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less severe, or if the small particles of ice, after they are formed, passi through a warmer stratum of air, they will be partially melted, and adhere, and thus the snow will fall in broad flakes. Neither snow nor hail can be formed unless the temperature be at or under 32° of the thermometer; but whenever the temperature sinks so low as this, water cannot exist in the liquid state; and its change from that state will be rapid in proportion to the minuteness of the single quantities. The upper strata of the air are, in consequence of its being more expanded, always colder than the lower; and thus a snow cloud, or hail-stones, may be formed there, even though the air lower down and surface of the ground may at the time be so warm as that snow or ice exposed to them would be instantly melted. The hail-stones which fall from a great height in warm climates, and at warm seasons of the year, appear from their structure to be first formed as drops of rain, and then crystallized in their fall through some cold stratum of the air; for if a large one be cut, there is a shell of very perfect ice on the outside, while the interior is fibrous, as if made up of small threads of ice with air in the interstices.

Rain, whether in the liquid state, or congealed as snow, falls much more frequently, and in greater abundance, in mountainous countries than on plains, even supposing them to be situated at the same distance from the sea. Thus, according to Mr. Dalton's account, the annual quantity of rain which falls at Keswick, in Cumberland, is more than three times that which falls at Upminster, in Essex,-the annual number of inches being 67 at the former place, and only 19 at the latter. Two causes conspire to produce this effect:-First, the unequal surface of a mountainous country subjects the air to continual varieties of temperature, and constant currents: for instance, if the sun is shining strongly upon one side of a mountain, while the other is in the shade, the same effect will be produced as if it were day on the illuminated side, and night on the other; a current will, in consequence, be continually rising from the warm side and descending to the cold, and the result will be, a wind blowing across the top of the mountain from the shaded side. From this cause clouds in all their varieties must have many more chances of being formed in a mountainous country than on a plain. Secondly, being formed, they will be attracted by the mountain as well by the general attraction of gravitation as by a different state of electricity subsisting between the mountain and them. Thus, around the summits of mountains we find, not only cirro-stratus, but cumulus, cumulostratus, and nimbus, collectively, with violent gusts of winds, blowing alternately in various directions, while it is calm and sunshine upon the plains, at no great distance.

DEW AND HOAR FROST.

These, which bear the same analogy to each other as rain or snow, are produced when the surface of the earth becomes suddenly colder than the air over

it. When the cold is in a moderate degree, dew is the result; and when it is greater, the dew forms into little spicule of ice, either just as it lights on the ground, or at its first formation in the air, according to the degree of temperature. There is no doubt, also, that the tendency to produce dew or hoar frost is increased by the quantity of humidity which happens to be in the air. Hence, dew falls more frequently in warm weather than in cold; and is more copious in the warm latitudes than in those where the temperature is lower.

RAINBOWS, HALOS, &C.

When the rays of the sun fall upon a mass of cloud, they are in so far reflected as to give to that cloud a light appearance, when viewed in a favorable position; but a very dark one, when seen on the side opposite to the sun. The power which a cloud has to stop and reflect back the light of the sun is of course in proportion to its density; and hence, the slightest piece of cumulus or cumulo-stratus conceals the body of the sun, while, through the light stratus of a summer morning, or the cirro-stratus which portends a storm, the disc of the luminary is still visible, though shorn of its beams, and surrounded by a train of reflected light in the fleecy parts of the cloud. When the cloud is once formed into drops and begins so fall, the light of the sun passes free through the intermediate space; but the portion which falls upon each drop is wholly reflected, and by this reflection it is divided into the prismatic hues of which light is composed, and into which it may be separated by passing the rays of the sun, or looking at the fire or a candle, through a three-cornered prism of glass, or even through a tumbler or drinking glass, any part of which is cut into diamonds. If in such circumstances a shower happens to be falling on one side of the spectator, while the sun is directly on the other side, the spectator will, upon turning his back to the sun, and looking at the cloud, perceive a rainbow, in which the colours will be displayed in beauty proportionate to the stillness of the time, and the size of the drops. The centre of the bow is a point directly opposite to the sun; and the bow extends from that centre to an eighth part of the circumference of the heavens every day, 45° being the angle at which the prismatic colours become visible by a single reflection. Of course, when the sun is just on the horizon, the bow will be a semicircle; it will gradually diminish as the sun is elevated, and if the elevation of the sun be more than 45°, the bow will not be visible. Rainbows are also produced by a second and even a third reflection. These lie without the primary bow; and have the colours alternately reversed, and are so much fainter, that the third bow is seldom visible. When it is mentioned that the rainbow never exceeds a semi-circle, the spectator is understood to be upon level ground; for, if he were so much elevated, on a tower or otherwise, as that he could see the rain falling at 45o under the level of his eye, the bow would then be a perfect circle. Rainbows may be formed in the

spray of cataracts, the foam of the ocean, dew upon the grass, a summer fog in rapid evaporation, or, indeed, in every case where light can be seen in front falling upon drops of moisture; and one may be at any time formed by sprinkling water between the observer and any dark surface which is directly exposed to the sun.

LIGHTNING AND THUNder.

Lightning, whether it takes place as a communication between one cloud and another, or between a cloud and the earth, is merely an effort of nature to restore the two substances between which the lightning passes to the same electric state; and it is violent in proportion to the different states previous to the discharge. The thunder, or noise, is the mere mechanical consequence of the lightning: the great heat which the lightning produces in its rapid course expands the air to almost a vacuum; and as even at the height to which thunder extends the pressure of the atmosphere is still not less than 13 or 14 pounds on a cubic inch, the tendency of the air to come together after the instantaneous fire passes, and the momentarily heated air condenses, may be equal to many thousands of tons, and quite sufficient to occasion a sound louder than that which is produced by any artillery. The sound of the latter is, indeed, of the very same kind as the former, only the combustion and expansion of the air in a discharge of artillery are produced by an artificial cause, while in the case of thunder the cause is natural. Thunder and lightning occur most frequently at those places and those seasons of the year where the heat is most intense; and a violent thunder-storm usually produces a change of the weather, by either disturbing or destroying the equilibrium of the atmospheric stratum. The lightning makes its way to the nearest point of that body which has a deficiency of electric excitement. Dry air does not convey it unless within a certain short distance, called the striking distance; and thus when a cloud, highly electrified, descends toward the earth, it may give out the electric fire with so much violence as to kill animals, burn combustibles, or even shatter rocks to pieces. It seeks for that substance in the earth which is the best conductor; and hence the summits of mountains are often ploughed into large furrows by the lightning.

If the time that elapses between observing the flash and hearing the sound be measured, the distance of the thunder may be counted by allowing 1142 feet to a second, or about 3 seconds to a mile. When very near the report is loud, and the danger considerable; but in all cases the danger is over the instant that the flash is seen. Powerful as it is, it is exhausted in an instant, and returns not again to the attack.

R. M.

CLIMATE;

OR,

THE DISTRIBUTION OF HEAT OVER THE
EARTH S SURFACE.

CLIMATE, so named from a Greek word, signifying "to incline," or "slope," and adopted at first, no doubt, because its differences are chiefly occasioned by the different inclination or slope at which the rays of the sun fall upon the earth's surface, was, by the earlier geographers, restricted to certain zones or belts of the surface, beginning at the equator, and proceeding toward the pole. The distinction between one of these climates and another, was a difference of half an hour in the length of the longest, or midsummer day; and as the difference of the length of that day increases faster than the latitude, the first climate, or that nearest the equator, measured 4 degrees, or about 295 miles from north to south, while the twentysecond climate, or that occupying the space between the parallels of Newcastle and Edinburgh nearly, extended over only 1 degree, or about 69 miles.

But though this application of the word climate, or rather this imaginary division of the earth's surface into climates, served to point out the distribution of solar light and heat, according to the average slope or angle at which these fall upon the earth, yet it by no means pointed out the actual degree to which those different latitudes are favourable or unfavourable to animal or to vegetable life; because, as will hereafter be shown, these depend upon many circumstances besides mere differences of latitude. Hence the word climate camc, by degrees, to have a more extended meaning'; and it is now used to denote that combination of circumstances which tend to make the earth desirable for man's habitation, and productive for his use, or the reverse; and, accordingly, we are accustomed to speak in general terms of a good climate or a bad climate, and to characterize particular modifications of it, as hot, cold, moist, dry, uniform, variable, healthy, un→ healthy, and a number of others, according to circumstances.

In this modern and more extended meaning of the term, the study of climate is at once one of the most amusing and instructive in which any man can be engaged. It is amusing, because it brings together all the results of our observations upon the influence of the celestial bodies, the atmosphere, and the weather, and connects them with the kinds of plants and of animals which are found to vary according as the operation of these ARTS, No. 5.

K

circumstances is varied; and not only this, but it shows us, that a great deal of the characteristics of the several nations and races of men are produced by the climates in which they live. The great heat of the regions under the equator darkens the complexion, enervates the body, and diminishes the nobler faculties of the mind; and in those regions, civilization, and the arts, and the sciences, have never been the production of the native inhabitants. It may be true that, to a considerable extent, the limited natural wants of the inhabitants of those warm regions may have a strong tendency to produce idleness and repress the active and inventive faculties. They need no clothing; they require no houses, and very little furniture ; the strong meats and stimulating liquors to which we are accustomed, and which we find, or at least believe to be necessary for counteracting the comparative coldness of our climate, and calling forth our energies, would in them produce the most violent and fatal diseases; and thus they are without that most important of all stimuli-the stimulus of necessity. On the other hand, it is found that, in regions approaching so near to the poles as that cold is strongly predominant, the vegetable tribes become fewer in number, smaller in size, and less adorned with beauty; and that man, sinking under the opposite extreme to that which relaxes his powers at the equator, becomes even more feeble and helpless, and has his mental powers even less developed than they are in the former regions. Thus the range of the latitudes, from pole to pole, furnishes us with a sort of key to the several degrees of human intelligence and comfort; and though upon the whole, the warmer regions seem to agree much better with the human constitution than the colder ones, the more energetic and intellectual part of the species has always been found in those climes which are comparatively temperate

Nor is it as a mere subject of curiosity, of cultivation of the mind, of extension of the powers, and of mental pleasure-that pleasure which the mere animal enjoyment of the world can neither impart nor disturb—that this branch of natural knowledge is deserving of general attention; for the different climates of the earth, with the different vegetables and animals that they produce, and the different wants and superfluities to which they give rise, make the world far more delightful-much more the source of activity and multiplied pleasures than if it had been one uniform Eden from pole to pole. In this is laid the foundation of commerce, and it is just because one region produces that which is desirable, but not producible in another region, that there is a basis laid for friendly and mutually profitable intercourse among all nations, and the highest interests of the whole human race tend to link them together as one great family.

Although it has been mentioned that the different degrees of influence which the light of the sun has upon different latitudes of the globe is not the sole cause of the variations of climate, yet it is a principal cause; and the only other cause which is in great or permanent operation, arises from

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