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 bones, 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."

Questions. What are sounds? Describe the external ear;-the tube of the external ear ;-the membrana tympani ;-the drum, or tym. panum ;-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 ?

1

CHAPTER XVIII.

RESPIRATION.

1. By Respiration, is meant the process of taking air into the lungs and throwing it out again.) (Inspiration is the act of drawing the air in); (expiration, that of forcing it out, Respiration is essential to all animal existence. It is in the lungs that the last change is produced in the assimilation of the food, by which it is converted into that vital fluid, which carries life, and strength, and nourishment to every fibre in the animal system.

2. (The parts concerned in respiration may be arranged into three divisions, viz., 1, the bones which form the respiratory cavity; 2, the muscles by which these bones are moved,

Thorax, or chest; a, the sternum; b, b, the spine; c, c, the ribs,

and the size of the cavity regulated; 3, the respiratory organs contained within the cavity,

3. The bones which enter into the composition of the chest, are the sternum, or breast-bone; twelve dorsal vertebrae, and twenty four ribs) By examining the preceding plate, Fig, 1., the sternum will be seen to terminate at its lower part in a triangular piece of cartilage, which lies directly over the stomach, and may be felt externally ;the seven uppermost, or true ribs, will be seen to extend the whole distance from the spine to the sternum, strips of elastic cartilage being interposed between the breast-bone and their anterior extremities, while the five lower, or false ribs, are merely attached to each other by slips of cartilage; the two lower, indeed, float loosely, without any attachment to the others, except by means of muscles.

4. I have already stated that the ribs are attached to the spine at an acute angle, so that they cannot be moved out of their ordinary position, without enlarging the dimensions of the chest. The articulation of the ribs, both at the spine and sternum, is effected by means of cartilage, thus allowing sufficient degree of motion for the purposes of respiration in a healthy state. The chest expands during inspiration for the reception of air; and during expiration, it contracts to expel air, which is no longer useful. There are, therefore, two motions required, an upward and an outward; the first increases the distance between the spine and sternum; the last, that between the ribs. Now these motions are both effected by the ribs, and they are so articulated, that they cannot perform the one without the other. By looking at the cut, then, it will readily be seen how where the ribs rise, the sternum will be pushed out, and of course the cavity of the chest enlarged.

5. The Muscles.-All the muscles which are attached to the bones just described, aid more or less in respiration, though the intercostal muscles are those chiefly concerned These run from one rib to another, filling up the spaces be