ACOUSTICS AND GRAVITATION. 49 



not successive. While the objective revolves 360, only during a few degrees 

 is there visibility. If the motor is slowed down, the flashes soon become 

 disagreeably intermittent. Possibly in the study of adiabatic expansion, the 

 revolving-telescope method may be useful. 



44. Observations with longitudinal pipes. The experiments with the form 

 of pipe, figure 68, were made somewhat carefully, as this may be considered a 

 standard form. The open pipe used was of wood, with an inner cross-section 

 2.8X2.8 cm. and length 24 cm. It responded perfectly to the fundamental, 

 the softest producible note showing about o.i fringe in double amplitude, the 

 loudest full note not more than a whole fringe. Blown so hard that the note 

 sharpened, the amplitude rather diminished. As in the above case, the mean 

 energy (io 8 X8.7n) lies between (1 = 24 cm. here) 



7 ergs/cm. s 



and ten times this quantity, or io 3 X8.7 ergs/cm. 8 . The maximum energy 

 would, therefore, be ir/2 times larger, which makes the energy content io 4 X 

 1.37 ergs/cm. 8 for the full note. The general shape of waves obtained was 

 sinuous only for long excursions of the objective. For short excursions the 

 zigzag forms and diagonal pattern were often striking. 



The pipe did not at first admit of the production of the octave quite free 

 from the fundamental. The wave-pattern consisted of fundamental waves, 

 along the contours of which octave waves passed to and fro. The embouchure 

 was therefore reset to give a clear, shrill overtone alone. The waves now 

 vanished, evidencing complete symmetry in the dense and rare nodes present. 



The pipe was then closed with a plate of glass and the corresponding experi- 

 ments carried out. Toward the fundamental, the interferometer was partic- 

 ulaily sensitive, suggesting waves even before the sound was in evidence. 

 The full note, though not loud, gave the same displacement of one fringe, at 

 maximum response. 



The surprising feature of this closed pipe, however, was the occurrence of 

 strong waves for the first overtone (fitth above octave). The two nodes, there- 

 fore, aie here not symmetrically rare and compressed. The double amplitude 

 of these waves for the very shrill note was 0.4 fringe, so that the differential 

 energy would be io 3 Xs.S er g s pei cubic centimeter. One would naturally 

 expect a fundamental present with the overtone to account for this, but 

 neither the waves nor the sound of pipe gave the least indication of the funda- 

 mental. It seems more probable that the node at the closed end of the pipe is 

 a semi-node, s, figure 72, while that nearer the open end is a full node, ss, and 

 that the difference found is the energy of the former. This would brinp the 

 energy of the full node (such as occurs necessarily in the open pipe) up to 

 io 3 Xn eigs/cm. s . 



The duodecimal overtone was also obtained sharply, but could not well be 

 sustained long enough tor measurement. 



