another, and may be regarded as constituting the proper atmosphere in its relations to life. The vapour of water is a fourth ingredient, and often a considerable one; but it is very variable, and is subject to laws of its own, and may be considered as a kind of atmosphere by itself-an atmosphere within the atmosphere.

Other things in the air are very numerous and very interesting. Some rather dart through it than exist in it, and have no appreciable weight-such as light, heat, the electric fluid, and perhaps the principle of sound. Some are gases, all different from one another, and from the oxygen, and the nitrogen, and the carbonic acid, and all possessed of curious properties. Some are rarified or vapourised essential oils-the substances which constitute odours. Some are invisible and inconceivably minute reproductive organs of the smallest plants, and of microscopic animals. Some are particles of inorganic matter, either so minutely divided as to be imperceptible by the naked eye, or occasionally so coarse and plentiful as to appear like smoke and dust. And some are unknown substances of great power and mystery, associated, no man can tell how, with the invigoration of plants and animals, and with sudden and temporary outbursts upon them of fatal diseases. But all these matters are mere admixtures in the atmosphere; and all, except the imponderable ones, occur in exceedingly minute quantities, and are exceedingly fitful and fluctuating.

The atmosphere envelopes the solid earth like a spherical stratum or all-encompassing sea. It holds on quite firmly, and partakes steadily of the earth's

revolutions; and, though subject to many powerful currents and commotions within itself, it is but very slightly affected, in the way the ocean is, by tidal forces. Its total height is believed to be about fortyfive miles. But its density at the bottom is sixty-four times greater than at or near the top, and decreases one half at every distance of about two and three quarter miles. Its total weight is computed to be upwards of five thousand billions of tons, or equal to that of a globe of lead sixty miles in diameter. Its pressure at the surface of the sea and of the lowest valleys averages about fifteen pounds on every square inch, but constantly varies, at any one place, from changes in the amount of its vapour, from expansions and condensations of its gases, from the effects of its currents and commotions, and from the action of heat, electricity, and other agents. The variations of pressure or of local weight are readily measured by the rise and fall of the mercury in the barometer, and furnish man with his best artificial means of anticipating changes of weather. The average pressure upon a human body of nearly fifteen square feet of surface, is about fourteen and a half tons; and this is essential to man's breathing and moving with health and comfort. The effects of the diminished pressure, and of the attenuated density at mountain elevations, are always painful; and at heights of from ten to fifteen thousand feet above sea-level, they commonly include excessive weakness, great difficulty of breathing, and sometimes acute agony, and a sense of suffocation. The pressure at ordinary levels is also a great force, and works wonders by its action, and is capable of being turned to mighty practical account in human mechanics.

But this will be better explained in a subsequent chapter.

Evaporation is continually going on from seas, lakes, rivers, moist soils, and all sorts of moist surfaces. Water passes into vapour at all ordinary temperatures, simply by absorbing heat; and the vapour formed at all ordinary temperatures is commonly invisible at the time of its formation, and becomes instantly diffused through the air. If no atmosphere existed, evaporation would still take place, and would maintain around the earth a constant yet very variable envelope of vapour. The rapidity of it, in particular conditions of the air, is often astonishingly great, and may sometimes be remarked in the quick drying of clothes, and in sudden dryings of the ground after summer showers; but, if it were not repressed and regulated by the weight and counteraction of the atmosphere, it would frequently be tremendous, and might, in one day, render fields, and fens, and shallow pools as dry as parched deserts. It is very various at different times, and very various in different countries, and it makes surprising differences on clouds and landscapes, and on crops and animals, by its varieties; yet, in general, it is greater in summer than in winter, greater in the day time than in the night, greater in a dry state of the air than in a moist state of it, greater even in a dry cold day of winter than in a moist warm day of spring, greater in warm and low countries than in cold and mountainous ones, greater in districts adjacent to seas and lakes and large rivers than in districts distant from any expanse of water, and greater in a wind than in a calm, in a brisk breeze than in sluggish puffs of air. All these differences work in the same general way as

the variations of temperature and the succession of the seasons, and play a powerful part in the glorious vicissitudes of clime and weather.

The quantity of water evaporated into the air throughout the year, is estimated at a depth over all the globe of about thirty-three inches, or at an aggregate bulk of about an hundred thousand cubic miles. Yet never does the vapour of more than a small portion of that prodigious quantity exist in the atmosphere at one time; and never or seldom does this constitute more than nearly a twentieth part, or less than about a two-hundredth part of the atmosphere's contents. It does not diminish rapidly in density upward, as the atmospheric gases do, but is quite as dense, or nearly so, at a considerable height, as at the surface or the ground. It is also a good deal lighter than the atmospheric gases; so that the atmosphere, when very humid, has a good deal less pressure than when very dry. How curious that, in rainy weather, when sensitive persons feel languid, and "as if a load were on them," they really sustain very many pounds less pressure than in fair weather, when they feel light and spirited! This may afford every one an idea of the happy effects of great pressure in low situations, and of the disastrousness of small pressure at lofty mountain elevations.

In some states of the atmosphere, invisible vapour forms into fogs and clouds, and, in other states, it passes at once into dew and rain. But what those states precisely are, or what are other states in which dews, and fogs, and clouds, and invisible vapour resolve back into one another, or congeal into hoar-frost, and snow, and hail, philosophers do not well know.

Electricity, and winds, and changes of temperature have mainly to do with them; but in what proportions, or under what precise laws, we cannot tell.

Dew is a deposition of vapour upon objects which are freely exposed to the atmosphere. It resembles the moisture which is formed on the walls of a room when they have a lower temperature than the humid air within, or that which is formed on the inside of windows when a fall of rain or hail suddenly diminishes the temperature of the air without. It commonly begins to form about sunset; and it abounds more in spring and summer than in the other seasons, more in valleys and in the vicinity of waters than in uplands and dry regions, and a very great deal more in hot tropical countries, which have few or no rains, than in the cold countries of the temperate zones which have a dripping climate. It always refreshes vegetation, and, in some burning districts near the equator, it supplies many plants with all the moisture they need, even in defiance of a scorching sun, and a perfectly arid soil. It forms also on stones, and even on metals, and no doubt gave rise to certain remarkable appearances which occasioned ancient poets and historians to speak of sweating statues and weeping images. The deposition of it is the gentlest of all atmospheric phenomena; and hence the beauty of the allusion of Moses, when he said that, in discoursing to the Israelites about the character of the Most High, his "speech should distil as the dew."

Hoar-frost, or rime, is simply frozen dew, or frozen fog. It abounds most on the low slopes of hill-screened valleys, and occurs oftener in autumn and winter than in spring. It commonly forms only when the atmos