ACOUSTICS AND GRAVITATION. 15 



The displacements of the achromatic fringes corresponding to the head of 

 mercury in B may be read off by a telescope provided with an ocular o.i mm. 

 micrometer. It is, however, advantageous to place the micrometer in the 

 wide slit of the collimator, the fringes being parallel to the scale-parts. To 

 obviate the need of adjusting the inclination of the fringes (as this frequently 

 changes), the slit-holder should be revolvable around the axis of the collima- 

 tor, the scale being parallel to the length of the slit and the fringes moving in 

 the same direction across the white, ribbon-like field. Fringes equal to a 

 scale -part in breadth are most convenient. 



14. Observations. Closed and open resonators. Spring interrupters dip- 

 ping in mercury were first used, having frequencies of n = i2 and 100 per 

 second, respectively. 



When the cock C is closed, there is no appreciable effect until the telephone 

 resounds harshly. In such a case there is marked dilatation in the resonator 

 R, increasing with the intensity of vibration. The successive readings (s r 

 fringes) are liable to be fluctuating; for instance, 



Current off 20 25 22 21 26 21 24 16 



Current on 30 34 30 30 31 34 26 27 



but the sense and mean value are definite. Since the head is (s f $ )_ = 



2 



s\/2 cm. (the displacement being 5 fringes of wave-length X) , this mean value s of 

 the many experiments for the given intensity of vibration is at once equiva- 

 lent to the pressure A = 2Xicf 4 cm. of mercury, or about jXicT 8 atmos- 

 phere. If but 500 ohms are put into the telephone circuit, however, 

 appreciable deflection here ceases. 



One might suppose (particularly in view of the irregular readings) that the 

 equilibrium position of the telephone-plate differs slightly when in vibration 

 from the position when at rest, and refer the above deviation to a cause of this 

 nature ; but in view of the results with pin-hole leaks presently to be described 

 and in which an effect of the supposed shift of the plate would be untenable, 

 the dilations are probably real. In the closed space R in B, temperature 

 and other discrepancies would inevitably be detrimental to a smooth record. 



Again, if the stop-cock Cis completely open, no effect whatever is obtained. 

 The bore of the small stop-cock in this case need not exceed 2 or 3 mm. All 

 the negative results which I obtained by other methods heretofore are thus 

 explained. 



15. Resonator all but closed. If now, from the open position, the plug of 

 the cock C is rotated gradually until the opening is reduced to the merest 

 crevice, the fringe deflection 5 will, on further slow rotation, be found to in- 

 crease from zero, with great rapidity, to a positive maximum. The deflec- 

 tion then falls off with similar rapidity through zero to the negative value when 

 the cock is again quite closed. I have indicated this result graphically in 

 figure 15, in which the abscissas show the degree to which the cock has been 



