STUDIES IN NATURE 



looking-glass ; it is no good to stand in front of 

 a wooden door or muslin curtain. 



By means of an echo we may make another 

 attempt to find the speed at which sound travels. 

 We can generally tell what object is sending the 

 air waves back to us, and if we measure how far 

 away it is, and see by the seconds-hand of a 

 watch how long a sound takes to get there and 

 back, we can calculate the speed. 



(50) 



