months. If space permitted, an interesting statement could he made of the changes which have taken place in vegetation in Greenland, and throughout certain northern parts of Europe,–also in Palestine, Greece, and other southern countries,–while we know that the earth's inclination upon its axis has been unchanged.
Mrs. Somerville remarks, that, though the temperature of any one place may be subject to very great variations, yet it never differs from the mean state more than a few degrees.
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
