ACOUSTICS. 



jiiinatingftn clastic fluid, with the same ve- 

 locity with which thev arrived at that ol). 

 When the walls of a passage, or 

 of an unfurnished room, arc smooth, and 

 pc -rtecth parallel, any explosion, or a 

 stamping 1 with the foot, coniinunir-. 

 impression to the air, which is reflected 

 from one wall to the other, and from the 

 second again towards the ear, nearly in 

 the same direction with the primitive im- 

 pulse : this takes place as frequently in 

 a second, as double the breadth of the pas- 

 sage is contained in 1130 feet; and the 

 ear receives a perception of a musical 

 sound, thus determining its pitch by the 

 breadth of the passage. On making 

 the experiment, the- result will be found 

 accurately to agree with this explanation. 

 If the sound is predetermined, and the fre- 

 quency of vibrations such, as that each 

 pulse, when doubly reflected, may coin- 

 cide with the subsequent impulse, pro- 

 pceding directly from the sounding body, 

 the intensity of the sound will be much 

 m< Teased by the reflection; and also, in 

 H less degree, if the reflected pulse coin- 

 cides with the next but one, the next but 

 two, or more of the direct pulses. The ap- 

 propriate notes of a room may readily be 

 discovered by singing the scale in it; and 

 they will be found to depend on the pro- 

 portion ofitslengthorbreadth to 1130feet. 



B\ altering our situation in a room, and 

 expressing a sound, or hearing the sound 

 of another person, in different situations, 

 or when different objects are alternate'/ 

 placed in the room, that sound may be 

 heard louder or weaker, and more ->r less 

 distinct. Hence it is, that blind persons, 

 who are under the necessity- of paying 

 great attention to the perceptions of their 

 seiiM- of hearing, acquire i he habit of dis- 

 tinguishing, from the .sound even of their 

 own voices, whether a room is empty or 

 furnished ; whether the windows are open 

 er shut : and sometimes they can even dis- 

 tinguish whether ar.\ pel-sou be in the 

 room or not A great deal of furniture in 

 a room checks, in a great measure, the 

 sounds that are produced in it, for they 

 hinder the free communication of the \ i- 

 brations of the air from one part of the 

 room to the other. The fittest rooms for 

 declamation, or tor m" nch as 



contain few ornaments that obstruct the 

 sound, and at the same time have the least 

 echo possible. 



A strong and continued sound fatigues 

 the ear. The strokes of heavy hammers, 

 *f artillen, &c. are apt to make people 

 deaf for a time : and it has been known 

 that persons who have been long exposed 



to the continued and confused noise of cef- 



tain manufactories, or of water-falls <>; 

 other noisy places, can hear what >s spo- 

 ken to them much beMer in the midst of 

 that noise than elsewhere. 



We shall conclude this article with an 

 experiment or two, for the amusement of 

 the younger part of our readers. 



Experiment 1. Place a concave mir. 

 ror, AB, fig. 6, of two feet in diameter, in 

 a perpendicular direction, and at the dis- 

 tance of about five or six feet from a par- 

 tition EF, in which there is an opening 

 equal in size to the mirror ; against this 

 opening must be placed a picture, painted 

 in water-colours, on a thin cloth, that the 

 sound may easily pass through it. Be- 

 hind the partition, at the distance of a few 

 feet, place another mirror (ill, of the same. 

 si/.e as the former, and directly opposite 

 to it. At the point C is to be placed the 

 figure of a man, seated on a pedestal, u ith 

 his ear exactly in the focus of the first 

 mirror; his lower jaw must be made to 

 open by a wire, and shut by a spring. 

 The wire must pass through the figure, 

 and under the floor, to come up behind 

 the partition. Let a person, properly in- 

 structed, be placed behind the partition, 

 nearthe mim>r; any OIK- ma\ now wh'.sjie-,- 

 into the ear of the image, with the assur- 

 ance of beinganswered. The deception Is 

 managed by giving a signal to the person 

 behind the partition, who, by placing his 

 ear to the focus I of the mirror Gil, will hear 

 distinctly what the other said, and moving 1 

 the jaw of the statue by the concealed wire, 

 will return the answer directly, which 

 will be heard distinctly by thefirst speaker. 



Ex.2. Let two heads of plaster f Pa- 

 ris be placed on pedestals, on opposite 

 sides of a room. A tin tube of an inch in 

 diameter must pass from the car of one 

 headthrough the pedestal underthe floor, 

 and gx> up to the mouth of the other. 

 When a person speaks low into the ear 

 of one bust, the sound is reverberated- 

 through the length of the tube, and will In- 

 distinctly heard by any one who shall 

 place his ear to the mouth of the other. 

 The end of the tube \\ hicli is next the ear 

 of the one head should be considerably 

 larger tlian that end which comes to the 

 mouth of the other. If there be two tubes, 

 one going to the ear, and the other to the 

 mouth of each head, two persons may 

 converse together, by appKing their 

 mouth and ear reciprocally to the month 

 and ear of the busts, while other people, 

 standing in the middle of the room, l>c- 

 t\\ ecu the heads, will not hew any part of 



