224 Proceedings of Indiana Academy of Science. 
The writer is now at work on a third method which promises more 
accurate results than either of the ones described in this paper. 
It may be noted that a photographic method of measuring sound 
speed eliminates sources of error found in all methods where the sense 
of hearing or any mechanical device is used to register the time of 
arrival of a sound wave, and where the distances traversed by the wave 
are large. There is no question as to personal error, time lag, wind 
velocities, differences in temperature, humidity, density, change of wave 
form, etc. The method gives the instantaneous speed at points up to the 
source of sound itself. These points will be discussed and data submitted 
in a later paper. 
The writer wishes to thank Professor Cogshall of the Department of 
Astronomy, of Indiana University, for his kindness in grinding and 
polishing the steel mirror used in this experiment. 
