THE ATMOSPHERE.—CLIMATOLOGY.
(From Part XIV. of Knowledge for the People; or, the Plain Why and Because.)
Why may the atmosphere be termed a fourth kingdom of Nature?
Because it extends its influence in an equal degree over the three kingdoms, the animal, the vegetable, and the mineral, operates upon each after a distinct manner, and appears rather to be independent, and allied to all of them, than to be rightly included within any one.
Why is a knowledge of the atmosphere important to the naturalist?
Because it serves to throw much light on the history and functions both of the animal and vegetable creation; for it is through this great medium that heat, light, electricity, oxygen, and the great springs of vital phenomena, are conveyed to all classes of organized matter. It is by means of this wonderful agent, that we gain the theory of respiration in all classes of creatures possessing animal life; and that we become acquainted with the migrations of animals, as well as many of their peculiar instincts and habits. It is the atmosphere that enables us to account for the periodical changes in the plumage of birds and the furs of animals, and the variety of colours to be found amongst them. By means also of the elasticity of the atmosphere, sounds and odours are transmitted to sensitive beings. Atmospherical phenomena, it may be safely inferred, attracted the observation of mankind in the earliest ages: we know that the Egyptians and the Greeks wrote upon the subject; the Jews too, a pastoral people, "could discern the face of the sky;" and even in our day, shepherds may be ranked among the weather-wise. "This is a fine morning, a soft day, or a cold evening," are modes of salutation with us, as commonly as is the "Salem Alikem" (Peace be with you!) amongst the inhabitants of the more serene countries of the East. Shenstone says, though with nearly equal spleen and truth: "there is nothing more universally commended than a fine day: the reason is, that people can commend it without envy."
Why do we call the atmosphere a fluid?
Because it has a tendency to move in all directions, and consequently rushes in and fills every space not previously occupied by a more solid substance. Hence we find, that every cave, crevice, place, and vessel, having communication with the atmosphere, if it be not filled with something else, is filled with air; against which it is no argument that we do not see it, as it is perfectly transparent, and consequently invisible.
Why do birds fly?
Because of the inertia of the atmosphere, which gives effect to their wings. Were it possible for a bird to live without respiration, and in a space void of air, it would no longer have the power of flight. The plumage of the wings being spread, and acting with a broad surface on the atmosphere beneath them, is resisted by the inertia of the atmosphere, so that the air forms a falcrum, as it were, on which the bird rises, by the leverage of its wings.
Why is air generally considered to be invisible?
Because, though a coloured fluid, and naturally blue, its colour acquires intensity only, or, in other words, becomes visible only, from the depth of the transparent mass. According to rigid Newtonians, air is transparent, or, rather, invisible; and the azure colour of the atmosphere arises from the greater refrangibility of the blue rays of light. Other philosophers imagine that the blue tint is inherent in air; that is, that the particles of air have the property of producing a blue colour, in their combination with light.
Why are the most distant objects in a prospect of a blue tinge?
Because their colours are always tinted by the deepening hues of the interjacent atmosphere. Again, the blending of the atmospheric azure with the colours of the solar rays, produces those compound and sometimes remarkable tints, with which the sky and clouds are emblazoned. Hence, the mountains appear blue, not because that is their colour, but because it is the colour of the medium through which they are seen.
Why do the Heavens appear blue?
Because of our looking at the dark vacuity beyond our atmosphere through an illuminated medium. Were there no atmosphere, it is universally admitted the appearance would be perfectly black, except in the particular direction of the sun, or some other of the heavenly bodies, and since the atmosphere is transparent, this blackness (if such an expression may be used) must be seen through it, only somewhat modified by the rays of light reflected by the atmosphere to the eye, from the direction in which we look. For this reason, the clearer or more transparent the atmosphere is, the darker is the appearance of the heavens, there being then less light reflected by the atmosphere to the eye. In the zenith, the appearance is always darker than nearer the horizon; and from the tops of high mountains, the heavens in the zenith appear nearly black.—Mr. B. Hallowell, in the American Journal of Science and Arts.
Why does the heat of temperature of different parts of the earth vary?
Because of the position of the place with respect to the equator, or rather to the ecliptic, or, more strictly still, with respect to the plane in which the earth revolves around the sun; for on this relation depends the temperature of the place, so far as it is produced, directly, by the influence of the sun. Maltebrun ascribes to it the following influences: 1, the action of the sun upon the atmosphere: 2, the interior temperature of the globe: 3, the elevation of the earth above the level of the ocean: 4, the general inclination of the surface, and its local exposure: 5, the position of its mountains relatively to the cardinal points: 6, the neighbourhood of great seas, and their relative situation: 7, the geological nature of the soil: 8, the degree of cultivation, and of population, at which a country has arrived: 9, the prevalent winds.
Why are the strata of air upon all mountains of successive coldness?
Because the air does not acquire immediately, by the passage of the solar rays, a considerable degree of heat. Thus, with the elevation of land, cold may be said to increase in very rapid progression. Winter continues to reign on the Alps and the Pyrenees, while the flowers of spring are covering the plains of northern France. This beneficent appointment of Nature considerably increases the number of habitable countries in the torrid zone. It is probable, that at the back of the flat burning coasts of Guinea, there exist in the centre of Africa, countries which enjoy a delightful temperature; as we see the vernal valley of Quito, situate under the same latitude with the destructive coasts of French Guyana, where the humid heat constantly cherishes the seeds of disease. On the other hand, it is the continued elevation of the ground, which, in the central parts of Asia, extends the cold region to the 35th parallel of latitude, so that in ascending from Bengal to Thibet, we imagine ourselves in a few days transported from the equator to the pole.—Maltebrun.
Why does the destruction of forests sometimes prove beneficial to a country?
Because a freer circulation of air is thus procured—but carried too far, it becomes a scourge which may desolate whole regions. We have a sad example of this in the Cape de Verde islands, not to mention others. It is the destruction of forests, and not a supposed cooling of the globe, which has rendered the southern part of Iceland more accessible to the dreadful cold which is too often produced by those masses of floating ice which are intercepted and detained by its northern coasts.—Ibid.
Why do mountains influence climates?
Because, although they cannot prevent the general motions of the atmosphere from taking place, they may, by stopping them in part, render particular winds more or less frequent throughout a certain extent of country. Maltebrun observes, there cannot be a doubt that the Alps contribute in securing to Italy its delightful and happy climate, its perpetual spring, and its double harvests.