186 



LECTURES TO SCIENCE TEACH EES. 



interval of time between the production of the light 

 and the reception of the sound through the water by 

 means of this apparatus. Inasmuch as light passed over 

 that small interval in an innnitesimally small space of 



FIG. i. 



time, the difference between the perception of the light and 

 the perception of the sound would give us the velocity of 

 sound through that small space of water. 



