and Propagation of Sound. 33 



familiar phffinomena; of these, two deserve notice as requiriijtr 

 further explanation. 



15. The echoes of a large empty room are often of a lower 

 pitch than the original sound; a whistle, for instance, although 

 answered by a similar note, will also excite a number of 

 echoes that are obviously lower. As this depth of echo is in 

 proportion to the size of the room (14.), there is reason to be- 

 lieve that it arises from the various stray pidses falling into 

 vibrations corresponding with their successive periods of re- 

 flection. Every room has its proper pitch of echo; and this 

 depends on the relation of the prevailing diameter of the room 

 to the velocity of sound through air. Thus, in a room 20 feet 

 square, or, better, a circular one 20 feet in diameter, the prin- 

 cipal echo of the room, besides the simple one, would consist 

 of about 56 vibrations in a second. 



16. Little reflecting cavities of a few inches' diameter must 

 necessarily be still higher in their echoes (14.), and it is these 

 echoes that give the tinkling sound to many hollow bodies 

 when struck. The remarkable reverberation of empty barrels 

 is a coarse instance; no sound of lower pitch will be reechoed 

 within them. If the mouth of a glass or earthenware bottle 

 be applied to the ear, and then tapped on the outside, each 

 stroke will appear tinkling ; and that this proceeds from the 

 internal echo, and not from the material, is plain from the 

 fact, that muffling the vessel in any v/ay by the hand, or by a 

 cloth, does not stop the tinkle of the interior, although heard 

 externally the stroke is a mere dead tap. The peculiar ring- 

 ing sound which accompanies blowing or whistling into a 

 bottle is referrible to the same cause, and is quite distinct from 

 the longitudinal vibration of the whole column of air excited 

 by blowing laterally as with a Pan-pipe. Spherical cavities 

 give the longest and most uniform echo, for the obvious rea- 

 son that the reflections are nearly of the same length (14.).* 



17. Very distant notes, such as street cries or the sound 



* I was led to these facts in seekinp; the cause of a similar phaenomenon 

 which occurs in the human body, and is an important sign of disease. In 

 pneumatoothorax, where a cavity is formed by air getting between the lungs 

 and the walls of the chest, coughing or speaking is often attended by a 

 tinkling echo, called by M.Laennec tintement metallique. I formerly rank- 

 ed as of this kind the tinkling sound which occurs in the ear itself when- 

 ever it is closed, and the hand, or whatever is used to close it, is tapped : 

 each stroke sounds like the clink of a piece of metal. But a? I find that 

 diminishing the cavity by introducing a small solid object far inio the ear 

 does not raise the tone, whereas forcing air through the Eustachian tube 

 so as to press on the tympanum destroys the sound, I am inclined to 

 think that the clink in question is produced by the vibration of the tym- 

 panum itself: accordingly I have found that in several who are slightly deaf 

 in one ear, this note of the tympanum is higher in the ear that hears best. 



Third Series, Vol. 6. No. 31. Jan. 1835. F 



