so that day and night alternately decline into each other by a gentle and pleasing gradation. By reason of the refractive quality of the atmosphere the sun appears to us as above the horizon before he actually arrives there; and though this phenomenon is of little importance to the inhabitants of the equatorial regions, in the polar regions, where the refractive power of the air is greatly aided by the thick vapours which are constantly floating in the air, the twilight and the latter phenomenon contribute greatly to cheer the long night of winter. Under the poles the dawn and twilight lasts each a month. The red or glowing dawn is occasioned by the vapours of the atmosphere reflecting only the red and yellow beams of the sun, which are also the lightest, and have inflexibility enough to penetrate through the atmosphere. The very refrangible rays seldom reach us.

Meteoric Stones.] M. Humboldt has, after a careful investigation of the nature and origin of those hard metallic bodies which occasionally appear to fall from the heavens, without any visible acquired projection from our earth, given it as his opinion that these irregular substances do not belong to our atmosphere. Some naturalists maintain that they are projected from the moon by some powerful volcanic agency; others, that they are detached portions of small moveable planets which circulate round our atmosphere. One hypothesis views them as fragments of a large planet which formerly existed between Mars and Jupiter, and of which the four small planets, Ceres, Pallas, Juno, and Vesta, are remaining portions; and another regards them as minerals in their primitive state, which have been ejected from the very interior of our globe, by volcanoes, situ. ated in the polar regions, which produce, at the same time, the phenomenon of the aurora borealis. A later and more probable hypothesis accounts for the origin of these bodies on the principles of chemical combination. The atmosphere contains various gaseous substances; and it is no more impossible that such formations should take place by their combination in the upper regions of the air, than that chemists should be capable of producing a solid substance by the mixture of two aeriform fluids, or that a gaseous fluid should under certain known circumstances undergo fixation.

Atmospherical Dust.] Professor Rafinesque, of New York, maintains that an imperceptible dust falls at all times from the atmosphere, and that he has seen it on Mount Etna, on the Alps, on the Alleghany and Catskill mountains in America, and also on the ocean. This is the same dust which accumulates in our apartments, and renders itself peculiarly visible in the beams of the sun. He has found it to accumulate at the rate of from one-fourth of an inch to one inch in a year, but in such a fleecy state that it could be compressed to one-third of its height. Hence he takes the average of the yearly deposite at about one-sixth of an inch.

Winds.] The winds are the most remarkable and important phenomena which the atmosphere presents. Wind is a motion of the air; and every thing which disturbs the equilibrium of the air excites wind. The principal cause of wind is a change of temperature in the atmospheric regions, whether caused by the action of the sun, or that of foreign particles introduced into it. Thus isolated clouds are often known to excite wind, and the action of thunder-clouds causes violent tempests. It is clear, that as the rays of the sun descend perpendicularly on the surface of the earth under the torrid zone, that part of it must receive a greater proportion of heat than those where they fall obliquely; the heat thus

acquired communicates to the air, which it rarefies and causes to ascend. and the vacuum occasioned by this operation is immediately filled by the chill air from the north and south. The diurnal motion of the earth gradually lessens from the equator to the poles, where it moves at the rate of 15 geographical miles in a minute: this motion is communicated to the atmosphere in the same degree; therefore, if part of it was conveyed instantaneously, from latitude 39°, it would not directly acquire the velocity of that at the equator: the ridges of the earth meet it, and give it the appearance of an east wind; the effect is similar upon the cold air proceeding from the north and south, and this similarity must be admitted to extend to every place particularly heated by the beams of the sun. The moon being a large body, situated comparatively near the earth, is known to affect the atmosphere in its revolutions; it cannot, therefore, be doubted, that some of the winds we experience are caused by her motion.

East Passage Winds.] Between the tropics, and even beyond them, to the 28th and 32d, and on the N.E. American coast to the 40th degree of latitude, a constant wind blows, which is called by sailors the passage wind. In the northern hemisphere this wind is more or less N.E., and in the southern S.E. near land, and particularly where the coasts are high, it is sometimes perceived to follow the direction of the coasts; but upon extensive plains, or beyond the land, it appears again. During rain, or thunder-storms, it is sometimes interrupted by calms and opposite currents of wind. On its extreme boundaries, and in the middle near the equator, where storms are generally rare, we observe frequent calms accompanied with rain, and alternating with light varying breezes. In this respect a part of the Atlantic, between the 4th and 10th degree of south latitude, is in bad repute amongst sailors, as the peculiar region of thunder-storms, calms, and violent gushes of rain, which alternate with sudden squalls.

Monsoons.] The passage-winds are most regular in the open ocean; but from this rule the Indian Ocean forms an exception. In the southern part of this body of water, to the 10th degree of south latitude, we find the east passage wind; but advancing northwards we meet with regularly alternating winds blowing from a free quarter. These have received the name of monsoons or moussons, from the Malay mussin' or, season.' Northwards from the equator, a strong S.W. wind, accompanied with storms and rain, blows from April to October; and during the remainder of the year, a gentle N.E. wind prevails. These two winds are the monsoons properly so called. Three kinds of winds blow during winter in the Indian Ocean, viz. the N.E. on the south of the equator; the NW. south of the equator to the 10th degree of latitude; and the constant S.E. passage wind to the south of this latitude. In summer there are only two winds, viz. the S.W. from the 10th degree of south latitude northwards; and the S.E. passage wind to the south of this. Variable winds occur throughout the whole year in a small district at the equator. When the monsoon is about to change, which takes place within two or four weeks after the equinox, it grows gradually weaker, and sudden calms and blasts of winds alternate with thunder-storms, water-spouts, whirlwinds, and the frightful typhoons or hurricanes, till the new monsoon sets in. This convulsed and perpetually shifting atmosphere extends from the coast of Africa to the Philippine islands, and New Guinea. The N.E. monsoon causes strong currents in the Indian Ocean,

which change with the monsoon. The cause of these winds has not yet been ascertained. Perhaps one of the principal causes may be found in the fact, that the Indian Ocean is surrounded on the north by a high coast, from which the clouds and vapours collected during the summer, recede in winter when following the sun's course.

Sea and Land Winds.] To the periodical passage-winds belong the sea and land-winds, which generally blow upon the mountainous lands and coasts of the torrid zone, but occur in hot weather in the temperate zone also, and even in Norway. In calm weather these winds change daily and regularly, at 9 A. M. in some places, and at noon, or 1 P. M. in others, a soft gentle breeze rises and blows from the sea till about midnight, when a similar breeze begins to blow from the land to the sea, which lasts till morning. Forster says, that the South Sea islands, notwithstanding their small extent, enjoy these winds. The strength and effect of these winds is generally proportioned to the size of the country from which, or towards which they blow; but they are seldom perceptible above a few miles inland or at sea. They are evidently produced by the rarefication of the air on the surface of the land during the day, and the reduction of its temperature at night. The sea preserving a more equable temperature than the land, communicates it to the air above it; but as the land and its superincumbent atmosphere can only heaten gradually, the decided changes in the temperature can only occur at considerable intervals, or at noon and midnight.

Changeable Winds.] The changeable winds are of most frequent occurrence in the temperate and cold zones. They blow irregularly both as regards strength, duration, and direction, and have in general a more rapid motion than the constant winds. Some of these winds, however, occur more frequently than others, and these are called reigning winds. To this class belong the west winds of the temperate, and the east winds of the cold zone. Some countries are more liable to violent storms than others. Thus the Cape of Good Hope, the southern parts of America, and New Holland, the whole of New Zealand and Japan, and several other countries, are subject to violent storms, particularly during the winter season. The equinoctial storms which generally occur about the period of the equinox, are dreaded even in European seas. The prevailing winds of Britain, as ascertained by the Royal Society of London, are as follow :

The same observations show, that the S.W. wind blows more upon an average in each month of the year, than any other, particularly in July and August; that the N.E. prevails during January, March, April, May, and June, and is most unfrequent in February, July, September, and December; the N.W. occurring more frequently from November to March, and less so in September and October than in any other months. In the 5th volume of the Statistical Account of Scotland, there is a table of seven years' close observation made by Dr. Meek, near Glasgow the average of which is stated as follows:

In Ireland the prevailing winds are the W. and S.W.

These variable winds cannot be so readily accounted for; yet it is evident that, though they seem the effect of capricious causes, they depend upon a regular system, arranged by the great Author of Nature. That accurate and successful observer, the celebrated Franklin, discovered, in 1740, that winds originate at the precise point towards which they blow. This philosopher had hoped to observe an eclipse of the moon at Philadelphia, but was prevented by a N.E. storm, that commenced at seven in the evening. This he afterwards found did not occur at Boston till eleven; and upon inquiry, he had reason to suppose it passed to the N.E., at the rate of about 100 miles an hour. The manner in which he accounts for this retrograde proceeding is so satisfactory, that we shall give it in his own words, particularly as his assertions are supported by recent observations, both in America and Scotland. He argues thus: "I suppose a long canal of water, stopped at the end by a gate; the water is at rest till the gate is opened, then it begins to move out through the gate, and the water next the gate is first in motion, and moves on towards the gate; and so on successively, till the water at the head of the canal is in motion, which is last of all. In this case all the water moves indeed towards the gate, but the successive times of beginning the motion are in the contrary way, viz. from the gate back to the head of the canal. Thus, to produce a north-east storm, I suppose some great rarefication of the air, in or near the Gulf of Mexico; the air rising thence has its place supplied by the next more northern, cooler, and therefore denser and heavier air; a successive current is formed, to which our coast and inland mountains give a north-east direction."-According to the observations made by captain Cook, the N.E. winds prevail in the Northern Pacific Ocean, during the same spring months they do with us; from which facts it appears the cold air from America and the north of Europe, flows, at that season, into the Pacific and Atlantic Oceans. Velocity of Winds.] The following is a table of the different velocities and forces of the winds, according to their common appellations : Velocity-1 mile per hour

2 3

4

5

www

Hardly perceptible.

Just perceptible.

Gentle pleasant wind.

10

15

20

50

100

{

Pleasant brisk gale.

Very brisk.

High winds.

Very high.

A storm or tempest.

A great storm.

A hurricane.

A hurricane that tears up trees, and carries buildings, &c. before it.

The force of the wind is nearly as the square of the velocity, or bu little above it, in these velocities. But the force is much more than in the simple ratio of the surfaces, with the same velocity, and this increase of the ratio is the more, as the velocity is the more. By accurate experiments with two planes, the one of 17 square inches, the other of 32, which are nearly in the ratio of 5 to 9, Dr. Hutton found their resistances, with a velocity of 20 feet per second, to be the one 1,196 ounces,

and the other, 2,542 ounces; which are in the ratio of 8 to 17, being an increase of between one-fifth and one-sixth parts more than the ratio of the surfaces.

Direction of the Winds.] The winds do not move in the same direction through the atmosphere. On the contrary, we frequently observe the wind of the upper regions blowing in a totally different direction from that of the lower. The very loftiest regions of the atmosphere enjoy probably uninterrupted serenity. The winds generally move in a horizontal direction, and hence they receive their names from the quarter of the horizon from which they blow; but we also find currents of air moving in a direction inclined to the plane of the horizon and at various angles. It is a remarkable fact, that violent currents of air pass along, as it were, within a line, without sensibly agitating that beyond them. An instance of this kind occurred at Edinburgh, when the celebrated aeronaut, Lunardi, ascended in his balloon, which was conveyed with great velocity, by the wind, at the rate of 70 miles an hour, while a perfect calm existed in the city and neighbourhood.

Hurricanes.] The most dreadful storms which occur in the atmosphere are the hurricanes by which the Antilles and Mascarenes are so often ravaged. The fury of these appalling phenomena is beyond all idea, they appear to be a horrid mixture and chaos of all the elements, lightning, thunder, rain, hail, and wind, and communicate to the air a velocity surpassing that of gunpowder, which sweeps away every object which it meets with in its course, and lays whole districts bare and desolate. Mr. Stewart, in his View of the Past and Present State of Jamaica, says:"A hurricane is usually preceded by awful and certain prognostics. An unusual calm prevails; not a breath of wind is felt; the atmosphere is close and sultry, the clouds wild, broken, and perpetually and rapidly shifting; at length a deep and portentous gloom gradually settles and overspreads the hemisphere; the sun is enveloped in darkness; a deep, hollow, murmuring sound, is indistinctly heard, like the roaring of a distant cataract, or the howling of winds through remote woods,-rapid and transient gusts of wind and rain speedily succeed,-various birds of passage are seen hastily driving along the sky, or are thrown down by the violence of these gusts,-even the cattle grazing in the fields, as if instinctively aware of the approaching danger, withdraw to the thickets for shelter. The blasts soon become more impetuous; at one moment they rage with unconceivable fury, and the ensuing instant seem as it were suddenly to expire. In a few hours the hurricane reaches its acme of violence-when all the winds of heaven, and from every point of the compass, winged with destruction, seem let loose from their caverns. The largest trees are thrown prostrate, or shattered and stripped of their foliage; the provision-grounds are laid waste; the sugar-canes levelled to the earth, and in the more exposed situations, torn up by the roots, and wafted about like chaff. Many of the dwellings are blown down, or unroofed, and their inhabitants too often either buried in the ruins, or driven forth to perish unsheltered. Nothing can be more appalling than the wild howling and threatening violence of a hurricane during the night, when the vivid and quickly-succeeding gleams of lightning, darting athwart the heavens, make darkness visible,' and heighten the horror of the scene." Fortunately, however, their duration is usually short, and their sphere of action very limited. Their usual prognostication is a small cloud which appears above some mountain, and spreads out with