NEw METHODS OF MEASURING THE SPEED OF SOUND PULSES 
NEAR THE SOURCE. 
By ARTHUR L. FoLrey, Waterman Research Professor and Head of 
Department of Physics, Indiana University. 
In the Proceedings of the Indiana Academy of Science for 1915 the 
writer showed that the relative speeds of sound pulses at some distance 
from the source and of different intensity are apparently the same. The 
experiments described threw no light on the question of the actual speed 
of a pulse at different distances from the source. This paper deals with 
a method, rather with two methods, of finding the actual and instanta- 
neous speed of the pulse at any point less than a meter or so from the 
source. The method could be used for greater distances by increasing 
the intensity of the spark producing the sound pulse, so as to give the 
wave sufficient intensity to cast a “shadow” on a plate or film. 
(Sse es see ee 
Figure 1 shows the arrangement of the apparatus used in this experi- 
ment. M is a plane steel mirror made by grinding and polishing the 
flat surface formed by cutting an axial longitudinal section 20 cm. 
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