This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
18. The transmission of sound is affected materially by the condition of the atmosphere, with respect to temperature, moisture, etc. During the night, when the air is still, and of uniform density and temperature, sounds are heard to a great distance ; but when it is loaded with vapors, as in a fall of snow or rain, sounds are more limited, and rendered confused and indistinct.
19. The density of the air has a great effect upon the transmission of sound. In a dry, cold atmosphere, at the level of the sea, sounds are transmitted to vast distances, while on high mountains, such as the Andes, or even Mont Blanc, the report of a pistol is not louder than that of an Indian cracker. The wind, also, has a great influence in aiding or retarding the transmission of sound. Sound is also reflected like light, and indeed is subject to the same laws, for the angle of reflection is always equal to the angle of incidence. Reflected sound is termed an echo. The rolling of thunder is supposed to depend partly on the sound being reflected from cloud to cloud, and through strata of air of different densities, though it also arises from a discharge of electricity, through a wide extent of air. In this case, as the sound from the point nearest the hearer reaches his ear first, and some moments elapse before that from the more distant arrives, there must consequently be a continued peal. Some of our large public houses and manufactories are fitted with pipes for conveying intelligence to distant apartments, attention being attracted by ringing a bell.
20. It has been doubted whether sound can be propagated from one medium to another, as from air to water. It is now known, however, that if a musket is discharged over a person who is under water, he will hear the report. The question has also arisen, whether sound can be propagated from water to the air again. This is easily proved by striking two stones together under water, although we are told that persons in a diving bell under water, could not hear a musket discharged immediately over it. But here the sound had to be communicated from the air to the water, and from the water to the air again. Sound travels at the rate of eleven hundred and forty two feet in a second, or a mile in four seconds. As light travels much faster, we see the flash of a gun before we hear the report. This will enable us to tell in a thunder storm how far we are from a thunder cloud ; as we have only to allow eleven hundred feet for each second, between the time when the flash is seen and the report heard, and one beat of the pulse for a second. In this way, too, the distance of a ship of war at sea is often ascertained by those on board of the vessel she is in pursuit of. Solids and fluids convey sound not only more perfectly, but also more rapidly than air. It is found that the velocity of sound in water, is about four thousand nine hundred feet in a second, being between four and five times more rapid than it is through air. Sound passes through tin at the rate of eight thousand one hundred and seventy five feet, and through iron, glass and wood, eighteen thousand five hundred and thirty feet in a second. This explains why, when a gun is fired at a distance over the surface of a frozen lake, we hear two reports after we see the flash ; first a sharp and loud one, transmitted by the solid ice, and then a weaker and duller one, through the air. Franklin, however, found by his experiments, that, after travelling about a mile through the water, sound lost some of its intensity, which indeed might be expected. Musical tones are said to be acute when the intervals between the vibrations are short, and grave when they are long. Thus a flute called an octave produces a shriller sound than the common flute ; a fiddle than a bass viol. The strings, too, of a violin, which are designed for high or acute notes, are smaller than the others, that their vibrations may be more rapid; while those which make the grave tones are large, and wound round sometimes with fine wire, to increase the weight, and make them vibrate more slowly.. It is the quality and variety of the sounds which, in musical tones, gives the hearer so much pleasure.
21. In the lower order of animals, hearing is performed by means of an apparatus, much more simple than in man. Some of them have merely a membranous sac, supplied with nervous threads. This is even the case in fishes, which require neither tympanum nor hones, nor any of the accessory parts found in land animals, as the undulations of water strike with greater force upon the organ of hearing than those of air. The apparatus of hearing in the frog is very singular, and designed so as to enable the animal to hear both in air and water.
22. Animals with long ears are able to move them by muscles for that purpose, and turn them to the point whence the sound proceeds. This may be seen in the horse, which turns his ear always in the direction of the sound. In stage horses, we often see the leaders turn their ears forward, while those behind turn theirs backward. Some men also have the power of moving their ears.
23. Like all our other senses, that of hearing is capable of much improvement by cultivation. The Indian in the forest, accustomed to listen to the approach of his enemies, or of his prey, acquires such acuteness of this sense, as to hear sounds which would be inaudible to those who live amid the din of civilized life. The blind also excel in the acuteness of hearing, and for this reason especially, acquire great skill in performing on musical instruments. Shakspeare thus describes a person destitute of musical taste.
" The man that hath no music in his soul, Nor is not moved with concord of sweet sounds, Is fit for treasons, stratagems, and spoils ; The motions of his spirit are dull as night, And his affections dark as Erebus ; Let no such man be trusted."
What are sounds ? Describe the external ear ;-the tube of the external ear ;-the membrana tympani;-the drum, or tympanum ;-the eustachian tube. How many little bones of the ear are there ? Their names and office ? How are low sounds perceived ? What is the use of the spiral passages ? How is sound conveyed ? Is it conveyed by solids ;-by water ? What is the stethoscope ? Can sound be propagated from one medium to another ? At what rate does it travel ? How can we tell the distance of sound ? What effect has the density of air on sound ? How is the lengthened peal of thunder explained ? How is hearing performed in the lower order of animals? Can hearing be improved by cultivation ?