of CO^. 



