The string of a violin or piano, when pulled to one side by its middle, and let go, will vibrate backwards and forwards, like a pendulum, and striking rapidly against the air produces tones, which are grave, or acute, according to its tension, size, or length,

The manner in which such a string vibrates, is shown by fig. 116.

Fig. 116.
C

d

If pulled from e to a, it will not stop again at e, but in pas.

sing from a to e, it will gain a momentum, which will car. ry it to c, and in re

turning, its momentum will again carry it to d, and so on, back. wards and forwards, like a pendulum, until its tension, and the resistance of the air, will finally bring it to rest.

The grave, or sharp tones of the same string, depend on its different degrees of tension; hence if a string be struck, and while vibrating, its tension be increased, its tone will be changed from a lower to a higher pitch.

Strings of the same length are made to vibrate slow, or quick, and consequently to produce a variety of sounds, by making some larger than others, and giving them different degrees of tension. The violin and bass viol are familiar examples of this. The low, or bass strings, are covered with metallic wire, in order to make their magnitude and weight, prevent their vibrations from being too rapid, and thus they are made to give deep or grave tones. The other strings are diminished in thickness, and increased in tension, so as to make them produce a greater number of vibrations in a given time, and thus their tones become sharp or acute, in proportion.

Under certain circumstances, a long string will divide itself into halves, thirds, or quarters, without depressing any part of it, and thus give several harmonious tones at the same time.

The fairy tones of the Eolian harp are produced in this manner. This instrument consists of a simple box of wood, with four or five strings, two or three feet long, fastened at

Explain fig. 116. On what do the grave or acute tones of the same string depend? Why are the bass strings of instruments covered with metallic wire? Why is there a variety of tones in the Æolian harp, since all the strings are tuned in unison?

each end. These are tuned in unison, so that when made to vibrate with force, they produce the same tones. But when suspended in a gentle breeze, each string, according to the manner or force in which it receives the blast, either sounds as a whole, or is divided into several parts, as above described. "The result of which," says Dr. Arnot, "is the production of the most pleasing combination, and succession of sounds, that the ear ever listened to, or fancy perhaps conceived. After a pause, this fairy harp is often heard beginning with a low, and solemn note, like the base of distant music in the sky; the sound then swells as if approaching, and other tones break forth, mingling with the first, and with each other."

The manner in which a string vibrates in parts, will be understood by fig. 117.

Fig. 117.

Suppose the whole length of the string to be from a to b, and that it is fixed at these two points. The portion from b to c, vibrates as though it was fixed at c, and its tone differs from those of the other parts of the string. The same happens from c to d, and from d to a. While a string is thus vibrating, if a small piece of paper be laid on the part c, or d, it will remain, but if placed on any other part of the string, it will be shaken off.

Wind.

Wind is nothing more than air in motion. The use of a fan, in warm weather, only serves to move the air, and thus to make a little breeze about the person using it.

As a natural phenomenon, that motion of the air which we call wind, is produced in consequence of there being a greater degree of heat in one place than in another. The air thus heated, rises upward, while that which surrounds this, moves forward to restore the equilibrium.

The truth of this is illustrated by the fact, that during the burning of a house in a calm night, the motion of the air towards the place, where it is thus rarefied, makes the wind blow from every point towards the flame.

Explain fig. 117, showing the manner in which strings vibrate in parts. What is wind? As a natural phenomenon how is wind produced, or, what is the cause of wind? How is this illustrated?

In islands situated in hot climates, this principle is charm. ingly illustrated. The land, during the day time, being under the rays of a tropical sun, becomes heated in a greater degree than the surrounding ocean, and consequently, there rises from the land a stream of warm air, during the day, while the cooler air from the surface of the water, moving forward to supply this partial vacancy, produces a cool breeze setting inland on all sides of the island. This constitutes the sea breeze, which is so delightful to the inhabitants of those hot countries, and without which men could hardly exist in some of the most luxuriant islands between the tropics.

During the night the motion of the air is reversed, because the earth, being heated superficially, soon cools, when the sun is absent, while the water being warmed several feet below its surface retains its heat longer.

Consequently, towards morning, the earth becomes colder than the water, and the air sinking down upon it, seeks an equilibrium by flowing outwards, like rays from a centre, and thus the land breeze is produced.

The wind then continues to blow from the land, until the equilibrium is restored, or until the morning sun makes the land of the same temperature as the water, when for a time there will be a dead calm. Then again the land becoming warmer than the water, the sea breeze returns as before, and thus the inhabitants of those sultry climates are constantly refreshed during the summer season, with alternate land and sea breezes.

At the equator, which is a part of the earth continually under the heat of a burning sun, the air is expanded and ascends upwards, so as to produce currents from the north and south, which move forward to supply the place of the heated air as it rises. These two currents, coming from latitudes, where the daily motion of the earth is less than at the equator, do not obtain its full rate of motion, and therefore when they approach the equator, do not move so fast eastward as that portion of the earth, by the difference between the equator's velocity, and that of the latitudes from which they come. This wind, therefore, falls behind the earth in her diurnal motion, and consequently has a relative motion towards the

In the islands of hot climates, why does the wind blow inland during the day, and off the land during the night? What are these breezes called? What is said of the ascent of heated air at the equator? What is the consequence on the air towards the north and south?

west. This constant breeze towards the west is called the trade wind, because a large portion of the commerce of nations, comes within its influence.

While the air in the lower regions of the atmosphere is thus constantly flowing from the north and south towards the equator, and forming the trade winds between the tropics, the heated air from these regions as perpetually rises up, and forms a counter current through the higher regions towards the north and south from the tropics, thus restoring the equilibrium.

This counter motion of the air in the upper and lower regions is illustrated by a very simple experiment. Open a door a few inches, leading into a heated room, and hold a lighted candle at the top of the passage; the current of air as indicated by the direction of the flame, will be out of the room. Then set the candle on the floor, and it will show that the current is there into the room. Thus while the heated air rises up and passes out of the room, that which is colder flows in, along the floor, to take its place.

This explains the reason why our feet are apt to suffer with the cold, in a room moderately heated, while the other parts of the body are comfortable. It also explains why those who sit in the gallery of a church are sufficiently warm, while those who sit below, may be shivering with the cold.

From such facts, showing the tendency of heated air to ascend, while that which is colder moves forward to supply its place, it is easy to account for the reason why the wind blows perpetually from the north and south towards the tropics; for, the air being heated, as stated above, it ascends, and then flows north and south towards the poles, until, growing cold, it sinks down, and again flows towards the equator.

Fig. 118.
de

Perhaps these opposite motions of the two currentswill be better understood by the sketch a figure 118.

How are the trade winds formed? While the air in the lower regions flows from the north and south towards the equator, in what direction does it flow in higher regions? How is this counter current in lower and upper regions illustrated by a simple experiment?

Suppose a b c to represent a portion of the earth's surface, a being towards the north pole, c towards the south pole and b the equator. The currents of air are supposed to pass in the direction of the arrows. The wind, therefore, from a to b would blow, on the surface of the earth, from north to south, while from e to a, the upper current would pass from south to north, until it came to a, when it would change its direction towards the south. The currents in the southern hemisphere being governed by the same laws, would assume similar directions.

OPTICS.

Optics is that science which treats of vision, and the properties and phenomena of light.

The term optics is derived from a Greek word, which signi fies seeing.

This science is one of the most elegant and important branches of natural philosophy. It presents us with experiments which are attractive by their beauty, and which astonish us by their novelty; and, at the same time, it investigates the principles of some of the most useful among the articles of common life.

There are two opinions concerning the nature of light. Some maintain that it is composed of material particles, which are constantly thrown off from the luminous body; while others suppose that it is a fluid diffused through all nature, and that the luminous, or burning body occasions waves, or undulations in this fluid, by which the light is propagated in the same manner as sound is conveyed through the air. The most probable opinion however, is, that light is composed of exceedingly minute particles of matter. But whatever may be the nature, or cause of light, it has certain general properties, or effects which we can investigate. Thus, by experi ments, we can determine the laws by which it is governed in its passage through different transparent substances, and also those by which it is governed when it strikes a substance through which it cannot pass. We can likewise test its nature to a certain degree, by decomposing, or dividing it into its elementary parts, as the chemist de composes any substance he wishes to analyze.

What common fact does this experiment illustrate? Define Optics. What is said of the elegance and importance of this science? What are the two opinions concerning the nature of light? What is the most probable opinion?