New Methods of Measuring. Pie 
ably due to the powerful air currents set up by the whirling electrode. 
Nevertheless it was possible to get three or four pictures on each film 
and to get sufficiently well defined sound pulse and rod shadows to permit 
of reliable speed calculations for waves of radius greater than 2 or 3 cm. 
The polish of the metal mirror was not sufficiently good to give well- 
defined wave pictures close to the source, where the wave is more or less 
confused with other spark effects. It was decided, therefore, to eliminate 
the mirror entirely and get the picture directly on a moving film. The 
mirror shaft was removed and in its stead was placed a shaft carrying 
an eight-inch flat-face steel pulley two feet in diameter. The film was 
fastened to the face of the pulley and rotated within 1 cm. of the open- 
ing O, across the center of which the rod R was fastened in a horizontal 
position and exactly in line with the sound and light sparks. The dis- 
tance on the film between the sound spark and the light spark shadows 
of R together with the pulley speed gave the time interval between the 
sparks. From the radius of the wave shadow together with the dis- 
tances from the light and sound sparks to the film the true radius of 
the sound pulse was calculated. As before, the quotient of radius by 
time gave the sound speed. 
The definition of both sound wave and rod shadows was much better 
in this case than when the rotating steel mirror was used. However, 
the experiment did not yield better results for waves of small radius, 
because it was impossible to rotate the film fast enough to make the 
distance between the rod shadows sufficiently large to be measured with 
accuracy, when the time interval was small. The film was thrown off 
and torn to fragments whenever the speed exceeded some twenty-five 
revolutions per second, regardless of the precautions taken to hold the 
film on the pulley. Even when both the edges and the ends of the film 
were cemented to the pulley, the film was thrown off at a speed of some 
twenty turns per second. The highest rotational speed was obtained 
when the film was held on the pulley by placing over it a strip of strong 
cotton net of about 5 em. mesh, with edges laced securely on the inside 
of the pulley rim. The string shadows were readily differentiated from 
the rod and wave shadows, and were not so objectionable as the writer 
feared they might be. 
On account of the limited speed at which the film could be rotated, 
the increase in the accuracy of the time interval measurements resulting 
from the better definition of the rod shadows was offset by the fact that 
the distance between the shadows was much less than by the rotating 
mirror method. Both the rotating mirror method and the moving film 
method gave results that show that if there is any difference between 
the speed of a sound pulse of the intensity used and the speed of an 
ordinary sound wave, the difference is less than two per cent. 
