lxi 



in a vacuum, without contact with objects outside of the vacuum, they 

 produce no sound, because they cannot affect the air. 



Many other substances convey sound equally as well as the air, but air 

 being the only element usually in contact with the ear, it is the natural 

 medium for man. The metals, water, wood, and many other substances, 

 are excellent conductors of sound. 



If it were possible for us to seethe particles of air when they are set in mo- 

 tion bv a harp-string making about eleven hundred vibrations per second, 

 -we would see them spaced off in equal distances of about one foot in length, 

 in every direction from the harp-string. In every alternate foot or space 

 Ave would see them moving towards the string, and in each intermediate 

 space we would see them moving from the string. The next instant the 

 atoms in all these spaces would have their motions reversed. Each alter- 

 nate set of atoms would be seen approaching and compressing each other, 

 and then instantly rebounding from each other and compressing the set on 

 the opposite side. These motions to and fro would be found to correspond 

 exactly in time with those of the harp-string creating them. It would be 

 seen that the impulse from the harp-string would be first imparted to the 

 set of atoms nearest to it, and by this set it would be imparted to the next 

 and so on out to the most distant ones in the system. Hence eleven hun- 

 dred vibrations would have to be made by the string before the air eleven 

 hundred feet distant would be set in motion. These vibrations of the air, 

 when they fall upon the tympanum, create the sense of sound. They 

 travel at the rate of about eleven hundred feet per second, the speed vary- 

 ing with the density and temperature of the air. The to and fro move- 

 ment of the atoms in each of these eleven hundred spaces, or waves of air, 

 is called the swingor vibration of the atoms. This swing is of much greater 

 amplitude near the string, and diminishes as the vibrations are more 

 and more distant. Hence the impulses upon the ear will be much more 

 energetic near the string than at a distance. The note sounded will there- 

 fore be louder. It will still be the same note, however, whether the string 

 be distant or near, because each wave created by it is of exactly the same 

 length, and hence the waves fall in exactly the same periods. It is the 

 rapidity of these impulses on the ear that determines the pitch of the note. 

 If the harp-string were shorter, or if it were lighter, or if it were more 

 tightly drawn, its vibrations would be more rapid. Then the waves would 

 be shorter, and more of them would be required to make up eleven hundred 

 feet in a second of time. Consequently, as the waves travel at the same 

 rate without regard to their size, they would come into the ear more rapidly 

 and a note of higher pitch would be the result. 



The same note sounded by a rapidly approaching steam whistle on a 

 locomotive has a higher pitch to the stationary listener before, than it has 

 after the whistle has passed and is retreating from him. If the listener be 

 himself on a train rapidly meeting the one on which the whistle is sound- 

 ing, the change of pitch at the moment of its passing the hearer will 

 be much more marked. The current of steam issuing from the contracted 

 opening in the whistle is thrown into rapid vibrations by being directed 



