The earth is surrounded in all directions by a covering of air about forty miles in thickness, which bears about the proportion to the earth itself that a coating one inch thick would to a ball seventeen feet through, being about 1/200th of its diameter. This air, though invisible, may at all times be felt while in motion (such is the wind), and by rapidly passing the hand backwards and forwards it may be perceived to press against and slightly obstruct it; this air is composed of about twenty parts oxygen and eighty parts nitrogen, together with about one part in every thousand of carbonic acid (consisting of carbon and oxygen in union) and a small proportion of vapour of carbonate of ammonia (ammonia in union with carbonic acid). It is the oxygen which enables animals to live in the air, as it is taken into their systems at every breath, it is thus constantly being consumed and would ultimately be so far diminished, that they could live no longer for want of it; but Providence has so arranged, that the necessary quantity is always being supplied by vegetables (which give out oxygen) to meet the deficiency. The air, although a gas, is yet capable of being weighed, of great compressibility, and of expansion to an unlimited extent, this causes the lower part (near the surface of the earth) to be much more dense than the upper regions, and those who have ascended to the tops of high mountains, have described the difficulty of respiration as being very great, owing to the rarefaction or lightness of the air. At the surface of the earth it presses with a weight of about fifteen pounds upon every square inch, but as this pressure is equal in every direction, it is not felt by us, nor does it crush the most fragile flower or insect; but, remove the pressure from one side of anything, and it will be found to press with violence upon the other. If the air were removed from the inside of a drum, the weight of the surrounding air would burst in the parchment and fill it. By way of experiment, fill a large basin with water, take a tumbler in the left hand and a piece of lighted paper in the right, hold the lighted paper for a moment under the inverted tumbler and immediately apply its open mouth to the surface of the water, letting it dip in about half an inch, the heat expands and consequently gets rid of some of the air in the tumbler, and as this cools again it resumes its original bulk; the pressure of the air on the surface of the water in the basin will force it up into the tumbler and nearly fill it (fig. 1). This is the principle of the barometer, which is a tube entirely exhausted of air and the weight of the atmosphere forces a column of mercury (which is many times heavier than water) up to the height of about twenty-nine inches, according to the weight of the air at any particular time; this weight varies according to the state of the weather, whether wet or dry; but the average weight of air is found to be perfectly stationary, and during the twenty years from 1816 to 1836, it was found at Paris not to have varied 1/1200th of an inch. Winds or currents of air proceed from several causes; when the sun shines on a large surface of the earth, it becomes heated and a column of hot air is pressed upwards, for hot air is lighter than cold, and the cold air all around, by its weight, forces it up and rushes in to fill the space; this space may perhaps be many hundred square miles in extent, so that a current of wind is caused to blow towards this spot from all the regions round. This is the general state of things at the equatorial parts of the earth and causes the "trade winds," which uniformly blow (the greater part of the year) from both north and south towards the equator. Another cause of local winds, is the condensation of vapours into water; when the air over a large region is saturated with moisture it is greatly expanded by it, and when the vapour is condensed and falls as rain, the air from the surrounding parts forces itself in to fill the space occupied by the rain while in a state of vapour, which is nearly one hundred thousand times greater than it occupies when in the form of water. There are many causes to determine the condensation of the vapour, which the atmosphere always holds in larger or smaller quantities, but cold is the chief agent, for when a current of air passes over seas or rivers, or the damp surface of the earth, it becomes loaded with moisture, and being capable of holding only a certain quantity (less when cold than hot), it follows that warm air when saturated with moisture must let some of it fall when it becomes cooled. This may occur from entering a colder region, or uniting with another current of air colder than itself. In tropical regions the air is so warm that it takes up a very large quantity of watery vapour and upon the coming of colder weather, the rainy season begins and the air empties itself of its superfluous moisture, causing the most tremen dous falls of rain in consequence (fig. 2). When the clouds are overcharged with moisture in cold climates or in cold weather, the vapour freezes as it condenses and forms snow, which under the microscope presents a series of the most beautiful star-like crystals (fig. 3); the same result is produced by the freezing of the vapour or fog near to the earth's surface, this is called "hoar-frost," which is (like dew) deposited on those parts of the surface most cooled by radiation. Hail is caused in all probability by the drops of rain passing through a cold stratum of air and becoming frozen as they fall. There are many wonderful accounts on record of great masses of ice falling from the clouds, some of them several feet thick; the cause of these phenomena (if they ever did occur) is not known. The curious occurrence of red or green snow, which has sometimes been known to fall, is owing to the snow being mixed with myriads of minute cellular plants called the Protococcus pluvialis, which in one state of its existence is green and in another red, and they colour the snow accordingly. The cause of their sudden accumulation in such vast quantities is not known, but may be sought for in some peculiar state of the atmosphere favourable to their growth, which in many places is prodigiously rapid.

The temperature of the air differs very greatly in different situations at the surface of the earth, the extreme difference being about 180 degrees; thus in winter, at the poles, the thermometer frequently stands at 60 degrees below zero, and at the equator, in summer, 120 degrees above, but, in the higher regions of the air, it is even colder than at the poles, for the sun shining on the earth heats it, and the air is heated by contact with it; this is its only source of heat, of which it receives less and less the further it is from the earth, so that at a distance (depending upon the situation and climate) ranging from 3,500 to 17,000 feet from the surface, the air is so cold that it is called "the region of perpetual snow," and all mountains whose tops reach above this altitude are covered with snow (formed by the freezing of the rain and vapour) from this point to their summits (fig. 4), and the sun (although it nearly always shines there, these regions being above most of the clouds) is not able to melt it, for the radiation of heat from its surface is greater than its absorption from the sun's rays, white substances absorbing but very slowly, and rough surfaces (as the snow) radiating very rapidly.

Dew is caused by condensation of the watery vapour held in the air; when the sun goes down the radiation continues from the surface of the earth, and those surfaces which radiate most rapidly, such as fields of grass, get cooled down below the temperature of the surrounding air, and therefore cause a condensation of its watery vapour. This may be imitated by filling a large glass goblet with very cold water, and bringing it into a very warm room, the outside of the glass vessel will become covered with dew, although previously quite dry; the same may be shown by filling a similar glass with water at the ordinary temperature, taking care that the outside is dry, then stirring in an ounce or so of nitre or Rochelle salt, which will cause the water to become cold, and the outside of the glass to be covered, as before, with dew. Dew falls fastest when there are no clouds in the sky, as radiation then takes place from the earth into space, and is not compensated for by a return of radiation from the clouds to the earth.

Clouds are formed by the partial condensation of vapour, and are borne along by the wind, instead of falling as rain; the reason of their not falling is this, when the air below the clouds is saturated with moisture it will absorb no more and the rain falls, but when it is warm and dry, and passed in a constantly renewed current, then the vapour is absorbed before it reaches the earth, and is carried off by the wind. Clouds, therefore, although they often appear stationary, are constantly altering their form and size, portions being absorbed while others are being formed. Fogs are the same as clouds, the vapour arising from wet grass, rivers, &c., being condensed as it ascends, by a current of cold air passing over them. The fogs in London have a brown colour, from admixture of smoke, dust, &c., with which the air is contaminated (fig. 5). Clouds are of various forms and sizes, and indeed of almost every variety, but certain kinds which are frequently seen, have received different names. The "cirrhus" comprises all the feathery white clouds which float high up in the air in fine weather; the "cumulus" consists of the large mountain-like clouds which are generally seen in summer; the "stratus," those horizontal layers of clouds low down in the horizon, so often seen at sunset; and the "nimbus" is the rain-cloud, of a dark grey or leaden hue, with sharp well-marked edges (fig. 6).

Clouds are amongst the most beautiful as well as useful things in nature, and it is one of the greatest proofs of the active benevolence of God, that all those things which serve man the most, are the most beautiful to contemplate. Without clouds there would be no rain, and without this no vegetation. In many parts of the tropical regions there is little or no rain, and in such parts desert places abound. Clouds are often of different states of electricity, and when they come near enough to each other, a transfer of the fluid takes place, accompanied with a flash of lightning (fig. 7) and a report, although this is not always heard at the same time that the lightning is seen, as sound does not travel nearly so fast as light; there is no danger from this kind of lightning. But it sometimes happens that a cloud in an opposite state of electricity to the surface of the earth is near enough to produce a flash of lightning between them, in this case it is extremely dangerous to be near; but when the thunder is not heard till some time after the flash is seen, there can be no danger, as it is then far away. Sound travels at about eleven hundred feet per second, therefore (in round numbers) it may be known that the lightning is one mile distant for every five seconds that elapse between the flash and the thunder. The notion which prevails that iron and steel attract lightning is entirely erroneous, they do not even conduct it so well as copper and many other metals.

Lightning conductors (fig. 8) are wires of iron or copper, made to project above the highest parts of buildings and carried down to the earth; they have the power of conducting the lightning down without injury to the building, for electricity (which lightning is) travels through metallic wires for any distance without disturbance or noise (as in the electric telegraph), but bad conductors are apt to be burst and rent by it if it be strong enough, creating flashes of light and reports. Lightning strikes most readily any projecting substance or point, the highest point is therefore selected for the protrusion of the upper part of conductors. In a plain or open space, trees and animals are often struck, as they form conductors (although very bad ones) to the lightning; being bad conductors, instead of allowing it to pass quietly to the earth as do the wires, they are generally destroyed by it. It has been often stated that we should select an open space in a thunder-storm, but this would render one liable to be struck, especially if there were no object near of larger dimensions, the best plan is to stand near to any trees or houses without touching them, the danger would not then be so great, the trees or buildings being taller, would be most liable to conduct the lightning to the earth. The danger of touching or leaning against a tree in a thunder-storm, arises from the person thus making himself (as it were) part of it and incurring the same risk.

Thunder-storms, which would at first thought appear to be of no utility to man, are indeed of great service; it is a common saying that thunder clears the air, this is the result of feeling and experience, but chemists have demonstrated the fact that a substance called "ozone" (a peculiar state of the element oxygen) is produced by thunder-storms; this ozone possesses the wonderful power of correcting (decomposing) putrid and unwholesome gases and exhalations which might otherwise produce fever, cholera, &c., it moreover destroys the ova or germs of many animals and vegetables which might otherwise be injurious to vegetation of more importance, the slight injury which these storms inflict here and there should weigh as nothing in the balance of utility with such universal good. Thus it is that the works of God have all the stamps of goodness, and this ought to inspire us with so much thankfulness as to overcome every fear for personal safety. Were these grand phenomena of nature, (as materialists would make us believe) the result of laws depending solely upon the physical co-operation of mere matter, good would be the exception instead of the rule, and most of them would produce effects, if not injurious at least not beneficial, but such is not the case, and throughout all the wonderful operations of nature there is not one but tends to good, for God often inflicts a small injury that a great good may result; we say a small injury, but it is questionable if the injury is not often a benefit, which appears to us injurious only because we do not understand it fully, every disturbing cause tends to produce some apparent disorder, such as storms, hurricanes, &c., some indeed so terrific as to destroy ships and houses, but what would be the result if the atmosphere were never disturbed from any cause? Why the lower stratum would become so loaded with impurities that it would be unfit to breathe, miasms and noxious gases would for ever remain a curse to the races of men and animals who might be doomed to inhabit such regions, but the very regions where these miasms are most likely to form are those about the tropics, and here it is that the greatest storms occur to remove them. Even the great deserts, which appear so useless to man, and which are uninhabitable to a great extent, have their office, and an important one too; they are to the earth what ventilators are to buildings, drawing the cold air from the poles to cool the regions that are too hot, and sending a current of heated air through the upper regions of the atmosphere (where it can do no injury to anything) to warm the colder parts of the earth, another instance of the wonderful care and goodness on the part of the Creator. The various and beautiful colours of the clouds, particularly at the rising and setting of the sun, are caused by refraction separating the white light into its primitive constituents, blue, red, and yellow light and their combinations, purple, orange, green, &c., and the more obliquely the rays impinge upon the earth the greater will be this refraction, this accounts for colours seldom appearing in the clouds at midday. But of all the beautiful effects of the refraction of light, the Rainbow (fig. 9) is the most glorious, it has been celebrated in all ages for its transient beauty. It is only seen when rain is falling in front of a brightly illuminated cloud, the sun being behind the spectator; it is a reflection of the sun by the cloud transmitted through millions of drops of rain, each of which acts as a prism, and produces rings of colour; for each of the rays of light (red, blue, and yellow) are refracted in unequal degrees, and therefore take separate places, forming the rings of colour seen in the rainbow.

The air is the great source from which all the nourishment of the organic creation, whether vegetable or animal, is derived; its carbonic acid is decomposed by the vegetables, which appropriate the carbon, turning over the oxygen to their companions, the animals; the ammonia of the air furnishes all the nitrogen of seeds and other nutritious parts of vegetables, which are eaten by animals; and water, the chief source of all nutriment, passes first from the air before it enters the soil, bringing both carbonic acid and ammonia to fertilise it. That all this nourishment is derived from the air is evidently shown by the formation and increase of mould in forests which have grown for centuries; this black mould is nearly all decayed vegetable matter, formed by the continuous fall and decay of leaves and trees, but which, instead of diminishing, increases. Now, where does all this come from? certainly not out of the earth, for it does not contain the necessary elements. Expose the surface of the bare earth for several centuries, and first small plants, then larger ones, will grow upon it, until the state of things described above takes place, the earth being just as rich in organic matter now as at the beginning, and much richer on the surface, where a thick stratum of black mould forms from the repeated fall and decay of leaves and wood; all the substance which the forest shall have drawn from the earth (with the exception of certain salts and earthy matters) must therefore have been derived from the air, which contains every ingredient necessary for its formation, while in the earth itself no kind of organic matter is ever found.

There was formerly a time when the carbonic acid of the air was in much greater abundance, and favoured the growth of those plants which thrive where there is plenty of water, as in swamps and marshes. They grew and decayed for a vast period of time, till a thick stratum of carbonaceous matter was deposited, which, after being buried (by some convulsion of nature) at a great depth, and pressed by the enormous weight of the superincumbent earth into a hard solid substance, is now being dug up by man, and forms that most valuable of all products of the mine, coal (fig. 10).

That coal is derived from decayed and altered vegetable matters, is pretty well proved; for many pieces of coal, if ground thin and subjected to the microscope, present a texture exactly such as can be seen in wood of the present date, and not only the ordinary structure, but in very many cases certain "dotted fibres," indicating that the wood belonged to the order of cone-bearing trees (ConiferÆ), and there is but little doubt that the constant deposit of such wood and its slow and gradual decay was the real source of all our coal. How wonderful, and how good, is the foreknowledge of God! Just at a time when the great forests of the earth are fast disappearing, and with them the only other source of fuel for fires, this reserve of the old forests, which was then useless, serves man as a most excellent friend—more useful than gold or silver, more precious than diamonds or rubies! But this world is one of perpetual change, and the coal thus brought forth from the depths of the earth, is by man being rapidly restored to the air from whence—thousands of years ago—it was derived; and it is not coal alone which is thus being restored, but every organic being in the whole creation, for at their decay they all enter this vast mausoleum of the dead!