ACOUSTICS AND GRAVITATION. 



19 



about 2,000 ohms were put in circuit. Three maxima and three minima (one 

 positive and two negative) are indicated. The maximum below f' could not be 

 reached. The strong one at c" was well marked and approachable from both 

 sides. The small one near g", though easily observed by continuously changing 

 the pitch, was difficult to record. 



The latter, however, is particularly interesting, as it introduces the strong 

 decrement at e" and a". I therefore re-examined it in figure 23 with less re- 

 sistance (1,000 ohms) in circuit, and the results came out more clearly. The 

 deep minimum at a" deserves further investigation, as it precedes a probably 

 very high maximum at the near c'". At least, this may be inferred from the 

 stimulation produced by an organ-pipe used on the outside of the apparatus. 



In figure 24 I repeated the work with a finer micrometer scale, placed in the 

 collimator to secure a larger range. The curves are far from smooth, owing to 

 the mental confusion produced by the wealth of chromatic intervals, to which 

 I have adverted. Something better than the electrical siren used will have to 

 be devised; but apart from this the results are very definite. 



i 



25 



3a 



f a c? aTe,' yaT cf e' 



e' g: a 1 e,' cL' e'(jf a? 



Adjusting the siren for the maximum c", the sensitiveness with different 

 resistances in circuit (2,000 to 9,000 ohms) was determined. The curve is 

 shown in series 9 , figure 1 8, and is the highest thus far obtained. The equation 

 rs = constant does not fit so well here, a result inseparable from the slightly 

 fluctuating note; for this makes much difference in the maximum. The mean 

 value is about rs = 80 X io 3 . Referred to a circuit resistance of 100 ohms, this 

 is equivalent to a deflection of 800 fringes and a pressure of Ap = o.o24 cm. 

 of mercury. In the course of the work this was at least doubled. 



Treated in the same way, the minimum at a", if the resistance in circuit 

 were but 100 ohms, would produce negative deflection of 400 fringes, equiv- 

 alent to a negative pressure increment of A= 0.012 cm. of mercury. This 

 also was later much increased. These are striking results deserving more 

 careful investigation than I have thus far been able to give them. The pres- 

 sure, decrements also, are real, as may be shown by inserting a stop-cock and 



