ACOUSTICS AND GRAVITATION. 39 



In narrow pipes (diameter 0.7 cm., length 5 cm.) certain harmonics may be 

 reversed and the others left unchanged when the telephone poles are changed. 

 Thus, with the short hard-rubber tube of figure 50, the curve above g' was not 

 subject to change on commutation, but below this there were marked differ- 

 ences. The e' of a few negative fringe displacements in figure 50 changed to 

 e' of about 50 positive displacements on reversing the current. Moreover > 

 the curve with 1,000 ohms in circuit as compared with the curve for 2,000 

 ohms, though preserving the same character as figure 50, differed in the 

 relative value of maxima and minima. 



It is, moreover, frequently possible to change negative fringe displacements 

 into increasing positive displacements by increasing the telephone current; 

 i. e., with the removal of extra resistance in circuit. The curves are thus de- 

 pendent on their general location on the incidental changes of current (as one 

 of their parameters), and this explains why on repeating a series of observations 

 the curve, as a whole, may have risen or been depressed. In a more searching 

 investigation the current, like the frequency n, would have to be definitely 

 measured in its relations to s. 



The most interesting inversion of this kind was made with the brass pipe 

 13 cm. long, i cm. in diameter, from which the peculiar graphs of figure 51 

 were obtained. To get the whole graphs within this field, 2,000 ohms were put 

 in the telephone circuit and there were a few other incidental modifications. 

 Apart from this, the graph, figure 59, obtained with the pin-hole probe for the 

 position / of the switch, is the same in character as the group in figure 5 1 , with 

 the strong maximum at a' and strong minimum at d" '. The curve for the 

 position // of the switch is very different. While the minimum near g' has 

 probably been retained, the former strong maximum at a' is obliterated. 

 Furthermore, the former negative minimum at d" has been reversed to a 

 positive maximum and enhanced. 



Similarly, the same pipe cut down to 10 cm., as recorded in figure 52, with 

 only positive fringe deflections, lost its %d" maximum on commutation 

 of current. 



Finally the pipe, cut down to 7 cm. and examined at 6 cm. of depth, gave the 

 graphs / and //, figure 60, for two positions of the commutator. The difference 

 at g' and a' is marked; but at the pipe-note e" the position / evokes an enor- 

 mous maximum, followed at once by an equally pronounced negative minimum, 

 whereas in the position // the minimum remained in the positive field. 



Other examples will be given among many obtained, in treating the open 

 organ-pipe on the interferometer. 



Referring for convenience to figure 59, let s n be the mean fringe displacement 

 at any frequency n. Then the observed displacements at n will be s n + A^ n 

 and 5 n As n , respectively, for the two positions, I, II, of the switch of the 

 telephone current. Hence the reversible effect impressed by the telephone is 

 2 As n , a quantity which vanishes between a' and c" (i. e., at &') and is positive 

 below and negative above &', so far as observed. It also vanishes at f'. Thus it 

 seems to me probable that on operating with a sufficiently strong magneto, the 



