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
