Fig. 2. A aiONOCHOKi), OR SONOMETER. 



SOUND. 



1. Definition. 2. Air the conductor. 3. Sound progressive. 4. Breadth 

 of sonorous waves. 5. Musical sounds, how characterised. 6. Pitch. 

 7. Loudness. 8. Timbre. 9, 10. Velocity. 11. Distances measured 

 by sound. 12. Density of air affects loudness. 13. Effect of atmo- 

 spheric agitation. 14. Sound conveyed through liquids. 15. Solids. 

 16. Effects of Elasticity of air. 17. Chladni's experiments. 18. 

 Monochoi-d. 19. Vibration of musical strings. 20. Cause of har- 

 mony shown. 



1. SOTJJTD is the sensation produced in the organs of hearing 

 when they are affected by undulations transmitted to them 

 through the atmosphere. These undulations are subject to an 

 infinite variety of physical conditions, and each variety is followed 

 by a different sensation. 



2. That the presence of air or other conducting medium is 

 indispensable for the production of sound, is proved by the fol- 

 lowing experiment. 



Let a small apparatus (fig. 1, p. 186) called an alarum, con- 

 sisting of a bell a, which is struck by a hammer 6, moved by 

 clockwork, be placed under the receiver of an air-pump, through 

 the top of which a rod slides, air-tight, the end of the rod being 

 connected with a detent which governs the motion of the clock- 

 work connected with the hammer. This rod can, by a handle 

 placed putside the receiver, be made to disengage the detent, so 

 as to make the bell ring whenever it is desired. 



185 



