move them, and to adjust the whole system to the several purposes of hearing, to hear loud or soft, discordant or agreeable sounds. The passage just behind the drum of the ear is called the vestibulum, being as it were, the anti-chamber or entrance to two other cavities. The first of these is called the labyrinth, and consists of certain semi-circular canals; and the second is called cochlea, from its resemblance to a snail shell. The labyrinth appears to have something of the mechanism and uses of a hearing trumpet; while the cochlea appears to be destined for the more delicate and refined uses of hearing, such as the forming and modulating of musical and harmonious sounds. There is one particular contrivance of the nerves ministering to this sense, that I must not pass over; which is, that the branches of one of the auditory nerves spread partly to the muscles of the ear, partly to the eye, partly to the tongue and instruments of speech, and inosculated with the nerves that go to the head and breast. By these means there is an admirable and useful consent between the parts of the body; it being natural for most animals upon hearing any uncouth noise, to erect their ears, and prepare them to catch every sound; to open their eyes, those constant, faithful sentinels, to stand upon the watch: and to be ready with the mouth to cry out and utter what the present exigency may suggest. And this is accordingly usual for most animals, when surprised or terrified by any frightful noise. Such is the nature and properties of sound, and such the admirable structure and uses of the ear. If it be asked why, when a word is pronounced it excites a certain idea, and not a simple tone? or how a sound can act upon the soul, and produce so many different notions? we are obliged, in this respect, to confess our absolute ignorance. We know enough, however, to be assured that the ear is one of the most convincing proofs in nature of the divine wisdom and goodness; and well may we exclaim with the devout David, Such knowledge is too wonderful for me: it is high and I cannot attain unto it. But I will praise Thee, O Lord, for I am fearfully and wonderfully made." 66 ACOUSTICS. ANALYSIS OF THE SCIENCE. 1. Acoustics is a science which instructs us in the nature of sound. It is divided into diacoustics, which explains the properties of those sounds that come directly from the sonorous body to the ear; and catacoustics which treats of reflected sounds. 2. Most sounds that affect us are conveyed to the ear by means of the air: but water is a good conductor of sound so also are timber and flannel. EXPERIMENTS. (1.) A bell rung under water returns a tone as distinct as if rung in air. (2.) If you stop one ear with a finger, and the other by pressing it close to a long stick or piece of deal board, and a watch be held at the other end of the wood, the ticking will be heard, be the stick or board ever so long. (3.) If you tie a poker or any piece of metal on to the middle of a strip of flannel about two or three feet long, and then press with the thumbs or fingers the ends of the flannel into your ears, while you swing the poker against an iron or steel fender, you will hear a sound like that of a heavy church bell. 3. A body while sounding is in a state of vibration, which it communicates to the surrounding air, the undulations affect the ear, and excite in us the sense of sound. 4. Sound travels at the rate of 1142 feet in a second, or about 13 miles in a minute. This is the case with all kinds of sounds: the softest whisper flies as fast as the loudest thunder. 5. The velocity of sound has been applied to the measurement of distances. EXPERIMENTS. (1.) A ship at sea in distress fires a gun, the light of which is seen on shore 20 seconds before the report is heard, therefore it is known to be at the distance of 20 times 1142 feet, or little more than 4 miles. (2.) I see a vivid flash of lightning, and in three seconds I hear a tremendous clap of thunder, I instantly know the thunder cloud is only about two-thirds of a mile distant, I should therefore retire instantly from any exposed situation. (3.) The pulse of a healthy person beats about 76 times in a minute, if therefore between the flash of lightning and the thunder I can feel 1, 2, 3, 4, &c. beats of my pulse, I know the cloud is 900, 1,800, 2,700, 3,600 feet from me. 6. Sound, like light, after it has been reflected from several places, may be collected into one point as a focus, where it will be more audible than in any other part; and on this principle whispering galleries are constructed. 7. Speaking trumpets and those made to assist the hearing of deaf persons, depend on the reflection of sound from the sides of the trumpet, and also by its being confined and prevented from spreading in every direction. 8. A speaking trumpet, to have its full effect, must be directed in a line towards the hearer: the report of a gun or cannon is much louder when fired towards a person, than one placed in a contrary direction. 9. An echo is the reflection of sound striking against a surface fitted for the purpose, as the side of a house, a brick wall, hill, &c. EXPERIMENTS. (1.) If a person stand about 65 or 70 feet from such a surface, and perpendicular to it, and speak, the sound will strike against the wall and be reflected back, so that he will hear it as it goes to the wall, and again on its return. (2.) If a bell situated in the same way be struck, and an observer stand between the bell and the reflecting surface, he will hear the sound going to the wall, and also on its return. (3.) If the sound strike the wall obliquely, it will go off obliquely, so that a person who stands in a direct line between the bell and the wall will not hear the echo. 10. The variety of sounds depends on the frequency of the vibration, on the quantity and force of the vibrating particles, and on the simplicity of the sounds; hence are derived the height, strength, and quality of sounds. 11. Music is any succession of sounds, that excites in a well disciplined ear certain agreeable sensations. Sound is therefore the subject matter of music, and it is harmony that makes sounds musical. 12. Harmony is the coincidence of two or more sounds, which by their union afford to the mind pleasure and delight. 13. Two strings of equal length, tension, and thickness, by performing their vibrations together, will sound the same note, or be in unison. Two pipes of the same length and diameter will agree in the same manner. Explanation. In the case of the strings, the air is struck by the body, and the sound is excited by the vibrations: in that of the pipes the body is struck by the air, but as action and re-action are equal, the effect is the same. 14. Large instruments and long strings produce grave or deep tones: small instruments and short strings produce acute and high tones. Examples. Let a musical string of any length be divided into two equal parts by a bridge in the middle: and the sound of each half is eight notes, or an octave, higher than the tone of the whole string.-Organ pipes produce grave or acute tones in proportion to their length and size. It is the shortest string of a harpsichord which yields the highest notes. QUESTIONS FOR EXAMINATION. 1. What is meant by the science of acoustics, and how is it divided? 2. How are sounds conveyed to the ear? Give an account of the experiments. 3. How is the sense of sound excited? 4. At what rate does sound travel? 5. To what purpose has the velocity of sound been applied? Give an account of the experiments. 6. Can sound be collected in a focus? 7. On what principles do speaking trumpets depend? 8. How must a speaking trumpet be situated to produce the full effect? 9. What is an echo? Give an account of the experiments. 10. On what does the variety of sound depend? 11. What is meant by music? 12. What is harmony? 13. When will two strings and two pipes be in unison? Give the explanation. 14. Which instrument produces deep, and which acute tones? Produce the examples. OPTICS. ON VISION. "With thought from prepossession free, reflect On solar rays, as they the light respect. In vain the Author had the eye prepar'd With so much skill, had not the light appear'd." Blackmore. If the construction of the universe were not so evident a proof of the existence of a supremely wise and benevolent Creator, as to render particular arguments unnecessary, the structure of the eye might be offered as one, by no means the least. This instance, amongst numberless others, demonstrating that the most exquisite performances of art, are infinitely short of those which are continually produced by the DIVINE MECHANIC. Of all the senses the sight is that which furnishes the soul with the quickest, most extensive, and most varied perceptions. It is the fertile source of the richest treasures of imagination, and it is to that principally, that the soul owes the ideas of beauty, and of that varied unity that enchants it. "Our sight," indeed, as observed by an admirable writer, "is the most perfect and delightful of all our senses. It fills the mind with the largest variety of ideas, converses with its objects at the greatest distance, and continues the longest in action, without being tired or satiated with its proper enjoyments. The sense of feeling can indeed give us a notion of extension, shape, and all other ideas that can enter at the |