Imágenes de página
PDF
ePub
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors]

Therefore, to discover to what climate a place is, whose latitude does not exceed 66} degrees, find the length of the longest day in that place, and subtracting twelve hours from that length, the number of half hours in the remainder will specify the climate.

PROBLEM XX. To find the limits of the climates.

Elevate the north pole to 23° 28', the sun's declination on the longest day; and turn the globe easterly till the intersection of the meridian with the

equator that passes through Libra comes to the horizon, and the hour of VI will then be under the meridian, which in this problem is the hour index, because the sun sets this day at places on the equator as it does every day at VI o'clock. Now turn the globe easterly, till the time under the meridian is 15 min. past VI, and you will find that 8° 34' of that graduated meridian is cut by the horizon; this is the beginning of the second climate; and the limits of all the climates may be determined, by bringing successively the time equal to half the length of the longest day under the meridian, and observing the degree of the graduated meridian cut by the horizon.

ZONES.

Zones is another division of the earth's surface, used by the ancients : that part which the sun passes over in a year, comprehending 234 degrees on each side the equator, was called by the ancients the torrid zone. The two frigid zones are contained between the polar circles. Between the torrid and the two frigid zones are contained the two temperate ones, each being about 43 degrees broad.

The latitude of a place being the mark of its position with respect to the sun, may be considered as a general index to the temperature of the climate: it is, however, liable to very great exceptions; but to deny it absolutely, would be to deny that the sun is the source of light and heat below.

Nothing can be more hideous or mournful than the pictures which travellers present us of the polar regions. The seas, surrounding inhospitable coasts, are covered with islands of ice, that have been increasing for many centuries : some of these islands are immersed six hundred feet under the surface of the sea, and yet often rear up also their icy heads more than one hundred feet above its level, and are three or four miles in circumference. The following count will give some idea of the scenery produced by arctic weather. At Smearingborough Harbour, within fifteen degrees of the pole, the country is full of mountains, precipices, and rocks; these are covered with ice and snow. In the vallies are hills of ice, which seem daily to accumulate. These hills assume many strange and fantastic appearances; some looking like churches or castles, ruins, ships in full sail, whales, monsters, and all the various forms that fill the universe. There are seven of these ice-hills, which are the highest in the country. When the air is clear, and the light shines full upon them, the prospect is inconceivably brilliant ; the sun is reflected from them as from glass; sometimes they appear of a bright hue, like sapphire ; sometimes variegated with all the glories of the prismatic colours, exceeding in the magnitude of lustre and beauty of colour the richest gems in the world, disposed in shapes wonderful to behold, dazzling the eye with the brilliancy of its splendour. At Spitsbergen, within ten degrees of the pole, the earth is locked up in the ice till the middle

of May; in the beginning of July the plants are in flower, and perfect their seeds in a month's time: for, though the sun is much more oblique in the higher lacitudes, than with us, his long continuance above the horizon is attended with an accumulation of heat exceeding that of many places under the torrid zone; and there is reason to suppose, that the rays of the sun, at any given altitude, produce greater degrees of heat in the condensed air of the polar regions, than in the thinner air of this climate.

Yet, if we look for heat, and the remarkable effects of it, we must go to the countries near the equator, where we shall find a scenery totally different from that of the frigid zone. Here all things are upon a larger scale than in the temperate climates : their days are burning hot ; in some parts their nights are piercing cold; their rains lasting and impetuous, lịke torrents ; their dews excessive; their thunder and lightening more frequent, terrible, and dangerous ; the heat burns up the lighter soil, and foams it into a sandy desert, while it quickens all the moister tracts with incredible vegetation.

The ancients supposed that the frigid zone was uninhabitable from cold, and the torrid from the intolerable heat of the sun; we now, however, know that both are inhabited. The sentiments of the ancients, therefore, in this respect, are a proof how inadequate the faculties of the human mind are to discussions of this nature, when unassisted by facts.

OF THE ANCIENT DISTINCTION OF PLACES BY THE

DIVERSITY OF SHADOWS OF UPRIGHT BODIES AT

NOON.

When the sun at noon is in the zenith of any place, the inhabitants of that place were by the ancients called ascii, that is, without shadow : 'for the shadow of a man standing upright, when the sun is directly over his head, is not extended beyond that part of the earth which is directly under his body, and therefore will not be visible.

As the shadow of every opake body is extended from the sun, it follows, that when the sun at noon is southward from the zenith of any place, the shadow of an inhabitant of that place, and indeed of any other opake body, is extended towards the north.

But when the sun is northward from the zenith of any place, the shadow falls towards the south.

Those are called amphiscii, that have both kinds of meridian shadows.

Those, whose meridian shadows are always projected one way, are termed heteroscii.

PROBLEM XXI. To illustrate the distinction of ascii,

amphiscii, heteroscii, and periscii, by the globe.

Rectify the globe to the summer solstice, and move the artificial horizon to the equator, the north point will be the most elevated at noon.

« AnteriorContinuar »