54 DISPLACEMENT INTERFEROMETRY. 



pipe, this elbowed form functioned badly. No waves were obtained for 

 the fundamental and the shrill duodecima. Nor would they be expected. 

 The fifth, which was not loud, gave about 0.4 fringe. 



Returning to the loud open pipes, whose semi-length is now 12 cm., the 

 energy content per cubic centimeter will be somewhat larger than above. 

 In the full fundamental there would be i o 3 X 2 7 ergs /cm. 3 ; but more than \ /4 

 was seen, and the usual values were 0.8 fringe, so that 2 X io 4 ergs/cm. s (allow- 

 ing for a slight increment from the interferometer) was probably not usually 

 exceeded. The same value holds for the shrill overtones of 0.8 fringe. Directly 

 compared, the amplitudes of the fundamental were the larger, agreeing with 

 the above inference. I have not been able to carry the experiment to a 

 conclusion, but it seems as if with the amplitude in excess of the equivalent 

 of one fringe the harmonic motion in the given pipe becomes unstable and 

 breaks up into the next overtone in succession. In the endeavor to prove 

 this I constructed diapason pipes of the 2-foot octave (c') about 60 cm. or 

 more long and 5 cm. in diameter, of brass and pasteboard. They were excited 

 by the device, figure 75, suitably secured to the pipe. The note was strong and 

 passed continuously (to use this phrase) into the overtone c" on increasing the 

 strength of the blast. To my regret, however, the fundamental c' again shook 

 the interferometer, as in the case of the reed pipes above, so that measurements 

 could not be made. 



One may finally ask how large a temperature effect A* should correspond 

 to the maximum compression Ap/p = 2Xio~ 2 given above. If c is the specific 

 heat at constant volume and / Joule's equivalent, 



JCp&t = p&P/p = 2 X io 4 

 whence 



2X10" 



=2.2 



42 Xio 6 Xo.i7Xo. 00129 

 The same result may be obtained from the equations of adiabatic expansion : 



T being the absolute temperature of freezing. 



This is an unexpectedly large temperature increment and would seem to be 

 easily measurable by the bolometric-telephone method, described in the last 

 report. It is probable that the reason there given for the negative result is 

 correct; i. e., in the case of these rapid alternations heat fails to penetrate the 

 bolometer wire adequately to become appreciable at the telephone. 



