LESSONS IN PHYSICS. 55 



upon the vibrations of cords, in accordance with the laws already 

 discussed. Vibrations of the air, giving musical sounds, may arise 

 from the vibrations of strips of metal, wood, or other substances, 

 and on this principle the construction of organs, horns, trumpets, 

 and other reed and wind instruments depends. In an organ 

 sounds are made by vibrating columns of air in pipes, sometimes 

 aided by the vibrations of a slender and elastic tongue called a 

 reed. The pitch of sound in pipes depends upon the length of 

 the pipes, or in case of reed pipes on the length and weight of the 

 reed. The air in the pipes of an organ vibrates as a whole and 

 in segments as well, so that there are overtones or harmonics in 

 pipe instruments as well as in stringed instruments. In the case 

 of the horn the sounds are made by vibrating columns of air, the 

 lips aiding the vibrations, as the reed aids the vibrations in the 

 organ pipes. 



The human voice is regarded by the best authorities as being 

 analogous to a reed-pipe, the vocal chords forming the reed, 

 and the cavity of the mouth the pipe; like the reed, it is rich in 

 harmonics, as many as sixteen having been detected in some 

 voices. But their number and relative intensities differ much in 

 different individuals, or even in the same person at different 

 times; and it is on this variety that the peculiarities depend by 

 which any one voice may be unmistakably distinguished from 

 every other. Voices in which overtones abound are sharp, and 

 even rough; those in which they are few or faint, are soft and 

 sweet. 



Sound waves move through all elastic media, as wood, iron, 

 the earth, water, air, etc. The velocity of sound waves varies 

 inversely as the square root of the density of the substance, and 

 directly as the square root of its elasticity. These laws are 

 doubtless true for gases whose structure is uniform throughout, 

 but are not true for the air. Through the air at ordinary tem- 

 perature sound waves move with a velocity of about 1,118 ft. 

 per second, varying somewhat with temperature and with the 

 amounts of water vapor in the air, etc. One great danger to 

 commerce is the difficulty of locating rocks, shoals, and other 



