ACOUSTICS AND GRAVITATION. 53 



g a b c' d' e' f g' a' f" 



o strong stronger max. strong weak o weak o o 



1.2 fringes 



Resonating pipes on the interferometer made no discernible difference. The 

 seat of activity is probably in the iron base of the apparatus acting as a sound- 

 ing-board. Loading it depressed the maximum to b. A totally different 

 interferometer in a new location showed the same behavior on the same base 

 (lathe-bed slide), with a maximum at 6. This discrepancy is an exceedingly 

 difficult one to eliminate, as it calls for a detection of the resonant member of 

 the interferometer, for which reason I abandoned further work with strong 

 reed pipes. 



With the diapason organ-pipes used above, there is much less danger of 

 direct influence. This is shown, for instance, in the balance obtained with 

 nodes of opposite sign. Moreover, I made control experiments by blowing 

 equipitched diapason pipes strongly in the neighborhood. There is, even here, 

 liable to be a little response. The tendency to assume wave-form may 

 be recognized; but it is much smaller than the pipe-note proper, and quite 

 absent in the overtones. Finally, the elbowed pipe, figure 79, which blows 

 -away from the interferometer, is additional guarantee. 



48. General result. The data obtained in these experiments with a di- 

 versity of pipes (cross-pipes, closed and open organ-pipes, tubes) when sound- 

 ing their full note, however shrill this might be in the overtones, rarely showed 

 a compression in excess of the equivalent of one fringe. In other words, there 

 was a remarkable constancy in the maximum amplitude throughout. Possibly 

 in the steam-whistle pattern (which I have not yet tried) this limit may be 

 exceeded. However, for the usual pipe, since the intensity * = a*/X 2 and a is 

 found nearly constant, the shrill notes of overtones are largely to be referred to 

 the decrease of wave-length. Thus in the closed pipe they would be 9, 25, etc., 

 times louder than the primitive, for equal response. 



To test this further, I constructed the open pipe, elbowed in the 

 middle, E, as shown in figure 79. The pipe is embouchured by the glass 

 plate g and there is a corresponding plate g' beyond the elbow, to allow the 

 component ray L of the interferometer to pass through. For the funda- 

 mental, the node is at E, so that a little more than a quarter-wave will be 

 seen, and a little more than a semi-node. For the octave (which like the 

 fundamental came out strongly and with the same pitch as for the straight 

 pipe) the nodes are at P and P', and a half wave-length is completely visible. 

 Large, handsome achromatic fringes were installed. In the course of several 

 settings of the embouchure, the best position for loud notes did not evoke 

 more than a fringe breadth between trough and crest; usually about 0.8 

 fringe was recorded,, while somewhat more than X/4 was observed. In cor- 

 responding settings of the embouchure for the shrillest overtone, the largest 

 -double amplitude of the zigzag waves was about 0.8 fringe. As a closed 



