THOUGHTS ON ELEMENTARY PRINCIPLES OF NATURAL

PHILOSOPHY.

SELECTION ON ACOUSTICS.

ACOUSTICS, or the science which treats of the nature and laws of sound, has considerable claims upon our attention.

In an open space, and through a serene atmosphere, sound is generally propagated in all directions, from the sonorous body. Sounds, even the most powerful, when thus transmitted freely through the air, diminish rapidly in force, as they depart from their sources, and, within moderate distances, wholly die away. What law this diminution follows is not yet ascertained'; and is indeed, in the present state of acoustics, incapable of determination. We can at present estimate the force of sounds by the ear alone; an instrument of comparison, whose decisions on this point vary with the bodily state of the observer, and whose scale expresses no definite relation but that of quality.

Though sound has in general, at its origin, a tendency to diffuse itself in all directions, it is sometimes propagated more forcibly in one direction than in others. A cannon seems much louder to those who stand immediately before it, than to those who are placed behind it. The same fact is illustrated by the speaking trumpet; the person towards whom the instrument is directed hears distinctly the words spoken through it, while those who are situated a little to one side, hardly perceive any sound.

Sound is, in a great measure, intercepted by the intervention of any solid obstacle between the hearer and the sonorous body. Thus, if, whilst a bell is sounding, houses intervene between us and the bell, we hear it sound but faintly, compared to what we do after having turned the corner of the building. From this fact, sound would seem to be chiefly propagated in straight lines. If, however, we speak through a tube, the voice will be wholly confined by the tube, and follow its windings, however tortuous; yet we are not from this to infer, that

air is the only medium through which sound can be transmitted. Various other bodies, both solid and fluid, are excellent conductors of sound.

If a log of wood is scratched at one extremity with a pin, a person who applies his ear to the other end will hear the sound distinctly. The trampling of horses, too, is heard at a much greater distance, by applying the ear to the ground, than by listening to the sound conveyed through the atmosphere. Several other instanecs of sound being transmitted through solids, will be men tioned before the conclusion of this article. Liquids are, in like manner, good conductors of sound. Mr. W. Arderon caused three persons to dive under water about the depth of two feet; in that situation they heard distinctly what he spoke to them. We are informed by professor Robinson, that, by plunging his head under water, he heard at the distance of 1200 feet, the sound of a bell which was rung in the same medium. cause then why sound is interrupted by the interposition of solid bodies, is not that the body so interposed is incapable of conducting sound, but that sound does not readily pass from one medium to another.

The

When a piece of ordnance is fired at the distance of some miles, a certain number of seconds intervene be tween seeing the flash and hearing the report; the sound of the cannon has therefore occupied this number of seconds in travelling, through that distance. Philosophers have endeavoured by an attentive observation of such facts, to determine the rate at which sound trav els through the air. The mean result of these observations gives to all sounds, whether loud or weak, a ve locity of about 1130 feet in a second of time. This velocity remains unchanged by those variations in the density of the atmosphere which the barometer indicates, but is increased by an elevation of temperature. The time which clapses during the transmission of sound to any moderate distance through solid bodies, is so minute as not to have been ascertained by any direct experi

ments.

The facts which might be adduced to prove that an impulse is communicated through the air by sounding bodies, are innumerable. The agitation which the explosions of artillery produce in doors and windows is fa

miliar to the most inattentive observer. Any person who has listened to the sound of an organ, must have perceived, that when the instrument was giving its deeper tones, the seat on which he was placed vibrated. The same fact is illustrated with respect to weaker sounds, by the discovery of Dr. Wallis, that if near a stretched musical string we sound on any instrument a note in accord with that of the string, the string will vibrate and give the same sound.

If, when artillery is fired at a distance, the particles of dust which float in a sun-beam are inspected with a microscope, they will be seen agitated at the same instant that we hear the explosion; and we are informed by a literary gentleman, that he has observed similar agitations in cobwebs. The impulse by which these bodies are affected, must therefore be propagated through the air with a velocity exactly equal to that of sound. The same coincidence cannot be exactly traced in solids, as these bodies transmit both sound and an impulse with such celerity, that the time which elapses during the transmission of either has not yet been estimated. In order to complete this analogy, it is only required that we should be able to prevent the transmission of an im pulse without changing the physical properties of the medium. This we can do in one case only: Let a person draw a string tight over his ears, and let another stretch the string, and suspend a watch to the remote end of it, the sound of the watch will be heard by the person over whose ears the string passes; if now the string be gradually relaxed, the sound becomes by degrees weaker, and ultimately imperceptible.

From these analogies we must conclude, that the propagation of sound depends entirely on the transmission of an impulse which sounding bodies communicate to the surrounding medium.

In reviewing the whole process which takes place in the production of sound by a sonorous body, till it makes an impression on the organ of hearing, we perceive its several steps depending solely on some motion accompanying each of them, and essential to it. From all this the inference is irresistible, that sound is merely a motion originating in the sonorous body, and communicated from it through the intervening medium, to the interior

parts of the ear; and that hearing is merely a sensation produced by the consequent affection of this organ. We have thus endeavoured, by a regular induction from the phenomena of sound, to discover its nature; a mode of procedure which is not only the most legitimate, but also the best fitted to give a distinct view of the present state of Acoustics, and of the evidence on which its theory is established."

REFLECTIONS ON THE FOREGOING.

In view of the theory of sounds and the manner in which the vibrations of the air act upon the tympanum, or drum of the ear, we may well admire the wisdom of Him who adapted the organs of hearing to impulses which should reach them. Yet after the most critical examination of the media which contribute to the sense of hearing, what is it that hears? Not that transparent membrane which receives the impulses of air, and, vibrates with them. This no more hears than the parchment in a drum head. Not the cavities of the head near it, which but echo the vibrations, There is an invisible and indescribable something, somewhere within, to which the consciousness of hearing belongs. Yet this spirit in man which goeth upward, is not, by the senses, distinguishable from the spirit of the beast, which goeth downward. How the disembodied spirit will receive impressions like those of hearing we know not. No doubt its sources of pleasure or of pain from something like the sense of hearing, will be incalculably increased. While in the world most persons are susceptible of pleasure from the harmony of musical sounds; with some it is otherwise. To immense numbers of intelligent moral agents in another state of existence, the revolutions of numberless systems of material worlds, may be a harmonious concert, imparting raptures as much more intense than any earthly concert, as the objects are more numerous and vast. On the other hand to some conscious spirits untuned by sin," the music of the spheres," may seem horribly discordant. From circumstances so awful, let us all pray and strive, for deliverance. Let us seek to understand, and relish the harmony of the Divine diş

pensations here, and we shall inherit the harmony of heaven hereafter.

The foregoing article was inserted in this number of The MONITOR, partly to introduce to our readers with more effect, the following very interesting and pious letter from Mrs. P. of R.- to Miss B.- of Boston. The writer is the widow of a distinguished clergyman and the mother of one whom God has honoured as an eminent instrument of building up Christ's kingdom in the world.

DEAR MISS

ALTHOUGH YOU are almost a total stranger to me, as I have had but only one short interview with you, yet as I trust you are a child of God and participating with me in the same privations and trials, I cannot forget you, and the sympathetic feelings of my heart constrain me to attempt to communicate something for your relief under the grievous malady with which you are oppressed. I have fully experienced the infelicity of deafness, and trust learned something of the benefit which may be derived from it. My hearing was injured when a child, but in a small degree. It was not noticed by my par. ticular acquaintance until I was more than twenty years of age; then, as I found the malady increasing, I was induced to use means to prevent it which proved very unfavourable and injurious to my hearing. But before this I had suffered much from anticipation of the threatened evil; my proud spirit recoiled at the idea of deafness, the mention of the word would thrill through my frame, and I termed the ringing in my ears the funeral dirge of all future enjoyment in life. It was indeed the principal means of weakening my attachment to the world, and constraining me to turn my attention to relig ion. Surely then it was in this instance productive of good. Since I have experienced the reality of deafness I have found it to be exceeding grievous, and much has it drawn into exercise the proud rebellious spirit within me. It appeared to be an evil from whence no possible good could be derived. Yet, after all my perverse mur