634. In islands, situated in hot climates, this principle is charmingly 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.

635. At the equator, which is a part of the earth continually under the heat of a burning sun, the air is expandod, 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 rela

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 reezes called? What is said of the ascent of heated air at the equaLor? What is the consequence on the air towards the north and south?

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

636. 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, and forms a counter current through the higher regions, towards the north and south from the tropics, thus restoring the equilibrium.

637. 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 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.

638. 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.

639. Perhaps these opposite motions of the two currents will be better understood by the sketch, figure 129.

Suppose a b c to represent a portion of the earth's sur face, 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

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?

Fig. 129.
de

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.

640. 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 signifies seeing.

This science involves some 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.

641. 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 experiments, we can determine the laws by which it is governed in its passage through differ

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?

ent 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 decomposes any substance he wishes to analyze.

642. To understand the science of optics, it is necessary to define several terms, which, although some of them may be in common use, have a technical meaning, when applied to this science.

a. Light is that principle, or substance, which enables us to see any body from which it proceeds. If a luminous substance, as a burning candle, be carried into a dark room. the objects in the room become visible, because they reflec the light of the candle to our eyes.

b. Luminous bodies are such as emit light from their own substance. The sun, fire, and phosphorus, are luminous bodies. The moon, and the other planets, are not luminous, since they borrow their light from the sun.

c. Transparent bodies are such as permit the rays of light to pass freely through them. Air and some of the gasses are perfectly transparent, since they transmit light without being visible themselves. Glass and water are also considered transparent, but they are not perfectly so, since they are themselves visible, and therefore do not suffer the light to pass through them without interruption.

d. Translucent bodies are such as permit the light to pass, but not in sufficient quantity to render objects distinct, when seen through them.

e. Opaque is the reverse of transparent. Any body which permits none of the rays of light to pass through it, is opaque.

f. Illuminated, enlightened. Any thing is illuminated when the light shines upon it, so as to make it visible. Every object exposed to the sun is illuminated. A lamp illuminates a room, and every thing in it.

g. A Ray is a single line of light, as it comes from a lu minous body.

What is a transpa

What is light? What is a luminous body? rent body? Are glass and water perfectly transparent? How is it proved that air is perfectly transparent? What are translucent bodles? What are opaque bodies? What is meant by illuminated? What is a ray of light?

A. A Beam of light is a body of parallel rays.

1. A Pencil of light is a body of diverging or converging rays.

k. Divergent rays, are such as come from a point, and continually separate wider apart, as they proceed.

1. Convergent rays, are those which approach each other, so as to meet at a common point.

m. Luminous bodies emit rays, or pencils of light, in every direction, so that the space through which they are visible is filled with them at every possible point.

643. Thus, the sun illuminates every point of space, within the whole solar system. A light, as that of a light house, which can be seen from the distance of ten miles in one direction, fills every point in a circuit of ten miles from it, with light. Were this not the case, the light from it could not be seen from every point within that circumfer

ence.

644. The rays of light move forward in straight lines from the luminous body, and are never turned out of their course except by some obstacle.

Let a, fig. 130, be a beam of light from the sun passing through a small orifice in the window shutter

Fig. 130.

b. The sun cannot be seen through the crooked tube c, because the beam passing in a straight line, strikes the side of the tube, and therefore does not pass through it.

645. All the illuminated bodies, whether natural or artıficial, throw off light in every direction of the same color as themselves, though the light with which they are illuminated is white or without colour.

This fact is obvious to all who are endowed with sight. Thus, the light proceeding from grass is green, while that proceeding from a rose is red, and so of every other colour.

What is a beam? What a pencil? What are divergent rays? What are convergent rays? In what direction do luminous bodies emit light? How is it proved that a luminous body fills every point within a certain distance with light? Why cannot a beam of light be seen through a bent tube? What is the colour of the light which dif ferent bodies throw off? If grass throws off green light, what becomes of the other rays?