210 ANNUAL OP SCIENTIFIC DISCOVERY. 



Henry, who fully corroborated the principles laid down by Captain Meigs, 

 and approved of their judicious application. The above gentlemen visited 

 many of the most important buildings in Philadelphia, New York, and 

 Boston, for the purpose of examining them in reference to their acoustic 

 qualities, which examination completely established the accuracy of the 

 observations of Captain Meigs. 



Experience shows that the human voice, under favorable circumstances, 

 is capable of filling a larger space than was ever, probably, enclosed within 

 the walls of a single room. Ilerschel, in his admirable treatise on sound, 

 gives a few instances which are instructive. Lieutenant Foster, on Parry's 

 third Arctic expedition, found that he could converse with a man across 

 the harbor of Port Bowen," a distance of 6,G96 feet, or about one and a 

 quarter miles. Dr. Young records that, at Gibraltar, the human voice has 

 been heard at a distance of ten miles. If sound be prevented from spread- 

 ing and losing itself in the air, either by a pipe or an extensive flat surface, 

 as a wall or still water, it may be conveyed to a great distance. Biot heard 

 a flute clearly through a tube of cast iron (the water pipes of Paris) 3,120 

 feet long. The lowest whisper was distinctly heard. In fact, the only 

 way not to be heard was, not to speak at all. 



The favorable circumstances seem to be a perfectly tranquil and uni- 

 formly dense atmosphere, absence of all extraneous sounds, absence of 

 echoes and reverberation, vicinity of reflecting surfaces, and perhaps, in 

 some measure, the presence of substances which conduct sound well. A 

 pure atmosphere, being favorable to the speaker's health and strength, will 

 give him greater power of voice and more endurance ; thus indirectly 

 improving the hearing by strengthening the source of sound, and also by 

 enabling the hearer to give his attention for a longer period un fatigued. 



The effect of echoes in a small room is generally unnoticed ; the echo 

 returns so quickly that the ear receives it as coincident with the original 

 sound, to which it in. that case merely adds strength, perhaps prolonging 

 it very slightly. If the room be larger, and the echoing wall so distant 

 that the interval is sensible, the echo makes confusion. If, on a calm day, 

 we advance towards a wall, producing, at each step, some sound, we wiJl 

 find a point at which the echo ceases to be distinguishable from the origi- 

 nal sound. The distance from the wall, or the corresponding interval of 

 time, has been called by Professor Henry the limit of perceptibility. This 

 limit will vary with the nature of the sound ; if the sound be sharp and 

 distinct, as that produced by striking some hard substance, we shall find 

 the limit of perceptibility less than for the more prolonged sound produced 

 by the voice. The limit will probably vary also with the acuteness of the 

 ear, some persons being probably able to separate sounds undistinguish- 

 able to others. The general limit is probably about thirty or thirty-five 

 feet. It should be ascertained exactly ; and in constructing a room for 

 public speaking, the height of the ceiling should not exceed this limit. 

 The sound will then be strengthened to the speaker himself by the echo. 

 The interval between the original and reflected sounds will be shorter for 



