ACOUSTICS. 



tninating an elastic fluid, with the same ve- 

 locii) with which they arrived at that ob. 

 stacle. When the walls of a passage, or 

 of an unfurnished room, are smooth and 

 perfectly parallel, any explosion, or a 

 stamping with the foot, communicates an 

 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- 

 ceeding directly from the sounding body, 

 the intensity of the sound will be much 

 increased by the reflection ; and also in 

 a 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 of its length or breadth tollSOfeet. 



By 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 alternately 

 placed in the room, that sound may be 

 heard louder or weaker, and more or less 

 distinct. Hence it is, that blind persons, 

 who are under the necessity of paying 

 great attention to the perceptions of their 

 sense of hearing, acquire the habit of dis- 

 tinguishing, from the sound even of their 

 own voices, whether a room is empty or 

 furnished ; whether the windows are open 

 or shut ; and sometimes they can even dis- 

 tinguish whether any person 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 vi- 

 brations of the air from one part of the 

 room to the other. The fittest rooms for 

 declamation, or for music, are such 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, 

 of artillery, &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 cer- 

 tain manufactories, or of water-falls, or 

 other noisy places, can hear what is spo- 

 ken to them much better 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 GH, of the same 

 size 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, with 

 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, 

 near the mirror; any one may now whisper 

 into the ear of the image, with the assur- 

 ance of being answered. The deception is 

 managed by giving a signal to the person 

 behind the partition, who, by placing his 

 ear to the focus I ofthe mirror GH, will hear 

 distinctly what the other said, and moving 

 the jaw ofthe statue by the concealed wire 

 will return the answer directly, which 

 will be heard distinctly by the first speaker. 



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

 ris be placed on pedestals, on opposite 

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

 diameter must pass from the ear of one 

 head through the pedestal under the floor, 

 and go 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 ofthe tube, and will be 

 distinctly heard by any one who shall 

 place his ear to the mouth ofthe other. 

 The end ofthe tube which is next the ear 

 of the one head should be considerably 

 larger than that end which comes to the 

 mouth ofthe 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 applying their 

 mouth and ear reciprocally to the mouth 

 and ear of the busts, while other people, 

 standing in the middle of the room, be- 

 tween the heads, will not hear any part of 

 the conversation. 



