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DISTANCE AT WHICH SOUNDS ARE HEARD.

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of a place in reference to latitude, had any effect upon the distance at which a sound may be heard. "These distances," he says, "being so considerable, give me reason to suspect, that sounds fly as far, or nearly as far, in the southern as in the northern parts of the world, notwithstanding we have a few instances of sound reaching farther distances. Also, there is this other reason of suspicion, that the mercury in the barometer riseth higher without than within the tropics, and the more northerly, still the higher, which may increase the strength of sounds." It is, however, we imagine, quite impossible to determine any law of transmission dependent on latitude. In a northern district, the air may be almost habitually so loaded with vapour as to stifle sound, and in a southern so clear that, even when rarified, it shall be easily conducted. The condition of the atmosphere therefore, altogether independent of place, is an important element.

The distance at which a sound may be heard will also depend upon another condition-the degree of divergence produced. The intensity of a sound is soon lost, under ordinary circumstances, by the spreading which it suffers. When a meteor bursts, or a gun is discharged, the sound diverges, and is not heard in any one direction more than another. This spreading of a sound must of necessity diminish its intensity, and in a proportion greater than the distance. The intensity of sound decreases, or so it would appear from calculation, as the square of the distance increases.

Now from these remarks it follows, that by preventing the divergence of the sound, we must increase the intensity at any given distance, and consequently cause it to be heard at

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places which it could not otherwise reach. A common eartrumpet acts upon this principle. It is a tube, with one opening so small as to be easily placed in the ear, and the other is, in comparison, very large. Its object is to make sounds proceeding from short distances audible to deaf persons, and is admirably suited for this purpose. The diverging sounds proceeding from the human voice are gathered together, if we may so speak, by the large opening; and the concentrated effect of the sounds, which are no longer at liberty to spread, is produced upon the ear.

Mr. Curtis, aurist to her Majesty, has recently greatly improved this instrument. The common trumpet is in very many cases found to have an effect by no mean proportional to the amount of hindrance that a person may have to the exercise of the organ of hearing. The instrument then becomes altogether useless. It is also inconvenient and exceedingly troublesome. These objections have, we think, been removed, and for the future Mr. Curtis's improvement will be preferred.

The speaking trumpet is not an instrument of modern invention, but has been from time to time altered and made more convenient. Alexander the Great is said to have had a tube, by the use of which a man might make himself heard at the distance of one hundred stadia. Kircher is in all pro

honour is, by some

Kircher says, in his

bability the modern inventor; but this writers, given to Sir Samuel Morland. "Phonurg," that he invented the tromba twenty-four years before it was described by Morland, and published it in his Misurgia." One of these he had in his chamber in the

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Roman College, and by its means could make himself heard

SOUND PROPAGATED THROUGH TUBES.

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to the porter, and receive answers to any questions he proposed. He also informs us, that he took a trumpet fifteen palms in length to the Mons Eustachianus, where he assembled two thousand persons to prayers by its assistance, some of them coming from a distance of five Italian miles. There is but little doubt that Kircher was the inventor of the tromba; but whether Sir Samuel Morland was acquainted with the invention previous to the publication of his own description, may be fairly doubted.

We might give numerous other instances in which the intensity of sound is preserved by similar means. Speaking tubes are now commonly carried from one apartment to another in large buildings; and with their assistance, persons may communicate with each other at distances to which the human voice could not otherwise reach. A series of pipes might be so arranged through a number of apartments, as to carry orders from one office to another without any personal interview, and at the same time to confine the information to that one place where it is required. The lateral divergence of sound is such, that we may suppose the sides of the pipes to be acted upon by the air; and consequently if an opening be formed in a side, the sound will be heard there as well as at the end. We may then suppose, that in consequence of the condensation of the air during the propagation of sound, the wave has both a lateral and a forward motion. Of this we have many examples in familiar musical instruments.

There seems to be scarcely any limit to the distances at which sounds may be heard, when propagated through tubes. M. Biot states, that the faintest whisper uttered at one end of the Paris conduit-pipes, could be heard distinctly at the

other, a distance of 3,120 feet. We have no doubt that pipes might be so arranged, as to give an almost instantaneous communication between all the government offices, which would save much time, and be in many instances of the greatest importance.

Mr. Curtis, of Soho-square, has invented an acoustic chair, (fig. 6,) for the benefit of the incurable deaf; it has, however,

[graphic]

another purpose,

FIG. 6.

for "by means of additional tubes," says the inventor," the person seated in it may hear distinctly, while sitting perfectly at ease, whatever transpires in any apartment from which the pipes are carried to the chair; being an im

PROPAGATION OF SOUNDS.

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proved application of the principles of the speaking pipes now in general use. The chair is of the size of a large library one, and has a high back, to which are affixed two barrels for sound, so constructed as not to appear unsightly; and at the extremity of each barrel is a perforated plate, which collects sound into a paraboloid vase, from any part of the room. The instrument thus contrived gathers sound, and impresses it more sensibly, by giving to it a small quantity of air. The convex end of the vase serves to reflect the voice, and render it more distinct. Further, the air enclosed in the tube, being also excited by the voice, communicates its action to the ear, which thus receives a stronger impression from the articulated voice, or indeed from any other sound."

From these remarks it will appear, that the distance at which a sound may be heard, will depend on the state of the medium conducting it, and the surface over which it passes, and also on the divergence it suffers. Evidence has been given of the increased intensity produced by the transit of sounds through tubes which prevent the divergence, and consequently retard the decay. There is yet one other fact deserving our attention, before we proceed to another subject.

Although sounds are usually propagated in every direction by an elastic medium, diverging from what may be called the sonorous centre, there are instances in which this does not happen. One example will be sufficient in illustration. Dr. Young has explained an interesting experiment he made with a tuning-fork, and it has been also mentioned by Sir John Herschel. A tuning-fork (fig. 7) is a piece of steel in the form of a pair of sugar-tongs, to which is attached a handle of the same metal. When either of the branches is

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