SOUND WAVES. 169 



light from mirrors, or as the heat waves in the reflectors. We have what we 

 term " sounding boards " in pulpits, and speaking tubes will carry sound 

 for us without loss of power. Echoes are merely reflected sounds. 



The velocity of sound is accepted as i , I oo feet in a second, which is 

 far inferior to the velocity of light. Fogs will retard sound, while water will 

 carry it. Those who have ever rowed upon a lake will remember how easily 

 the sound of their voices reached from boat to boat, and Dr. Hutton says 

 that at Chelsea, on the Thames, he heard a person reading from a distance 

 of a hundred and forty feet. Some extraordinary instances could be deduced 

 of the enormous distances sound is said to have travelled. Guns have been 

 heard at eighty miles distant, and the noise of a battle between the English 

 and Dutch, in 1672, was heard even in Wales, a distance of two hundred 

 miles from the scene of action. 



Sound always travels with uniform velocity in the air in the same tem- 

 perature. But sound ! What is the cause of it ? How does it arise ? These 

 questions can now be fully answered with reference to the foregoing obser- 

 vations. Phenomena of vibration render themselves visible by light, heat, 

 and sound, and to arrive at some definite ideas of sound vibrations we may 

 compare them to waves, such as may be produced by throwing a stone 

 into a pond. 



There are, so to speak, " standing " waves and " progressive " waves. 

 The former can be produced (for instance) by thrumming a fiddle-string, 

 and when the equilibrium of the cord is disturbed, the position of the 

 equilibrium is passed simultaneously by the string-waves. In water the 

 waves or vibrating points pass the position of equilibrium in succession. 



Waves consist of elevations and depressions alternately, and when we 

 obtain two " systems " of waves by throwing two stones into water, we can 

 observe some curious effects. It can be seen how one series of depres- 

 sions will come in contact with the other series of depressions, and the 

 elevations will likewise unite with the result of longer depressions and 

 elevations respectively ; or it may very well be that elevation will meet 

 depression, and then the so-called " interference " of waves will produce 

 points of repose. These points are termed nodes. The waves of the string 

 proceed in the plane of its axis ; water waves extend in circles which 

 increase in circumference. 



The progression or propagation of sound may be said to begin when 

 some tiny globule of matter expands in the air. The air particles 

 strike one against the other, and so the motion is communicated to the air 

 waves, which in time reach the ear. But the velocity of the sound is not 

 equal in all substances. Air will convey it around our earth at the rate of 

 765 miles an hour, or 1,090 feet in a second. That is, we may accept such 

 rate as correct at a temperature of 32 Fahr., and at a pressure of thirty 

 inches, and the velocity increases almost exactly one foot per second for 

 each degree of temperature above 32. Therefore on an average, and speak- 

 ing in "round numbers," the estimate of 1,100 feet in a second maybe 



