SENSE OF HEAEING. 885 



a string of less tension be thrown into vibration near one of higher 

 tension there will be no response, for no membrane or string can recip- 

 rocate a note lower than its own fundamental note. By the term funda- 

 mental note we mean the lowest note which any string or membrane can 

 produce. If a higher note be sounded alongside of a string of low 

 tension the latter, as before stated, will divide itself, and the divisions 

 will be separated by points of rest, called nodal points, while the 

 intermediate parts of the string in vibration are termed loops. 



These remarks are true not onty of strings, but also of membranes. 

 If some sand be sprinkled on a membrane stretched across a drum-head, 

 so as to be capable of a musical tone, and then another note of precisely 

 the same pitch be struck near it, the sand will begin to dance on the sur- 

 face of the drum-head, thrown into vibrations of reciprocation, and will 

 accumulate on the lines of rest — the nodal lines ; but if we sound near 

 it a note higher than that of the membrane stretched across the drum, 

 then the membrane, instead of vibrating across its whole surface, will 

 divide itself, and the sand will collect in dark lines on the nodal points 

 as before. If we sound near the membrane a note lower than its 

 fundamental note, it will not respond. 



A column of air may be thrown into vibrations of reciprocation by 

 sounding a note in proximity to it. 



Sounds are also propagated by resonance. If an instrument capable 

 of producing a musical note while vibrating be placed in contact with a 

 medium whose molecules are capable of being thrown into vibration, the 

 second substance will vibrate and increase the intensity of the original 

 sound, even if the medium with which we place the sounding instrument 

 in contact is not itself capable of producing musical tones. Thus, when 

 we strike a tuning-fork under ordinary circumstances in the air the sound 

 is but faintly heard, while if we place the fork in contact with a piece 

 of wood, the sound is greatly increased in intensity. The woody fibres 

 are thrown into vibrations which reciprocate the original sound. 



Sound is also propagated by conduction, and this occurs when any 

 sonorous body during its vibrations is brought in contact with any 

 medium capable of being thrown into vibration. A familiar example of 

 this is found in the fact that while the tuning-fork held in the air is but 

 indistinctly heard, if placed in contact with the bones of the head it is 

 heard distinctly. In this case the sound is conducted from the tuning- 

 fork to the nerve of hearing through the bones of the head. All media 

 do not conduct sounds with the same degree of rapidity. Solids conduct 

 better than fluids, fluids better than gases, while in a vacuum no sound 

 whatever can be conducted. 



That a sound may be appreciated, it is evident that it must be con- 

 ducted to the terminal filaments of the auditory nerve in the labyrinth, 



