ACOUSTICS AND GRAVITATION. 



35 



telephone plate. The note evoked by the blower was now a faint e' and a 

 strong flat e"'. The survey in pitch at a depth of 4 cm. showed a minimum at 

 <z', a small maximum at b', a minimum near c", and a highly developed maximum 

 at a', a small maximum at b', a minimum near c", and a highly developed 

 maximum again at the flat e", all fringe displacements being pressures. In 

 other words, the curve scarcely departed in character from the groups, figure 52 , 

 though the displacements were smaller as a whole. 



The survey in depth below the mouth of the tube revealed no pressure 

 decrements. The curve ( i ,000 ohms in circuit) rose rapidly from zero, reaching 

 a maximum at about i mm. from the plate of about 30 fringes at 1,000 ohms, 

 very much as represented in figure 50. With the pin-hole but i mm. below the 

 plane of the mouth, however, the fringe displacement was already one-fifth 

 of the maximum. 



The final experiments were made with a modern bipolar telephone (100 

 ohms in circuit) and a somewhat wider (1.25 cm. in diameter) and longer (19 

 cm. as far as the telephone plate) tube. Blown when in place on the telephone 

 with the pipe-blower, it responded weakly to g' and strongly to e'". An ex- 

 ample of the results of the survey, with the pin-hole valve at depths 2,4, 8, 12, 

 16, 18 cm. below the mouth of the tube, is given in figures 53 and 54. The chief 

 maximum is here at a' (instead of the blown g') loud to the ear as well as to the 

 probe. The subsidiary maximum is near f". 



At a' the tube behaves strangely, since at different depths a' may be either 

 a maximum or a minimum. This result is so peculiar that a special examina- 

 tion of the distribution of pressure in depth was made as shown in figure 55, 

 for two different adjustments with 100 ohms in circuit. The occurrence of 

 minima near the depth of 16 cm. is well borne out in each case. 



feftfdftff ocm.4- 8 



30. Symmetrical induction. In the preceding paragraph it was inferred 

 that the large fringe displacements obtained were, in a measure, referable to 

 the non-symmetric induction supplied to the telephone by the break inter- 

 rupter. To test this a small magneto was installed and a survey made be- 

 tween g" and c" in case of the rubber pipe to which figure 50 refers. No de- 

 flection could be detected except at d", which came out very definitely with 

 7 fringe displacements. This relatively small value, however, was quite in 

 keeping with the corresponding weakness of the magnetic induction. In other 



