IN POPULAR PHYSICS. 41 



The audience break up the echoes which interfere with the 

 original sound. Wires strung across a lofty room often serve 

 the same purpose to a slight extent. 



19. What is the object of the sounding -board of a 



By its vibrations and those of the body of air which it in- 

 closes, it re-enforces the sound of the wires. 



20. During some experiments, Tyndall found that a 

 certain sound would pass through twelve folds of a dry 

 silk handkerchief, but ivould be stopped by a single fold of 

 a wet one. Explain. 



(See TyndalTs Lectures on Light, p. 325, for a series of experiments show- 

 ing the action of moisture on the propagation of sound-waves.) 



21. What is the cause of the musical murmur often 

 heard near telegraph lines? 



It is produced by the vibration of the wires. These are 

 thrown into motion by the wind and other causes. 



22. Why will a variation in the quantity of ^vater in 

 a goblet, when caused to sound, make a difference in the 

 tone ? 



It changes the area of the vibrating portion of the glass. 



23. At what rate (in meters) will sound move through 

 air at sea-level, the temperature being 2O C. ? 



Sound moves at the rate of 1,090 feet at C. The differ- 

 ence is nearly 2 feet for each degree C. 



1,090 feet + 40 feet =1,130 feet. 



OPTICS. 



235 1. Why is a secondary bow fainter than the pri- 

 mary ? 



The primary is produced by one reflection and two refrac- 

 tions ; the secondary, by two reflections and two refractions. 

 The additional reflection weakens the ray 



