recurs. It is seen at irregular intervals, and there are no signs which foretell its coming. It is perfectly impossible to calculate the periods of its return; and human beings are so accustomed to find every natural law working with exact regularity, that their astonishment at this eccentric visitor is never diminished. Even the animals of the regions where the Aurora's recurrence is most frequent, show a fear and a restlessness during the time it lasts, exactly like that exhibited by animals during a thunderstorm.

In the Middle Ages superstition made use of these appearances to excite the passions or overawe the minds of the people. They were pointed to as "signs of the times "heralds of war or pestilence; encouragements to begin an attack, or warnings to desist from fighting. The populace was made to believe that they were outward signs of God's fiery wrath against them for some national sin, or even for some opposition which they had evinced against their rulers. In the autumn of the year 1870, when some of the most remarkable of these lights appeared in different parts of the country, many people said they came in consequence of the war between France and Prussia; others, that it was a sign that England would be obliged to fight against one or the other of these countries, or perhaps both of them.

CHAPTER XLV.

THE ATMOSPHERE.

THE Atmosphere is the whole body of air which surrounds the earth; it forms a kind of outer covering from 40 to 50 miles high.

Air is a fluid, and birds move in it by their wings as fishes do in water by their fins. So the atmosphere may be described as one vast sea extending over the whole surface of the globe. It has its currents and streams,

both warm and cold. The winds are the currents, and the whirlwinds the eddies, in this sea of air.

The air being heavy, presses down the lower parts by the weight of the upper, and the particles are more closely packed-the air is denser. So the atmosphere is thinner, or (as it is generally called) rarer, the higher we ascend from the surface of the earth. In the same way, water at a considerable depth is more dense than near its surface.

The atmosphere is of important service to the inhabitants of the globe, and is one of those marvellous contrivances which attest the Hand of an Allwise and Beneficent Creator.

1. It is essential to the support both of animal and vegetable life. Atmospheric air is made up of two substances mixed together in exactly that way which is best adapted to this purpose. Animals as they inhale it are continually consuming one of its ingredients, and exhale* in return a vapour which, if breathed without mixture, would be destructive of animal life. This very vapour is that which vegetables inhale during the day, while they exhale the original ingredient fitted for animals to consume again. During the night, however, the reverse takes place in vegetables, which then inhale and exhale the same vapours as animals do. Hence a plant in a room is wholesome in the daytime, but unwholesome at night.

2. The atmosphere is an agent for conveying sound. "The air," it has been said, "is the 'carrier of sound'; it is also the bearer of speech, which is the means of communicating ideas, of maintaining social intercourse among men. The earth robbed of its atmosphere presents itself to the imagination as a desert brooded over by silence."t

* To inhale, is to take in by breathing; to exhale, to give out by breathing. † Humboldt.

3. The atmosphere collects and diffuses heat. The higher we rise above the surface of the earth, the more rare the air becomes, and therefore there is less heat. Hence, in ascending mountains, the cold increases with the height, and snow lies throughout the whole year upon the tops of high mountains at the equator.

4. It also diffuses light; and so places upon which the sun does not actually shine are not left in total darkness. In a cloudy day there is still light, because light is diffused by the atmosphere and by the clouds.

5. The weight of the atmosphere causes a considerable pressure at the surface of the globe. About 14 pounds upon every square inch is the average pressure, but at every place this pressure is liable to variations arising from movements in the air.

We might at first fancy that this would be a heavy load upon us, but we must remember that air is a fluid, and presses equally in all directions; therefore it supports rather than depresses us. Water is a fluid denser and heavier than air; but in water we feel lighter, because we are borne up by it. This is why it usually rains when the pressure of the atmosphere is comparatively low, because there is not enough of it to hold up the drops of rain. At such times we often feel oppressed, and say the air is heavy, whereas really the atmosphere is lighter than usual, and we want its support.

6. Impure exhalations, which are lighter than atmospheric air, are continually being carried by it away from the surface of the earth, where they would injure or destroy life. A well long shut up, when first reopened, would be fatal to any one who went down it, because it would contain a vapour destructive of life; but after it has been opened a short time, the atmosphere presses in, drives off the noxious vapour, and occupies its place.

7. But the most important duty of the atmosphere is that of gathering up and rendering back to the earth

the water, which is so necessary to all upon the earth. The air is capable in its ordinary state of holding suspended in it a certain quantity of moisture; the warmer the air, the more moisture it can retain. Water, therefore, is constantly ascending in the form of vapour from the sea, from lakes, rivers, and all moist places on the earth. This is called Evaporation. This goes on to such an extent, that the Caspian Sea, which has no outlet to the ocean, does not overflow, although the mighty Volga and other rivers are continually pouring their waters into it. This, too, is the reason why the ocean itself, the receptacle of so many vast streams, still preserves its ancient limits.

Since warm air holds more moisture than cold, evaporation takes place more rapidly in hot places; hence, in warm weather ponds are dried up and brooks fail.

This also explains the nature and origin of dew. During a hot summer day, the moisture from the earth is drawn up into the air; but at night, when the surface of the earth cools, the air round it is chilled, and gives up some of its moisture. The earth being cooler than the air, the vapour forms into drops, just as drops are formed in a cool glass of water, if surrounded by the vapour of steam.

The dew is greatest in a clear cool night following a sultry day; clouds prevent the heat of the air from escaping. It is greatest when the difference between the temperature of the day and night is greatest. This is the case in our climate in spring and autumn; therefore with us there is most dew at these seasons.

Some substances part with their heat more rapidly than others, and on these more dew forms. The mercury in the thermometer is generally lower upon grass than it would be if the instrument were placed upon gravel, and in a garden the grassplots are often wet with dew, while the gravel-walks are dry.

The vapour which is thus taken up is deposited in the form of rain or snow on the higher places of the earth. Snow covers the tops of high mountains, and, melting as it descends the sides, furnishes water to supply rivers. Water in other parts penetrates crevices in the earth, when it meets with rocky caverns and beds which form natural cisterns and channels, from which it comes forth in springs and wells in various places.

CHAPTER XLVI.

CLIMATE.

THERE is the utmost variety in the productions of the earth. Plants and animals of different kinds are found in different quarters of the globe, and many which flourish in one part cannot be sustained in another.

These varieties of plants and animals correspond to the varieties of climate upon the earth. The climate of a place depends upon the quantity of heat and moisture which is found there.

Some of the causes of the varieties of climate are the following:

1. The Heat of the Sun.-This is the chief cause. The sun's rays have most power where they are vertical; tropical countries are therefore very hot. The farther we go from the Equator the less vertical are the sun's rays, and so we pass gradually from countries where snow is unknown, to the polar regions where there is ice all the year round.

But so long as the sun is above the horizon of any place, that place is receiving heat; when the sun is below the horizon, it is losing heat. Therefore, since in countries distant from the equator the days are longer in summer, the heat in summer is much greater than in winter. This accounts for the short but hot summers of northern countries.