ACOUSTICS AND GRAVITATION. 



25 



differential fringe deflection, owing to the different resonance volumes at R 

 and R'. The result, however, was absolutely negative. The fringes showed 

 no motion whatever, either at any frequency (12 to 500) nor when all resist- 

 ance was removed from the telephone circuit. Each reservoir (R, R') thus 

 acts like an open-air communication with reference to the other, so that the 

 effect of the pin-hole valve vanishes. On using either side separately (tb'R' 

 closed, b open, or tbR closed, b' open) the normal behavior at once appeared. 



The only way of securing a differential effect detected was by elongating 

 either branch (b, for instance) by inserting a long piece of rubber tubing. 

 Thus, for 40 cm. and 80 cm. of interposed length, displacements cf 5 and 10 

 fringes were obtained from the unsymmetrical adjustment. No doubt this 

 is merely equivalent to stopping, partially, the access to either chamber, 

 R or R'. 



A variety of correlative experiments were made with the simple (non- 

 differential) apparatus, figure 14. I have already referred to the absence of 

 fringe displacement (nearly) when the copper foil carrying the pin-hole is 

 cemented to the mouth of a funnel- tube. Small flasks with a lateral tubulure 

 to be joined to t", figure 14, and the device c secured with a rubber cork in the 

 neck, similarly gave deflection of 7 to 15 fringes only ; test-tubes with lateral 



a 



V 



34 



C! 



t 



tubulure not above 20 fringes in contrast with a normal deflection of 40. Thus 

 a reservoir, within and immediately preceding the pin-hole, removes its 

 valve effectiveness in promoting pressure and virtually opens the reservoir to 

 the air. 



On the other hand, the prolongation of the quill-tube t", if the diameter is 

 not increased, is almost indefinitely permissible. Thus, by inserting 40 cm., 

 80 cm. t 120 cm. length of eighth-inch rubber hose between t" and the pin-hole 

 c, no marked difference in efficiency of the valve-like action could be detected. 



In all experiments with closed regions, care must be taken to allow for the 

 temperature or air-thermometer effect resulting from the entrance of light into 

 the resonator. It is small, but gradually increases in the lapse of time. Such 

 results are also obtained when screw-cocks are put in the branches b or b' and 

 very slightly opened. 



The experiments made suggest a method of obtaining an effect which is at 

 least apparently differential. For this purpose the cock C, figure 33, is to be 

 closed, so that c is inactive, and the pin-hole c', figure 14, to be inserted either 

 into the branch b f , figure 33, or b. With a normal fringe displacement of 40, 

 the pin-hole in b' gave a displacement of 38 fringes, in b correspondingly 27 



