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has been frequently noticed by poets. In rainy and changeable weather it has a light fleecy texture, and is irregular in the form of its component parts, approaching to the cirro-stratus.

The Cumulo-stratus, or twain-cloud usually presents an horizontal base upon which the cloud appears heaped or piled up, it is of common occurrence previous to rain, and sometimes changes into the the nimbus or rain-cloud. In our figure, the cumulostratus is shown at the left and nimbus at the right. The cumulo-stratus is usually formed out of the cumulus, which grows denser and spreads out laterally until it overhangs its base, while the tops remaining distinct seem like so many snow-capped mountains, or rocks piled up. When the cumulo-stratus increases in density and blackness, indicating rain, the nimbus or rain-cloud is formed. As soon as the actual rain commences the blackness is changed into a dark gray or slaty color. After the rain the clouds separate and form again cumuli, cirri, and cirro-cumuli, which float in the upper regions of the atmosphere, while the broken fragments of the nimbus sail along in the currents of wind below.

It often happens that a cloud, in descending, enters a stratum of air wartner than itself and is again converted into vapor and absorbed, a remarkable appearance of this sort is observed on the Table Mountain at the Cape of Good Hope. “Its flattop, called the Table Land, is about two miles in length from east to west, and of various breadths, but nowhere exceeding a mile. The height is estimated at 3500 feet above the sea. It is a common saying among the inhabitants of Cape Town, that when the Devil spreads his table cloth upon the mountain you may look for a strong south-east wind. In the whole system of meteorology there is not a more infallible prognostic. The Devil's tablecloth is a thin sheet of white vapor which is seen reaching over the edge of the precipice, while the sky all around is clear and unclouded. The rapidity of its descent, resembles that of water pouring over the face of a rock. The air, at the same time begins to be agitated in the valley, and in less than half an hour, the whole town is involved in dust and darkness. Instantly the streets are deserted, every window and door is shut up, and Cape Town is as still as if it were visited by the plague. Sometimes, instead of a sheet of vapor an immense cloud envelopes the mountain, and stretching out on all sides like a magnificent canopy, shades the town and adjacent country from the sun. The inferior boundary of this cloud is regulated, probably, by various circumstances, among others, by the strength of the wind, and the temperature of the air in the Table Valley. The influence of the latter is to be inferred from the fact, that though the cloud never descends more than half way into the hot parched amphitheatre of Cape Town, it may be observed on the side of Camp's Bay, rolling down in immensc volumes to the very sea, over which it sometimes stretches farther than the eye can follow it. Nothing can be more singular than the appearance of this cloud. It is continually rushing down to a certain point on the side of the mountain and there vanishing. Fleeces are seen from time to time, torn from its skirts by the strength of the wind, floating and whirling, as it were in a vortex, over the town, and then gradually dissolving away. But the main body remains, as it were, mailed to the mountain, and bids defiance to the utmost efforts of the gale. There is a constant verdure maintained on this mountain from the moisture deposited from the atmosphere.

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cLiMATE. 147

CHAP TER IV'.
Climate.

“The body, moulded by the clime, endures
Equator heat, or hypoborean frost,
Except by habits foreign to its turn,
Unwise you counteract its forming pow'r.”
Armstrong.

In the present chapter we shall very briefly consider the prominent causes which affect the climate of the various portions of the earth's surface. The subject is a very important one and we can do little else than give the great outlines, and must therefore refer the reader to the miore elaborate works of Leslie, Daniell, and Kaemtz.

The primary cause of all heat upon the surface of the earth, and its superincumbent atmosphere, is the sun, whose rays may also be regarded as the source of all life upon our planet. In the preceding pages, we have, somewhat at length, illustrated the manmerin which the sun apparently changes its position in the heavens, traversing during the year through the twelve signs of the Zodiac, in the path called the ecliptic, which is inclined at an angle of 230 28' to the celestial equator, which it crosses in two opposite points called the equinoctial points.

The celestial equator, we have shown to be in the same plane as the equator upon the earth, consequently, as the earth turns on its axis, it will happen that the rays of the sun, whenever it may be situated in the celestial equator, i. e. at the time of the equinoxes, will fall vertically, or perpendicularly upon all those places situated upon or near to the terrestrial equator. Twice a year, viz: on the 21st of March and the 21st of September, the sun is in those points of the ecliptic which cross the equator, and at this time its rays are vertical at noon at the equator, as we have just described. At these seasons of the year the changes from winter to spring, and summer to autumn, commence, and the sun is said to be crossing the line, The distribution of heat in the neighborhood of the equator is tolerably equal, for twice during the year, viz: March 21st and September 21st, the sun's rays fall vertically, and they do not fall very obliquely at any time between these two periods. From the 21st of March, the sun begins to move northword of the equator apparently, until at the 21st of June, its angular distance from the equator amounts to 23° 28′. This is the angle which the line SS' makes with the line E E!, see the figure on page 57, the former representing the plane of the ecliptic, the latter the plane of the equator. At this time, as the earth turns on its axis, the sun is vertical at noon at all those places which lie in a circle drawn upon its surface parallel to the equator, and at an angular distance of 23°28' north of it. This circle is called the tropic of Cancer, for a reason we have already explained. From the 21st of June to the 21st of September, the sun approaches the equator, which it crosses on the latter named day, it then moves farther south, until, on the 21st of December, its angular distance from the equator becomes 23° 28', and, if we suppose a circle drawn upon the earth at a distance of 23°28' from the equator, but south of it, the sun will now be vertical at all places situated on or near to this circle, which, for reasons already given, is called the tropic of Capricorn. All places therefore lying upon these tropics, receive once in the year, the sun's rays perpendicularly at mid-day, this being on thc 21st of June for the tropic of Cancer and the 21st of September for the tropic of Capricorn. At all places within these two tropics the sun is vertical at noon twice in the year ; and at all places without or beyond them, it is never vertical. The nearer we approach the tropics, leaving the equator, the more marked are the different seasons of the year, and for the following reason; once during the year, as we have just remarked, the sun's rays fall vertically at the tropics, and once they make an angle of 479 or twice 23°28', with the direction of the plumb-line, and which is the angle S S'C', see figure on page 57, falling consequently, with considerable obliquity. The hot

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test and coldest seasons being separated by a period of half a year, differ very considerably from each other in their temperature. The whole terrestrial zone lying between these two tropics is called the hot zone, or torrid zone. When the sun’s distance from the equator north is the greatest possible, i.e., when it is in the tropic of Cancer or at the point VI, see figure, page 80, the north pole of the earth is illuminated, and the south pole in darkness, as represented in the figure, page 109. If we suppose a circle traced upon the earth as shown at c d, it is evident that as the sun now illuminates all within this circle, the day will be to a spectator situated upon it, 24 hours in length, or in other words visible during a complete rewolution of the earth on its axis. A similar circle shown at gh, indicates the position in the southern hemisphere where the longest day is 24 hours. These two circles are called, the former the Arctic, and the latter the Antarctic, the former is situated 23°28' from the north pole, and consequently 66° 32' north of the equator, and the latter at the same distance from the south pole, and south of the equator. The terrestrial zones included between the tropics and the polar circles, are called the northern and southern temperate zones. The four seasons of the year are most strougly characterized in these zones, and the general rule for the dimunition of heat is, directly as the distance from the equator. Within the polar circles are the northern and southern frigid zones. As the earth turns upon its axis from west to east, the sun is apparently caused to rise in the east, move over the heavens, and set in the west, thus producing the alternation between day and night. During the day, the surface of the earth is warmed by the rays of the sun, but when these are withdrawn at night, the heat is radiated to the heavens and lost, during the night therefore the surface of the earth is cooled. We shall presently see that the vicissitudes in climate varying with the latitude, are mainly due to the unequal lengths of day and night. Under the equator the days and nights are very nearly equal, throughout the year, each lasting 12 hours. As soon, however, as we leave the equator, the length of the day varies according to the season of the year, and the difference between the day and night becomes more striking as we approach

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