shorter than the nights-opposite circumstances subsisting in the southern hemisphere. From the equator to the

polar circles the length of the longest day increases with the latitude, from twelve to twenty-four hours, the sun appearing for twenty-four hours without setting, at all places under the arctic and antarctic circles, at midsummer, which occurs at opposite periods of the year in the two hemispheres. From the polar circles to the poles the longest day lengthens with the latitude into weeks and months; and at the poles one season of day and night divides the year. The following Table contains the length of the longest day for different geographical latitudes :

56. Seasons.-The different seasons which we regularly experience-the rigour of winter, the mildness of spring, the glow of summer, and the moderate warmth of autumn, so refreshing by their contrast, depend upon the regularly-occurring differences in the length of day and night, and the periodic variations in the altitude of the sun. When the days are long, more heat is received from the solar influence than what is parted with during the short nights, which accumulating, produces the temperature of summer, with its resulting phenomena: on the contrary, when the days are short, less heat is received; and the nights being long, more is parted with, producing the temperature of winter-a medium temperature marking the intervals when the days and nights are either equal or but slightly differ. But another influential cause of seasonal change is the varying altitude of the sun, whose meridian elevation at Paris, for example, is 64° 38′ above the horizon at midsummer, and but 17° 42' at midwinter. In the latitude of London, the sun's meridian altitude varies from 62° in summer to 15° in winter. Now the height of the

sun above the horizon is one of the most important elements in the study of its heating action; for the solar influence becomes less powerful in proportion to the obliquity of its direction. It has been calculated, that when the height of the sun is 40° 30', only two-thirds of the rays that reach the atmosphere find their way to the earth; when 21° 30′, only half; the rest being absorbed by the atmosphere, or reflected into celestial space. Hence the sun having his greatest altitude above the horizon when the days are long, his rays then being most direct; and the least altitude when the days are short, his rays falling most obliquely; the two causes conspire to produce the different seasons of winter and summer. At the equator, the day and night being always equal, and the meridian elevation of the sun exhibiting no great divergences, heat is tolerably equally distributed throughout the year, and our strongly contrasted seasons are unknown. But as we recede from the equator towards the polar circles, seasonal changes become marked, because the length of day and night, and the solar altitude, are subject to more striking differences.

II. MATHEMATICAL DIVISIONS OF THE Earth.

57. In order conveniently to describe the globe, pointing out the position of places upon its surface, and the effect of its motions, imaginary lines or circles are drawn round it. Great circles divide the globe into two equal parts, or hemispheres, as the equator, etc. Small circles divide the globe into two unequal parts, as the tropics, etc. Every circle is supposed to be divided into 360 equal parts, called degrees; every degree into sixty equal parts, called minutes; and every minute into sixty equal parts, called seconds. Degrees are marked with a small cipher, °; minutes with a dash, '; and seconds with two dashes, ". Thus 30° 4′ 45′′ are read 30 degrees, 4 minutes, 45 seconds. On a great circle of the earth, a degree is 60 geographical or about 69 English miles; on a small circle it is less, according to the situation.

58. Great Circles.-The great circles are the equator, the horizon, the meridians, the ecliptic, and the two colures.

59. The equator, equidistant from the poles, separates

the globe into the northern and southern hemispheres. The latitude of places is reckoned from it, north and south; and their longitude on it, east and west.

60. The horizon separates the visible half of the celestial concave from the invisible, and is either rational or sensible. The rational, or true horizon, by which the rising and setting of all the heavenly bodies is determined, is an imaginary plane passing through the centre of the earth parallel to the eye of the spectator. The sensible, or apparent horizon, is the circle which terminates our view, where the earth and sky appear to meet. To a person standing on a plain, or in a boat at sea, this circle is only a few miles in diameter. If the eye be elevated 5 feet above the sea, or plain, the radius of the sensible horizon will be less than 24 miles; if 6 feet, it will be just 3 miles; and will enlarge or contract, according as the station is elevated or depressed. The cardinal points of the horizon are east, west, north, and south; the zenith is the upper pole of our horizon, the point in the heavens directly over our heads; the nadir is the lower pole, diametrically opposite to the zenith.

61. The meridians, or lines of longitude, are circles passing through the poles, and cutting the equator at right angles, dividing the globe into eastern and western hemispheres. There are twenty-four meridians commonly drawn on globes, each 15° apart, equal to a difference in time of one hour. But every place is supposed to have a meridian passing through it; and when the sun comes to the meridian of any place not within the polar circles, it is then noon, or mid-day (meridies) at that place. The sun is then at his greatest height above the horizon for the day, which is therefore called his meridian altitude.

62. The ecliptic represents the apparent annual track of the sun among the fixed stars, caused by the track which the earth really describes in its annual revolution. It intersects the equinoctial, or the line of the earth's equator produced on the concave surface of the heavens, cutting it obliquely at two opposite points, called the equinoxes, or equinoctial points, and passes northward 2340, passing southward to the same extent, the amount of the obliquity. The ecliptic is so termed because eclipses can only happen when the moon appears in or very near this circle. It is

divided into twelve equal arcs of 30° each, called signs, named from the constellations or groups of stars through which the sun appears successively to pass, which, with the days on which the sun enters them, are as follows:

The spring and summer signs are called northern because north of the equinoctial; the autumn and winter signs are south of the line.

63. The colures are two meridians which pass through the poles, and divide the ecliptic into four equal parts, marking the four seasons of the year. One of them inter

secting the equinoctial points, Aries and Libra, is thence called the equinoctial colure: the other intersecting the solstitial points, Cancer and Capricorn, is called the solstitial colure. The solstices are the points of the sun's greatest declination north and south, and are so denominated because at either of them, which respectively mark the longest and shortest days, the sun does not sensibly change his declination for several days together, or vary in meridian altitude. The term signifies the standing still of the sun, (sol the sun, sisto to stand.)

64. Small Circles.-The small circles are the tropics, the polar circles, and the parallels of latitude.

65. The tropics are two circles parallel to the equator or equinoctial, and at the distance of 23° from it north and south, equal to the obliquity of the ecliptic. The northern. is called the Tropic of Cancer, and the southern the Tropic of Capricorn, because they coincide with the ecliptic in the beginning of those signs. They are termed tropics (TpETW, to turn) because the sun, arrived at them in his apparent annual course, seems to turn away again southward or northward towards the equator, as the case may be.

66. The polar circles surround the poles, at a distance from them exactly equal to the distance of the tropics from the equator, or 230. That which surrounds the north pole is styled the Arctic Circle (aрктus, a bear), from the circumpolar constellation so denominated; that which sur

rounds the south pole is called the Antarctic Circle (opposite to the Arctic). When the sun is at the northern tropic, as in June, his rays extend 234° beyond the north pole, and fall short of the south pole by that extent. Consequently, at that time, no part interior to the Arctic Circle is carried by the rotation of the globe out of the sphere of illumination, while no place interior to the Antarctic Circle is carried within it. The same circumstances occur, but inversely, when the sun is at the southern tropic in December: the whole space within the Antarctic Circle is illuminated, while the corresponding space in the opposite hemisphere is left in darkness.

67. Parallels of latitude are small circles parallel to the equator, north and south, drawn on artificial globes through every 10° of latitude; but every place on the surface of the earth is considered as having a parallel of latitude drawn in through it.

68. Zones.-The tropics and polar circles divide the terrestrial surface into five great zones or belts, one torrid, two temperate, and two frigid, characterized by differences as to temperature. The torrid zone, included between the tropics, a space 47° in breadth, has the sun vertical at midday to every place within it twice in the year, the solar rays striking the surface perpendicularly; and as at other times their direction is never very oblique, this region is powerfully heated, whence its name (torridus, hot). The two temperate zones, one northern and the other southern, each 43° in breadth, are comprehended between the tropics and the polar circles. They never have the sun vertical, and are characterized by a lower temperature than the preceding belt; but at the same time, the variations in the length of day and night, and in the sun's meridian altitude, occasion strong seasonal contrasts within these limits. The two frigid zones, each 2310 in radius, included within the polar circles, being deprived of the influence of the sun for long intervals, and never receiving his rays otherwise than very obliquely, are regions appropriately styled frigid (frigidus, cold).

69. Climates.-In order to give a more particular description of the position of portions of the earth's surface in relation to the sun, geographers have divided it into a number of small belts parallel to the equator, distinguished