ACOUSTICS AND GRAVITATION. 21 



telephone circuit. The results were most striking and appeared about as 

 follows : 



g'tob' c" d'toef f g" pitch 



max. estimated 

 5 25 at -200 35 o fringes 



There is thus an enormous maximum dilatation somewhere in the range of 

 frequency d" to e" which, from the hovering character of the deflection, is not 

 further determinable. This amounts to a pressure decrement of A = 6 X io~* 

 cm. of mercury with 2,000 ohms in the telephone circuit. At 100 ohms it 

 would have been about a millimeter of mercury. Thus the introduction of the 

 pin-hole valve has enormously accentuated the simple phenomenon in 14 

 and guarantees its trustworthy character. 



It remains to place the two shanks of the U-tube, figure 14, in communi- 

 cation, respectively, with the two sides of the pin-hole O in figure 25; i. e., to 

 join t with the space above m' ; but for this purpose the plate g would have 

 to be sealed, for which operation I was not quite ready. The use of a pin- 

 hole within and without is usually prejudicial to sensitiveness. Thus at 1,000 

 ohms, the deflection of 47 fringes for this condition was quite doubled when 

 the inside pin-hole was removed. 



With the apparatus, figure 14, and the cock C opened at the critical point, 

 a diapason c" blown in the vicinity of the cock was quite appreciable and the 

 octave c'" three times as effective (15 fringes). In another adjustment, while 

 d" gave almost 35 fringes of negative displacement (dilatation), g" gave a 

 positive displacement of 45 fringes and d'" (shrill overtone) nearly 100 fringes, 

 also positive. There is some misgiving in interpreting these data, as the open 

 mouth of the pipe must usually be close to the mouth of the cock; but as d" 

 was still appreciably effective 6 to 12 inches away, the results are probably 

 trustworthy. The difference in sign of the compression corresponding to c" 

 and c'" is particularly to be noticed. 



21. Apparent removal of pressure decrements. The slow growth of rel- 

 atively enormous pressure decrements here recorded is so surprising that fur- 

 ther experiments are needed. To begin, one may ask whether the telephone 

 plate, held as usual by strong screw-pressure between annular plates of hard 

 rubber, is adequately air-tight. I therefore removed the telephone and sealed 

 all these parts with cement, rigorously. 



On replacing the telephone with the adjustment, as in figure 25, the be- 

 havior had in fact changed, the negative deflection being of the small value 

 indicated in 14, without growth in the lapse of time. In other words, the 

 presence of the pin-hole c' within the closed region was now ineffective. 



We may summarize the results, so far as at present in hand, in figures 26 and 

 27. In an air region R, closed on one side by a vibrating telephone plate T 

 and on the other by a quiet plate U, the pressures are distributed as if there 

 were a maximum at T and a minimum at U. If the region R, figure 26, com- 

 municates with the atmosphere by a pin-hole of critical diameter, the pres- 



