MONOCHORD. 



first heard by the beasts of the field, because their ears are gene- 

 rally near the ground, and they then by their agitation and 

 alarm give warning to the inhabitants of the approaching cata- 

 strophe. Savage tribes practise this method of ascertaining the 

 approach of persons from a great distance. 



16. The velocity with which sound is transmitted through the 

 air varies with its elasticity ; and where different strata are ren- 

 dered differently elastic by the unequal radiation of heat, the 

 agency of electricity, or other causes, the transmission of sound 

 will be irregular. In passing from stratum to stratum differing 

 in elasticity, the speed with which sound is propagated is not 

 only varied, but the force of the intensity of the undulations is- 

 diminished by the combined effects of reflection and interference, 

 so that the sound, on reaching the ear, after passing through 

 such varying media, is often very much diminished. 



The fact, that distant sounds are more distinctly heard by night 

 than by day, may be in part accounted for by this circumstance, 

 the strata of the atmosphere being during the day exposed to 

 vicissitudes of temperature more varying than during the night. 



17. The solids composing the body of an animal are capable of 

 transmitting the sonorous undulations to the organ of hearing, 

 even though the air surrounding that organ be excluded from 

 communicating with the origin of the sound. 



Chladni showed that two persons stopping their ears could 

 converse with each other by holding the same stick between their 

 teeth, or by resting their teeth upon the same solid. The same 

 effect was produced when the stick was pressed against the breast 

 or the throat, and other parts of the body. 



If a person speak, directing his mouth into a vessel composed 

 of any vibratory substance, such as glass or porcelain, the other 

 stopping his ears, and touching such vessel with a stick held 

 between his teeth, he will hear the words spoken. 



The same effect will take place with vessels composed of metal 

 or wood. 



If two persons hold between their teeth the same thread, stop- 

 ping their ears, they will hear each other speak, provided the 

 thread be stretched tight. 



18. Of the various forms of apparatus which have been con- 

 trived for the production of musical sounds, with a view to the 

 experimental illustration of their theory, that which is best 

 adapted for this purpose, called a monochord or sonometer 

 (fig. 2) consists of a string of catgut or wire attached to a 

 fixed point, carried over a pulley, and stretched by a known 

 weight. Under the string is a hollow box or sounding- 

 board, to the frame of which the pulley is attached. The string 



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