Without this atmospheric covering of ours, it is considered that the temperature of the earth at its surface would be the same as that of the celestial spaces, supposed to be at least 76° below zero, or possibly, says Humboldt, 1400° below! Human life, without our atmosphere, could not exist for a single moment.

It is computed, that, if the annual heat received by the earth on its surface could be equally distributed over it, it would melt, in the course of a year, a stratum of ice 46 feet thick, though it covered the whole globe, and as a consequence the amount of unradiated heat would render it uninhabitable.

The relative position of the sun affects temperature, rather than its distance. In winter the earth is three millions of miles nearer the sun than in summer, but the oblique rays of the former season reach us in less quantity than the more direct The distribution of land and water, the nature of the soil, the indentation of bays, the elevation of land above the sea-level, insularity, etc., all, as we have already suggested, have a modifying influence on temperature.

The atmosphere possesses also a reflecting and refracting power, arising from its varying density, and, perhaps, in the latter case, somewhat from its lenticular outline.

But for this property we should have no twilight. The sun, instead of sending up his beams while 18° below the visible horizon, would come upon us out of an intense darkness, pass over our sky a brazen inglorious orb, and set in an instant amid unwelcome night.

Reflection is the rebound of the rays of light or heat from an opposing surface at the same angle as that at which they fall upon it. These are called angles of incidence and reflection, and are equal.

Refraction is the bending of a ray passing obliquely from a rarer into a denser medium. This may be observed when a rod is placed slantingly in a vessel of clear water; the part immersed will appear bent or broken. This is ordinary refraction. Terrestrial refraction is the same thing, occurring whenever there is a difference of density in the aerial strata.

The atmosphere absorbs some portion of the light which it receives. It is not all reflected or refracted or even penetrative.

Objects seen under various degrees of light, either convected or retarded by different media, appear near or distant, distinct or confused. Thus, we are often surprised at the apparent nearness and brightness of an opposite shore or neighboring island, in some conditions of the air, while at other times they seem distant and lie in shadowy obscurity.

The looming up of a vessel on the water is another common instance of the principle of refraction.

It has been noticed by almost every one, that, during the warm and moist nights of summer, the moon, as she rises above the horizon, appears much larger than when at the zenith. So the setting sun is seen of apparently increased size. Sir John Herschel asserts that the appearance is an illusion, and so do some others. Professor Carey says, that, if we look through a paper tube at the moon when on the horizon, the paper being folded so as to make the aperture of its exact size, and then look again at it when it reaches the zenith, we shall find there is no difference.

On the other hand, an experiment is offered by a German Professor, of the name of Milo, of this kind: If we look through a tube so constructed as to have one side filled with spirits of wine and the other with common air, the half of the object seen through the former will be found to appear much larger to the eye than the other half seen through the latter.

It is laid down, that, where extraordinary refraction takes place laterally or vertically, the visual angle of the spectator is singularly enlarged, and objects are magnified, as if seen through a telescope. Dr. Scoresby, a celebrated meteorologist and navigator, mentions some curious instances of the effects of refraction seen by him in the Arctic Ocean.

Many remarkable phenomena attend this state of the atmosphere, known as the Fata Morgana of Sicily, the Mirage of the Desert, the Spectre of the Brocken, and the more common exhibitions of halos, coronæ, and mock suns. The Mountain House at Catskill has repeatedly been seen brightly pictured on the clouds below. Rainbows are also due to this condition of the atmosphere.

We might occupy the remainder of the space allowed us by enlarging on various topics which belong to this part of our subject. The twilight gray, the hues of the evening and morning sky, the peculiarity of the red rays of light, the scintillation of stars, their flashing changes of colors, are all meteorological in their character, as well as strikingly beautiful and interesting.

Polarity of light is another of the wonders of which Meteorology takes cognizance. The celebrated Malus, in 1808, while looking at the light of the setting sun shining upon the windows of the Luxembourg, was led to the discovery that a beam of light which was reflected at a certain angle from transparent and opaque bodies, or by transmission through several plates of uncrystallized bodies, or of bodies crystallized and possessing the property of double refraction, changed its character, so as to have sides, to revolve around poles peculiar to itself, and to be incapable of a second reflection. The angle of polarity was found to be 54°.

The beam of polarized light was also found to have the peculiar property of penetrating into the molecules of bodies, illuminating them and, enabling the eye to determine as to their structure. The production of beautiful spectres, prismatic colors of gorgeous hues, and the most remarkable system of rings, has followed the discovery, and important results are expected from the continuation of the researches. It has already enabled the astronomer to determine what heavenly bodies do or do not shine with their own light. The subject is still under investigation.

Color from light comes also under the notice of the meteorologist. The received opinion is, that there is no inherent color in any object we look at, but that it is in the light itself which falls upon and is reflected from the object. Each object, having a particular reflecting surface of its own, throws back light at its own angle, absorbing some rays and dispersing others, while it preserves its own. In this sense it may be said that the rose has no color,–its hues are only borrowed. If the idea should be carried out, it would certainly destroy much of the poetry of color. Thus, in praising the modest blush which crimsons the cheek of beauty, we should destroy all its charm, if we attributed it to a sudden change in the reflecting surface of the epidermis,–a mere mechanical rushing of blood to the skin, and a corresponding change in its angle of reflection!

Without light, however, there is no color. Agriculturists and chemists understand this. Plants without light retain their oxygen, which bleaches them.

The theory of color has never been fully agreed upon. Some writers maintain that the character of its hues depends on the number of undulations of a ray. Goethe's theory is substantially, that colors are produced by the thinning or thickening and obstructing of light. Brewster contends that there are but three primary colors,–red, yellow, and blue. Wollaston finds four,–red, yellowish green, blue, and violet. But this, as well as the consideration of the solar spectrum of Newton, is more the specialty of Optics. The atmospheric relations of color are more apposite to our purpose.

The color of the clouds, which may be occasionally affected by electricity, is owing to the state of the atmosphere and its reflecting and refracting properties.

The color of snow is white because it is composed of an infinite variety of crystals, which reflect all the colors of light, absorbing none, and these, uniting before they reach the eye, appear white, which is the combination of all the colors.

Wind, the atmosphere in action, though not picturesque, is always wonderful, often terrible and sublime. The origin of wind, its direction and its force, its influence on the health of man, his business, his dwelling-place, and the climate where he perpetuates his race, have attracted the profound attention of the greatest philosophers.

To the rarefaction of the air at the equator, and the daily revolution of the earth, is attributed the origin of the Trade-Winds, which blow from the east or a little to the north of east, north of the equator, and east or south of east after we are south of the equator. The hot current of ascending air is replaced by cold winds from the poles.

But why are we not constantly subject to the action of north winds, which we rarely are? Because of the diurnal motion of the earth, which at the equator equals one thousand miles an hour, the polar winds in coming down to the equator do not have any such velocity, because there is a less comparative diurnal speed in the higher latitudes. The air at the poles revolves upon itself without moving forward;–at the equator, the velocity, as we have mentioned, is enormous. If, then, says Professor Schleiden, we imagine the air from the pole to be carried to the equator, some time must elapse before it will acquire the same velocity of motion from west to east which is always found there. Therefore it would remain behind, the earth gliding, as it were, from beneath it; or, in other words, it would have the appearance of an east wind. Lieutenant Maury adopts the same explanation. It is, indeed, that of Halley, slightly modified.

The warm air, ascending from the equatorial regions, rushes to the poles to be cooled in turn, sliding over the heavy strata of cold air below.

The northern trade-wind prevails in the Pacific between 2° and 25° of N. Latitude; the southern trade, between 10° and 21° of S. Latitude. In the Atlantic the trades are generally limited by the 8th and 28th degrees of N. Latitude. The region of calms lies between these trades, and beyond them are what are styled the Variables. In the former the seaman finds baffling winds, rain, and storms. Occasionally, from causes not yet fully explained, north and south periodical winds break in upon them, such as the Northers which rage in the Gulf of Mexico.

There are many curious facts connected with the Trades, and with the Monsoons, or trade-winds turned back by continental heat in the East Indies, the Typhoons, the Siroccos, the Harmattans, land and sea breezes and hurricanes, the Samiel or Poison Wind, and the Etesian. The Cyclones, or rotary hurricanes, offer a most inviting field for observation and study, and are an important branch of our subject. But we are obliged to omit the consideration of these topics, to be taken up, possibly, at some other opportunity. The theory of the Cyclones may be justly considered as original with our countryman, Mr. Redfield. Colonel Reid, Mr. Piddington, and other learned Englishmen have adopted it; and so much has been settled through the labors of these eminent men, that intelligent seamen need fear these storms no longer. By the aid of maps and sailing-directions they may either escape them altogether, or boldly take advantage of their outward sweep, and shorten their passages.

We have yet to ascertain the causes of the many local winds prevailing both on the ocean and the land, and which do not appear to be influenced by any such general principle as the Trades or the Monsoons.

The force of air in motion gives us the gentle breeze, the gale, or the whirlwind. At one hundred miles an hour it prostrates forests. In the West Indies, thirty-two pound cannon have been torn by it from their beds, and carried some distance through the air. Tables of the velocity of winds are familiar to our readers.

Let us next advert to the connection of the atmosphere with Vapor and Evaporation. The vapor rising from the earth and the sea by evaporation, promoted by dry air, by wind, by diminished pressure, or by heat, is borne along in vesicles so rare as to float on the bosom of the winds, sometimes a grateful shade of clouds, at other times condensed and gravitating in showers of rain. Thus it enriches the soil, or cools the air, or reflects back to the earth its radiated heat. At times the clouds, freighted with moisture, present the most gorgeous hues, and we have over us a pavilion more magnificent than any ever constructed by the hand of man. These clouds are not merely the distilleries of rain, but the reservoirs of snow and hail, and they are the agents of electric and magnetic storms.

Notwithstanding their variety, clouds are easily classified, and are now by universal consent distinguished as follows.

In the higher regions of the air we look for the Cirri, the Curl Clouds. They are light, lie in long ranges, apparently in the direction of the magnetic pole, and are generally curled up at one extremity. They are sometimes called Mackerel Clouds. They are composed of thin white filaments, disposed like woolly hair, feather crests, or slender net-work. They generally indicate a change of weather, and a disturbance of the electric condition of the atmosphere. When they descend into the lower regions of the air, they arrange themselves in horizontal sheets and lose much of their original type. The Germans call them Windsbäume, or wind-trees.

The Cumulus is another form of cloud, which floats along in fleecy masses, in the days of summer, but dissolves at night. Sometimes it resembles a great stack or pile of snow, sometimes it has a silvery or a golden edge, as if we saw a little of the lining. Sometimes they lie motionless in the distance, and are mistaken by mariners for land. They rest upon a large base, and are borne along by surface-winds. Their greatest height is not more than two miles. They carry large quantities of moisture with them, and, when preceding rain, fall rapidly into other shapes.

The Stratus, or Fall Cloud, is horizontal in its figure, lies near the earth, and its length is usually greater than its breadth. It floats in long bands with rounded or sharpened points, and is seen rising from rivers or lakes, at first as a fog. In the morning it indicates fine weather. The Fall Cloud never discharges rain.

This comes only from the Nimbus, which is quite unlike the others. It puts on a dark gray color, has irregular transparent edges, and increases rapidly so as to obscure the sky. It appears to absorb the other clouds, to be a union of their differently electrified particles, which are attracted to each other, form drops of water, and descend as rain.

Of the first three forms we have three modifications or varieties. The Cirro- Cumulus is a congeries of roundish little clouds in close horizontal position, varying in size and roundness, and often, to use the words of the poet Bloomfield, appearing as

"The beauteous semblance of a flock at rest."

The Cirro-Stratus is more compact than the Cirrus,–the strata being inclined or horizontal. It is sometimes seen cutting the moon's disc with a sharp line. The Cumulo-Stratus, or Twain Cloud, is denser than the Cumulus, and more ragged in its outlines. It overhangs its base in folds, and often bears perched on its summit some other form of cloud, which inosculates itself with it. Sometimes a Cirro-Stratus cloud comes along and fastens itself to it parasitically. It is one of our most picturesque forms of clouds.

Within the last two years we have twice observed in the city of New York, during the summer afternoons, large masses of clouds coming over from the southwest, and hanging rather low, which could not be well placed in any of the classes already described, or recognized as such by meteorologists. They consisted of a great number of hemispherical forms of large diameter, hanging vertically from a Stratus cloud or plane above them, and to which they appeared attached. They were regular in shape, and very distinct; they barely touched each other, and were of a gray color. They might be compared to a hay-field turned upside down, with innumerable hay-cocks hanging below it. Unfortunately, the circumstances under which the spectacle was observed did not; admit of any resort to the barometer, thermometer, or anemometer. Should further observations verify these remarks, it might perhaps be proper to style this variety the Hemispherical.

Dew is another atmospheric product. It is the condensation of the warmer vapor of the atmosphere, in calm and serene nights, and in the absence of clouds, by the cold surface of bodies on which it rests. In some countries it is copious enough to supply the want of rain. The earth radiates its own acquired heat, grows colder than the atmosphere, and so condenses it.

What is thermometrically called the dew-point is that degree at which the moisture present in the atmosphere, on being subjected to a decrease of temperature, begins to be precipitated or condensed. It is the same as the point of saturation. Daniell calls it "the constituent temperature of atmospheric vapor." It is our criterion for ascertaining how much moisture there is in the air, and at what degree of heat or cold it would be precipitated. When the air is saturated, a dry bulb and a wet bulb will read alike.

The dew-point has been a puzzle to most persons. Very few treatises explain it satisfactorily. The definition just given, though explicit, is not quite enough. For it will be perceived that an ordinary subtraction of the degrees of temperature on a wet thermometer, which had cooled down by evaporation, from the actual temperature indicated by a dry thermometer, will not give us the dew- point.