TRANSMISSION OF SOUND WAVES 97 



Experiment 84. Punch holes in the bottom of two clean tin 

 cans, and to each tie one end of a stout string about one hundred 

 feet long. Each taking a can, let two pupils separate until the 

 string is pulled tight (Fig. 72). Can you talk in lower tones 

 through the can than through the air ? What passes along the 

 string ? 



108. Speed of Sound Waves. We have, perhaps, seen 

 a man strike a blow at a distance and waited some time 

 before hearing the sound. This is because time is 

 needed for the sound waves to travel through the air. 

 Just as the ripples can be seen to move away from the 

 spot where a pebble is dropped in still water, so the 

 sound waves in air move away from a vibrating body 

 at a speed which can be measured. This speed is a little 

 greater in a warm than in a cold atmosphere. Through 

 air at ordinary temperatures sound waves travel about 

 1125 feet per second. A mile would be covered in about 

 five seconds. 



Experiment 85. Stand at some known distance (e.g. half a 

 mile or more) from a steam whistle which is soon to be blown. 

 Note the time when the " steam " appears (using a stop watch 

 if possible), and see how many seconds pass before you hear 

 the sound. Reduce the result to feet per second and compare 

 with the rule. 



Note other similar things whistles on distant trains or boats, 

 guns fired, blows struck, or engines puffing. Thunder is caused 

 by lightning and both occur at the same instant. Could you tell 

 how distant is the lightning by hearing its thunder ? How would 

 you do this ? 



109. Reflection ; Echoes. Roll a ball against a board ; 

 it bounds off at once. The ball is said to be reflected, 

 and the angle at which it leaves the board is the same as 



