PHYSICAL FORCES 117 



synchronous with the primary ones, beauty and per- 

 fection of timbre is the result. When certain bodies 

 are in vibration, they will elicit corresponding move- 

 ments^ called sympathetic vibrations from other bodies. 

 Thus certain wires of a piano may be made to vibrate 

 without being touched save by the waves of air set in 

 motion by another musical instrument emitting the 

 notes with which such wires of the piano correspond. 

 Even the pendulum of a clock that has stopped may be 

 set in motion by the pendulum of another clock standing 

 against the same wall and duly oscillating. 



The vibrations accompanying sound may be perceived 

 by other senses than the sense of hearing. Thus the 

 vibrations of a tuning-fork are visible, and those of an 

 organ-pipe may be very distinctly perceived by touch, 

 as also may the oscillations of a deep-toned cord. 



Air is a bad conductor of sound, which can be much 

 better transmitted by liquids or even by solids. Water 

 will transmit sound more than four times as fast as air, 

 and wood or iron will carry it seventeen times faster. 



ELECTRICITY. The physical energies yet noticed, 

 must have forced themselves on man's observation from 

 the first, but of those which remain to be considered 

 (apart from certain isolated phenomena) a knowledge 

 has been acquired only during the last few centuries. 



If sealing-wax be rubbed briskly for some seconds 

 with a piece of cloth or flannel, and then be held over 

 small fragments of torn paper within a distance of a 

 quarter of an inch, such fragments will immediately 

 rise and adhere to it. The same thing will occur if a 

 glass rod be similarly rubbed with a silk handkerchief. 



If a small ball, made of pith, be freely suspended by a 

 silk thread from some supporting object, it will be 



