Buch lesen: «The Atlantic Monthly, Volume 06, No. 33, July, 1860»
METEOROLOGY
A GLANCE AT THE SCIENCE
The purpose of this article is to present, in a brief and simple manner, the leading principles on which the science of Meteorology is founded,–rather, however, in the spirit of an inquirer than of a teacher. For, notwithstanding the rapid progress it has made within the last thirty years, it is far from having the authority of an exact science; many of its phenomena are as yet inexplicable, and many differences of opinion among the learned remain unreconciled on points at first sight apparently easy to be settled.
Meteorology has advanced very far beyond its original limits. Spherical vapor and atmospheric space give but a faint idea of its range. We find it a leading science in Physics, and having intimate relations with heat, light, electricity, magnetism, winds, water, vegetation, geological changes, optical effects, pneumatics, geography,–and with climate, controlling the pursuits and affecting the character of the human race. It is so intimately blended, indeed, with the other matters here named, as scarcely to have any positive boundary of its own; and its vista seems ever lengthening, as we proceed.
Without dwelling upon the numerous consequences which flow from meteorological influences, let us see what is properly included under the subject of Meteorology. And first, of the Atmosphere.
This is a gaseous, vapor-bearing, elastic fluid, surrounding the earth. Its volume is estimated at 1/29th, and its weight at about 43/1000ths, that of the globe. It is composed of 21 parts in weight of Oxygen and 77 of Nitrogen, with a little Carbonic Acid, Aqueous Vapor, and a trace of Carburetted Hydrogen. There are numerous well-known calculations of the proportions of the various constituents of the atmosphere, which we owe to Priestley, Dalton, Black, Cavendish, Liebig, and others; but that given by Professor Ansted is sufficiently simple and intelligible. In 10 volumes or parts of it, he gives to
and he adds a trace of Ammoniacal Vapor. It is usual to state the proportions of air as being 1 Oxygen to 4 Nitrogen.
It is a curious fact, that, while there are six varieties of compounds of nitrogen and oxygen, but one of these is fitted to sustain life, and that is our atmosphere.
It is well enough to note, that, when we use the word volume or measure, in speaking of the atmosphere or any gaseous body, we adopt the theory of Gay- Lussac, who discovered that gases unite with each other in definite proportions whenever they enter into combination. This theory led to important results; for by knowing the elements of a compound gas, we easily determine its specific gravity.
It has been attempted to apply the principle to organic bodies; but it has not yet been carried to a full and satisfactory conclusion. It may be noticed, too, that Dalton affirmed that simple substances unite with each other in definite weights to form compound substances, thus supporting the idea of Lussac. These discoveries were made about the same time, Dalton having the credit of originating them. Various modifications of the principle have been from time to time presented to public attention.
Whether the constituents of the atmosphere are chemically or mechanically combined,–one of the things about which the learned are not fully agreed,–it is found to be chemically the same in its constituents, all over the world, whether collected on mountains or on plains, on the sea or on the land, whether obtained by aëronauts miles above the earth or by miners in their deepest excavations. On the theory of its mechanical combination, however, as by volume, and that each constituent acts freely for itself and according to its own laws, important speculations (conclusions, indeed) have arisen, both as regards temperature and climatic differences. It should be observed, that volume, as we have used the word, is the apparent space occupied, and differs from mass, which is the effective space occupied, or the real bulk of matter, while density is the relation of mass to volume, or the quotient resulting from the division of the one by the other. Those empty spaces which render the volume larger than the mass are technically called its pores.
Has the composition of the atmosphere changed in the lapse of years? On this point both French and German philosophers have largely speculated. It is computed that it contains about two millions of cubic geographical miles of oxygen, and that 12,500 cubic geographical miles of carbonic acid have been breathed out into the air or otherwise given out in the course of five thousand years. The inference, then, should be, that the latter exists in the air in the proportion of 1 to 160, whereas we find but 4 parts in 10,000. Dumas and Bossingault decided that no change had taken place, verifying their conclusion by experiments founded on observations for more than thirty-five years. No chemical combination of oxygen and nitrogen has ever been detected in the atmosphere, and it is presumed none will be.
The atmosphere possesses, as may be readily imagined, many important characteristics. One of these is Weight.
This is demonstrated by simple, yet decisive experiments. The discovery of the fact is attributed to the illustrious Galileo, but to modern science we owe all the certainty, variety, and elegance of the demonstration. A vessel containing a quantity of air is weighed; the air is exhausted from it and it is weighed again. An accurate scale will then detect the difference of weight. A cubic foot of air weighs 1.2 oz. Hence a column of air of one inch in diameter and a mile in height weighs 44 oz.
The atmosphere is supposed to have an elevation of from 45 to 50 miles, but its weight diminishes in proportion to its height. The whole pressure at the surface of the earth is estimated to be 15 lbs. to the square inch; a person of ordinary size is consequently pressed upon by a weight of from 13 to 14 tons. Happily for us, the pressure from without is counteracted by the pressure from within.
The weight of the air is of great importance in the economy of Nature, since it prevents the excessive evaporation of the waters upon the earth's surface, and limits its extent by unalterable laws. Water boils at a certain temperature when at the earth's surface, where the weight of the atmosphere is greatest, but at different temperatures at different elevations from the surface. At the level of the sea it boils at 212°. On the high plains of Quito, 8,724 feet above the sea, it boils at 194°, and an egg cannot be cooked there in an open vessel. At Potosí the boiling-point is still lower, being 188°, and the barometrical column stands at 18°. Indeed, the experiment is often exhibited at our chemical lectures, of a flask containing a small quantity of water, which, exhausted of air, is made to boil by the ordinary heat of the hand.
Fahrenheit proposed to ascertain the height of mountains by this principle, and a simple apparatus was contrived for the purpose, which is now in successful use. The late Professor Forbes of Edinburgh, whose untimely death the friends of science have had so much reason to deplore, ascertained that the temperature of boiling water varied arithmetically with the height, and at the rate of one degree of the thermometric scale for every 549.05 feet. Multiplying the difference of the boiling-point by this number of feet, we have the elevation. The weight of the atmosphere, as indicated by the barometer, is also a means for ascertaining the height of mountains or of plains; but correction must be made for the effects of expansion or contraction, and for capillarity, or the attraction between the mercury and the glass tube, at least whenever great exactness is required. Tables for the convenience of calculation are given in several scientific works, and particularly in a paper of Professor Forbes, Ed. Trans. Vol. 15. Briefly, however, we may state, that between 0° and 32°, 34 thousandths of an inch must be allowed for depression or contraction, and between 32° and 52° 33 thousandths. The weight of the atmosphere is not only affected by rarefaction, but by currents of air, which give it a sudden density or rarity. Those who have ascended mountains have experienced both these changes.
A common experiment to prove the weight of air is that of the Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a merchant of that city. It is a part of every complete philosophical apparatus. It consists of brass caps, which, when joined together, fit tightly and become a globe. The air within being exhausted, it will be found difficult to separate them. If the superficies be 100 square inches and the height of the mercury be 30 inches, the atmosphere will press on these hemispheres with a weight of 1,475 lbs, requiring the efforts of seven or eight powerful men to tear them asunder. One of these instruments, of the diameter of a German ell, required the strength of 24 horses to separate it. The experiment was publicly made in 1650 at the Imperial Diet at Rendsborg, in the presence of the Emperor Ferdinand III. and a large number of princes and nobles, much to their astonishment.
As compared with water, the air (the barometer indicating 30°, and the thermometer 55°) is 833 times lighter.
It is this weight of the atmosphere which counterbalances that of a column of mercury 29 inches in height, and a column of water 32 to 34 feet in height.
The old quaint notion of Nature's abhorring a vacuum was found to be practically only an assertion that the air had weight. The ordinary pump, commonly called the suction-pump, is constructed on this principle. The weight of the atmosphere at the level of the sea is found to be the same all over the world.
We find the atmosphere with another characteristic,–Elasticity.
However it may be compressed, air returns, on liberation, to its original volume, and while thus perfectly elastic it is also the most compressible of bodies. This elasticity arises from the repulsive force of its particles, and is always equal to the compressive force which it balances. A glass vessel full of air, placed under a receiver and then exhausted by the air-pump, will burst into atoms. Water, on the other hand, is almost the reverse. Twenty cubic inches, introduced into a cannon whose sides are three inches thick, cannot be compressed into nineteen inches without bursting it. This non-elastic property of water, with another, that of communicating, when under the action of any force, an equal pressure in all directions, led to the invention of the hydraulic press.
The elasticity of the air enables fishes to rise and sink in water, through the action of the air-bladder.
The sudden compression of air liberates its latent heat, and produces fire. On this principle the pneumatic tinder-box is constructed.
Brockhaus says that air has as yet been compressed only into one-eighth of its original bulk.
For every degree of heat between the freezing-point and the boiling-point, 32° and 212°, the expansion of air is about 1/490th part, so that any invention which seeks to use rarefied air as a motive power must employ a very intense degree of heat, enough to fuse many kinds of metals.
To the celebrated Mr. Boyle and to Henry Cavendish, both of Great Britain, we are indebted for most of what we know of this particular property of the air.
Density, or closeness, is another quality of the atmosphere. It has been found to be 770 times less than that of water, and 770 cubic inches of air weigh as much as a cubic inch of water. It is in direct ratio with its elasticity, and there are tables by which it may be determined at different altitudes. At the surface of the earth, this density is indicated as 1; at 2-1/2 miles, as 1/2; at 5 miles, as 1/4; and so on, the difference being in a geometrical progression.
As we proceed in the consideration of our general subject, we shall find, under the appropriate heads, that density is not without material influence on reflection and refraction, on transparency and the transmission of light, the presence or absence of moisture, and the amount of heat at the earth's surface,– -and we might add, on health, and the increase or diminution of the vital energies.
Temperature is another branch of our subject, and one involving a series of subordinate topics on which volumes have been written, and to which are still devoted the labors of the most learned men of our day. In this place, merely an out-line can be attempted.
Temperature is the degree of heat or cold in the particles of all bodies, which is perceptible by sensation, and is measurable by their expansion or contraction. It is the key to the theory of the winds, of rain, of aerial and oceanic currents, of vegetation and climate with all their multifarious and important differences. While the inclined position of the earth on its axis and its movement in its elliptical orbit influence the general amount of heat, it is rather to the consequences of these in detail that we are called when we speak of temperature. If the sun shone on a uniformly level surface, everywhere of the same conducting and radiating power, there would be but little difficulty in tracing the monotonous effects of temperature.
The reformer Luther, as eccentric as he was learned and sincere, is reported to have said, that, if he had been consulted at the Creation, he would have placed the sun directly over the centre of the world and kept it there, to give unchanging and uniform light and heat! It is certainly much better that he was not consulted. In that case, every parallel of latitude would have been isothermal, or of equal mean annual temperature. The seasons would have been invariable in character. Some portions of the earth would have been scorched to crispness, others locked up in never-changing ice.
Vegetation, instead of being universal, would have been confined to a narrow zone; and the whole human race would have been driven together into one limited habitable space, to interfere with, incommode, and destroy each other. The arrangement is best as it is.
We find very important modifications of temperature, occasioned not only by astronomical influences, but by local causes and geographical characteristics. For while, as a general rule, the nearer we approach the equator, the warmer we shall be, yet temperature is greatly affected by mountains, seas, currents of air or water, by radiation, by forests, and by vegetation. It is found, in fact, that the lines of temperature, (the happy conception of Humboldt,) when they are traced upon the map, are anything but true zones or circles.
The line of the greatest mean warmth is not coincident with the equator, but falls to the north of it. This line at 160° W. Long, from Greenwich is 4° below the geographical equator; at 80° it is about 6° north, sweeping along the coast of New Granada; at 20° it comes down and touches the equator; at 40° E. Long., it crosses the Red Sea about 16° north of the equator, and at 120° it falls at Borneo, several degrees below it;–and the points of the greatest heat, in this line, are in Abyssinia, nearer the tropic of Cancer than to the equator. On the other hand, the greatest mean cold points, according to the opinions of Humboldt, Sir David Brewster, and others, do not coincide, as would seem natural, with the geographical poles, but they are both to be found in the northern hemisphere, in Latitude 80°, 95°E. Long. and 100° W. Long. from Greenwich. The western is ascertained to be 4-1/2° colder than the eastern or Siberian. If this be the fact,–but it is not positively admitted,–an open sea at the pole may be considered as probable, on the ground of its having a higher mean temperature than is found at 80°. Kaemptz places one of these cold points at the north of Barrow's Straits,–the other near Cape Taimur, in Siberia. Burghaus, in his Atlas, transfers the American cold pole to 78° N. Lat. It is perhaps too early to determine rigorously the true temperature of these points.
A noticeable fact also is this,–that places in the same latitude rarely receive the same amount of heat. Quebec, in British America, and Drontheim, in Norway, enjoy about the same quantity, while the former is in 47° and the latter in 68° N. Lat. The mean winter temperature of Pekin, 39° 45' N. Lat., is 5° below the freezing-point; while at Naples, which is north of Pekin, it seldom, if ever, goes below it, and Paris, 500 miles farther north, has a mean winter temperature of 6° above the freezing-point. The city of New York, about 11° south of London, has a winter temperature of much greater severity. The mean temperature of the State of New York, as determined by a long series of observations, is 44° 31'.
The mean temperature of countries is found to be very stable, and but very small variations have been detected in modern times. But that there have been important climatic changes, since the Christian era, cannot be doubted, unless we doubt history. Not many centuries ago, it was a common thing for all the British rivers to freeze up during the winter, and to remain so for several 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 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.
