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short time after sunset, darkness comes on. In fig. 48, § 229, which represents the latitude of London, the arc FXP is the portion the sun describes in the night of June 21; where, X R being less than 18°, there will be no absolute night, the twilight never failing, even at midnight. This will explain what the almanac means by "perpetual day," beginning or ending at such a date.

SECTION IV.

THE SEASONS.

46. WERE the earth to revolve round the sun with its equator coincident with the plane in which it revolves, or, which is the same thing, with its axis of rotation perpendicular to the ecliptic, there would be no change of seasons; the sun would be ever over the equator; north and south would be equally illuminated by his rays. The inclination of the axis from a perpendicular to the ecliptic, at an angle of 23° 28',* is the cause of the variety of spring, summer, autumn, and winter.

47. In fig. 37, Plate IV., let A B C D represent the earth's annual path round the sun s, with the earth in four different positions, 90° distant from each other; the earth's axis remaining parallel to itself in all positions, as there represented. Beginning with the position A, the winter quarter of the northern regions, we perceive that the sun is vertical over those places

* The mean obliquity of the ecliptic for the year 1852 is 23° 27′ 29′′.

23 degrees south of the equator. The circle they describe in the daily revolution of the earth, t c, is termed the tropic of Capricorn, so named from the sign the sun enters at that time. The boundary separating the light and dark hemispheres will reach beyond the south pole 23 degrees. The circle at that distance from the southern pole is termed the antarctic circle, a'r'; another at the same distance from the north pole is the arctic circle-viz. a r. An inspection of the figure will establish the following particulars :

1. In the position A, it will be summer to the inhabitants of the southern regions; the rays of the sun shining more directly on them, in consequence of the southern pole being turned towards him.

2. The inhabitants of the earth within the antarctic circle will not lose sight of the sun during the daily revolution of the earth, inasmuch as they do not retire within the darkened hemisphere.

3. Those of the arctic circle do not see the sun during the diurnal revolution, inasmuch as they fail to advance within the enlightened portion of the earth.

4. The countries above the equator will have shorter days than those below: the tropic of Cancer, for instance, will describe t'y' in the daylight, while the tropic of Capricorn will describe the much larger arc t y. And since the motion of the earth is equable, the days will be shorter in the former position than in the latter.

5. The spaces included within the arctic and antarctic circles are termed the FRIGID ZONES; between the tropics, the TORRID ZONE; those between the tropics and polar circles, the TEMPERATE ZONES.

The position of the earth in the spring quarter is

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represented at D. Here the sun shines over the equator. At this time day and night will be equal; for the boundary of light and darkness passing through the poles, every country on the globe passes through the enlightened portion in the same time as through the darkened half. The same circumstances will recur at

B, in the autumnal quarter.

At c, the sun being over the tropic of Cancer, it will be summer to the northern regions; and the circumstances of the inhabitants of the northern and southern hemispheres will be the reverse of those described above.

48. It is a singular circumstance that, in our winter, the earth occupies that part of her orbit which is nearest the sun, as is proved by his disc subtending a greater angle at that period of the year: the difference of the distance, however, falling short, as it does, of that in summer by only, can produce but little increase in the sun's heat. The principal causes of the increase of temperature during the summer quarter would seem to be, first: the sun's rays, which fall in a very slanting direction during the winter, give place to those which fall more nearly vertical; and therefore a given portion of the earth's surface will receive many more of them, and thus acquire a higher temperature. Again, the heat acquired by the earth during the long days of summer is not radiated or given out during the short nights of that season, but goes on accumulating from day to day: hence it is that in general our hottest weather is not at midsummer, but in July or August, after the earth has

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