274 NATURE OF SOUND. 



Inversely as the square root of the density. At the 

 freezing temperature, sound travels through oxygen with a 

 velocity of 1040 feet, and through hydrogen with a velocity 

 of 4164 feet per second. 



(a.) It is a very common mistake to think that an increase of 

 density causes an increase of velocity. It is known, e.g., that sound 

 travels more rapidly in water than in air ; that water is more dense 

 than air ; hence, say the superficial, sound travels most rapidly in 

 the densest bodies. It does not follow. Other things being equal, 

 the denser the medium, the less the velocity of the motion. A little 

 reflection will show that this must be so ; experiments will verify 

 the conclusion. In wave motion, the particles of the medium con- 

 stitute the thing that is moved. With a given expenditure of energy, 

 a number of light particles is moved more rapidly than an equal 

 number of heavy particles ( 157). 



427. Effect of Temperature Upon Velocity. 



An increase of the temperature of the air increases its 

 elasticity and decreases its density. We might, therefore, 

 expect sound to travel more rapidly in warm than in cold 

 air. Experiment confirms the conclusion. Tliere is an 

 added velocity of about 1.12 feet for every Fah- 

 renheit degree, or of about 2 feet for every centi- 

 grade degree of increase of temperature. (The 

 freezing temperature is 32 F, or C.) 



428. Noise. A noise may be momentary or con- 

 tinuous. A momentary noise consists of a single pulse in 

 the medium produced by a single and sudden blow. It 

 has neither period nor wave length. A continuous noise 

 consists of an irregular and rapid succession of pulses. 

 The ear is so constructed that its vibrations disappear very 

 rapidly, but the disappearance is not instantaneous ; if the 



