damp one, for the air can hold only so much moisture, and drinks up less when it is already more or less moist. The roads take very long to dry up in winter, but they dry very quickly in June.

6. As evaporation goes on continually all over the world, the mass of water thus raised into the air in invisible vapour is necessarily very great indeed. That which rises in a year from the Atlantic Ocean alone would make a solid mass thirty miles square.

9. DEW, MIST, AND CLOUDS.

1. When the vapours raised from the surface of the earth become visible in the sky we call them clouds; when they come nearer us, and hover on hill tops, or hang in valleys, they are called mists or fogs. When they fall invisibly in the evening and during the night, and rest in drops on the leaves and blades, we call them Dew.

2. All these-dew, mist, fog, or cloud-are thus only condensed watery vapour, formed exactly as we saw the mist formed on the cold tumbler. A mere dimness showed itself first; then the minute globules of water of which this dimness consisted, ran together into larger beads, and these again joined into drops which ran down the glass.

As the air cools from the absence of the sun, or from a cold current coming against a warmer one, the vapours floating in it are condensed, or thickened into a haze, and as the cold increases this haze runs into larger and larger drops; for haze is made up of small drops, and grows denser or thicker as the drops increase in size, forming clouds fleecier or darker as the drops are smaller or larger.

3. Dew. The name of dew is given to the moisture that falls in the evening and through the night, showing itself on all the leaves and blades of the landscape, in shining drops, in the light of the morning. It was thought in ancient times that, as dew falls only on clear nights, and as such nights are commonly cold, the moon, planets and stars pour down cold on the earth, and that this cold causes the dew. It shows how hard the true explanation of the simplest thing in nature is beforehand, even when it seems quite easy after it is once found out, that, although wise men were trying their best from the earliest ages to tell the cause of dew, it was not discovered till about sixty years ago (1814— 1817). Dr. Wells, of London, then proved that dew was caused

by the condensation of the watery vapour of the air, on substances which have become cooled by the radiation* of their heat.

4. During the day, the earth, and all things on it, receive a large amount of heat from the sun, and this heat, when the sun has gone down, begins to radiate or ray off into the air, till leaves, stones, and all things get more or less cold, as they give out their heat more or less quickly. The same thing exactly then takes place as we saw in the case of the cold tumbler, and the moisture in the room; the cold leaf, or twig, or blade, or stone, condenses the vapour in the air and is covered with moisture, which soon gathers into drops, and, then, is seen as Dew.

5. Some bodies give out their heat more quickly and freely than others, and by thus being colder, have dew formed on them more readily. It is so with glass, which is wet with dew, when gravel, which gives out its heat slowly, has very little. This law explains all the differences in the fall of dew on different surfaces. The degree of temperature-that is the point on the scale of heat and cold, at which the vapour in the air begins to be condensed, is called the Dew-Point.

6. Fog and Mist are another form in which the unseen watery vapour always in the air becomes visible. They are often formed on hills and mountain tops by the passing over them of a warm moisture-laden wind when their heat has been given off at night, or when cloudy skies have kept the sun from warming them. When morning once more heats the surface over which these mists or fogs rest, or when the sun breaks out, they melt away. A river flowing from a cold to a warm region is often covered with fog, caused by its waters being colder than the air over them, and condensing the vapour in it; and, on the other hand, fog often rests upon a river flowing from a warm to a cold region, because the warm surface of the waters gives off more vapour than the air over it can hold, or from the coldness of the air condensing what thus rises. Meadows are often veiled with fog when a river flowing through them is free from it, through the earth giving off its heat so much more quickly than the water, that it falls to what may be called the Fog-Point, while the river is yet above it. Newfoundland, the Western Islands of Scotland, Iceland, and other regions, where there is often a great difference between the temperature of the land and of the moist air passing over it, are very much visited by fogs. In the northern Atlantic they are caused very often in

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Any body which loses heat is said to radiate it-that is, to send it forth from it in rays, as the sun sends forth its beams.

summer, by icebergs floating south from the arctic regions, and condensing the vapours in the warm air around them.

7. Clouds are the watery vapour in the atmosphere condensed by rising to higher and colder regions, or by cold currents of air coming in contact with it, for there are currents of colder or warmer air flowing like so many rivers, from time to time, overhead. The sky is clear at night because the clouds descend towards sunset, as the heat lessens, and are dissolved by coming into contact with the warm air near the earth. Evaporation growing less as the day draws to its close, and almost ceasing by night, the sky is often cloudless in the mornings. But as the day advances, vapours again begin to rise with the kindling sun, and the varied scenery of lighter or heavier clouds repeats itself anew, the thinner vapours rising high, the denser floating nearer the earth, according to the weight of water they contain.

10. RAIN, SNOW AND HAIL.

1. Rain. You know how the vapour on the window-panes of a crowded room, gathers into smaller and then into larger drops, till at last it runs down to the sash below. In the same way the clouds, which are made up of drops of water finer than dust, grow darker and heavier by these small drops running together into larger, and still larger ones, till, at last, they are too heavy for the air to hold up, and fall through it as Rain, to the ground, washing the air from its impurities, and supplying the moisture needed for the growth of all vegetation.

2. The immediate causes of rain are various. It may be caused by clouds rising into colder regions of the air; by their drifting against a cold mountain peak, or by warm moist clouds being floated on air currents from the equatorial or tropical regions towards the arctic, or cold. Forests have a great effect in producing it, and hence, a wooded region grows each year more luxuriant, while an arid, treeless region, can hardly be changed to a wooded one, because the great heat of the naked soil, and the consequent dryness of the air over it, drive away the clouds that would have given it rain. There are also special causes in some localities, which are stated in section 4 below.

3. Some regions are nearly or quite rainless. On the coast of Peru, in the valleys of the rivers Colorado and Columbia, in North

America,-in the Sahara desert, in Africa, and in the desert on Gobi, in Central Asia, rain is almost unknown. On the other hand, the rainfall on the Khasia Hills, which are part of the Himalaya, on the North East of India, is no less than six hundred: inches, that is, fifty feet, a year! Five hundred inches fall there during the continuance of the South-West monsoons, or periodical winds, which last for seven months in the year. This is the rainiest district in the world. At Stye Head Pass, in the Lake District, the yearly rainfall is about two hundred and twenty-four inches; at Seathwaite, in Lancashire, it is one hundred and eighty-three inches. The wet winds of the Atlantic are the cause of this excessive moisture, for on the East Coast, which the clouds from the Western Ocean do not reach, the fall is only from twenty to twenty-eight inches.

4. The total want of rain in the Sahara is due, among other causes, to the fact that the winds prevailing there come from Asia, and expend much of their moisture before they reach it. It is the same with the great rainless deserts of Asia, the winds that blow over them having lost much of their rain before they reach them. There is a belt of almost continual rains, extending round the world a little north of the Equator, embracing part of Central America and of the north of South America, the regions on the line of Sierra Leone in Africa, and some of the islands and regions of the Far East. Bordering on this zone comes another, in which the rainy season is periodical, as in Brazil on the South, and India on the North of the Equator.

5. Snow is moisture frozen in the air by cold winds. The flakes are composed of minute six-pointed crystals of ice, of a thousand beautiful forms. The white colour of snow is caused by the blending together, in the snow crystals, of rays of all the prismatic colours which make up light, white being the colour of unbroken or perfect rays of light. When looked at separately, the crystals exhibit different colours.

6. Snow is about ten or twelve times lighter than water, so that when it falls evenly, the quantity of rain it represents is about a tenth of the depth of the snow.

7. Snow is of great use in many ways. A great amount of heat is given out from the moisture in the air, as it passes into snow, and thus the air itself is made warmer than it otherwise would be. When the snow has fallen, it protects the soil and the seeds and plants in it from cold, as it does not let the heat escape readily; or, in other words, it is a bad conductor of heat. The ground

beneath snow is often from twenty to thirty degrees warmer than the surface of the snow itself; sometimes, indeed, even forty degrees warmer.

8. Red Snow and Green Snow have been met with in different parts, especially in the Arctic regions. They are caused by the presence of minute vegetation of these colours.

9. Hail seems to be simply frozen rain drops, the varying sizes of the hailstones being apparently caused by a number of frozen drops being refrozen into a lump as they strike against each other in passing through the air. This is the simplest explanation, but it is questioned by some whether it accounts for all the facts seen in connection with hail storms. A sudden rush of very cold air through a portion of the atmosphere laden with moisture, is doubtless the ordinary cause.

10. Hailstones are sometimes of great size. In India, they are often as large as pigeons' eggs; and not a few weigh as much as a pound. In 1832, a mass of ice fell in Hungary about a yard in length, and two feet in thickness. In England, hailstones, though occasionally larger, are generally about the size of peas.

11. CIRCULATION OF WATER BETWEEN AIR
AND SEA.

1. How is it, that notwithstanding the vast amount of water raised from the ocean, and from lakes, by evaporation, they seem as full as ever after ages? The only answer must be, that as much water comes down from the air by the condensation of vapours into dew, rain, snow, &c., as rises into it. But how is this balance maintained?

2. The quantity of rain which falls upon the sea, must be much greater than that which falls on the land, because there is about three times more sea than there is land. The loss by evaporation from its surface is thus made up in part, but this is not the only way in which water returns to it.

3. The average depth of the yearly fall of rain in England is about thirty-one inches, so that if it all fell continuously, and remained on the ground, the whole country would be two feet and a half under water by one year's rain. How vast, then, must be the quantity of rain that falls over the whole land surface of the globe in a year, for England is very dry, compared with many countries.