of the earth, from west to east, causes the sun to appear to revolve about the earth, from east to west. So, as the annual revolution of the earth changes its position, and presents now the northern, now the southern hemisphere to the sun, the sun itself appears to advance alternately toward the north and toward the south. It seems to leave the equator in March, and pass northward till June; then to move southward, crossing the equator in September, and reaching its southern limit in December, when it returns northward, and comes again, in March, to the equator.

The periods (June 21st and December 22d) when the sun seems to stop in its apparent course to the north or south, and turn back, are called the solstices; and the points, B and D, which the earth then occupies, are called the solstitial points.

12. The parallels of 2340, to which the sun is vertical at the limits of its apparent journey, north or south [see Fig. 2], are called tropics; the northern, the TROPIC OF CANCER; the southern, the TROPIC OF CAPRI

CORN.

They are styled tropics, from the Greek porn (trop ́ē), a turning; because when the sun becomes vertical to either, it turns back toward the equator. The northern is known as the Tropic of Cancer, since it lies beneath the constellation in the heavens called Cancer or the Crab; and the southern, as the Tropic of Capricorn, because it lies beneath the constellation called Capricornus or the Goat.

SECTION II.

DAY AND NIGHT.

1..The succession of day and night is caused by the diurnal motion of the earth.

The earth being a globe, the sun can illuminate only one half of it at a time. It follows, therefore, as the sun occupies a fixed point in the heavens, that if the earth were without motion, the same hemisphere would always be presented to the sun; and consequently would have constant day, while the opposite hemisphere would have constant night.

But as the earth revolves on its axis, each portion of its surface is turned

Questions.--Of its diurnal motion? Of the change in its position with respect to the sun? Periods of the sun's apparent progress in a northerly and southerly direction? 12. What are the tropics, and their names? 1. Cause of the succession of day and night?

* Latin, sol, the sun; and sisto, to stop.

alternately toward the sun and from the sun; thus causing a succession of light and darkness, or the alternations of day and night.

UNEQUAL LENGTH OF DAY AND

NIGHT.

2. The unequal length of day and night is caused by the annual motion of the earth, and the inclination of its axis.

Referring to Figure 2, it will be seen that while the earth is at A and C, or about the 21st of March and 22d of September, the illuminated half of its surface extends from pole to pole; and hence the days and nights, except very near the poles [see the following paragraph], are everywhere of equal length. These periods, therefore, are known as the equinoxes; points A and C, as the equinoctial points.

and the

But as the earth passes from one of these points, through half its orbit, to the other, that pole which is inclined toward the sun is constantly illuminated, while the opposite pole is constantly in darkness. The year at the poles, therefore, has but one day and one night, each six months long.

As the earth advances toward B and D (the solstitial points), the sunlight gradually reaches beyond one of the poles, and retires from the other. Hence the length of the days is then constantly increasing in most of the hemisphere which surrounds the illuminated pole, while it is lessening in the opposite hemisphere. It results from this that the longest days in the one hemisphere, and the shortest in the other, occur at the solstices.

But as the earth passes from B and D to the equinoctial points, A and C, the limit of sunlight in both hemispheres continually approaches the poles. Thus, the days at this period become shorter in the hemisphere about the illuminated pole, and increase in the opposite one.

But whatever position the earth occupies in respect to the sun, one half the equator is always exposed to the solar rays. Hence, throughout the year, the days and nights at the equator are of equal length, or twelve hours each. In reference to this fact, the equator is sometimes termed the equinoctial line.

It is apparent, then-since the days are twelve hours long at the equator, and six months at the poles-that, departing from the equator toward either of the poles when inclined to the sun, the length of the days constantly increases; and proceeding from the poles toward the equator, it constantly diminishes.

SUMMARY

OF PRECEDING EXPLANATION,

ETC.

3. About the 21st of March and 22d of September, the days and nights, except very near the poles, are every where of equal length.

Questions.-2. Of their unequal length? 8. When are they equal in nearly every part of the earth?

* Latin, equus, equal; and nox, night.

4. A year at the poles consists of one day and one night, each six months long.

5. From about the 22d of December to the 21st of June, or while the sun appears to move northward, the days are constantly lengthening in the northern hemisphere, and shortening in the southern: but during the remainder of the year, or while the sun appears to move southward, the length of the days in the southern hemisphere is constantly increasing; and in the northern, constantly diminishing.

6. The days and nights at the equator are always of equal length, or twelve hours each.

7. While the sun is north of the equator, the days are longer as we go toward the north pole; but while south of the equator, their length increases southward.

At the polar circles [see definition of polar circles in the following paragraph], the longest days are twenty-four hours each. From the polar circles to the poles, the days lengthen into weeks and months. In latitude 67°, the longest day is one month; in latitude 70°, two months; and in latitude 80o, four months.

8. The parallels which mark the limit to which light or darkness extend beyond the poles [see Fig. 2], at the time of the longest days or nights, are called polar circles. The northern is distinguished as the ARCTIC CIRCLE; the southern, as the ANTARCTIc Circle.

The term Arctic is from the Greek, apxrog (ark'tos), a bear; and is applied to the north polar circle, because it is situated beneath the constellation of stars called the Great Bear. Antarctic is from the Greek, avri (an'ti), opposite, and apxros (ark'tos).

9. The polar circles are about 2340 distant from the poles, or as far from them as the axis of the earth is inclined from a perpendicular to the plane of the earth's orbit.

SECTION III.

TEMPERATURE OF THE EARTH'S SURFACE.

1. The outward warmth of the earth is derived chiefly from the sun.

Questions.-4. Length of day and night at the poles? 5. When do the days lengthen in the northern hemisphere, and shorten in the southern? When the reverse? 6. Length of days and nights at the equator? 7. When are the days longer as we go toward the north pole? Toward the south pole? 8. What are polar circles? Their names? 9. Distance from the poles? 1. Source of the outward warmth of the earth?

2. As the earth turns on its axis, the side presented to the sun is constantly gaining heat, while the opposite side is constantly losing it. The degree of heat or cold, at any place, depends, therefore, in a great measure, upon how long it is exposed to the sun, or how long turned from it; or on the comparative length of the days and nights.

3. Hence, setting aside other causes, we may conclude that the difference between the extremes of heat and cold is greatest near the poles, and less and less toward the equator.

4. The temperature of the different latitudes is modified in a still greater degree by the directness or obliquity with which the sun shines upon them.

5. The vertical rays of the sun produce most heat; because, as represented in the opposite figure, a certain number of them are concentrated upon a smaller space than an equal number of oblique, or slanting, rays.

6. Now, as the sun is always vertical to some point within the tropics, the inter

tropical regions are the warmest parts of the globe; and since, as we leave them, the rays fall more and more obliquely, the extra-tropicalt regions grow colder as we approach the poles.

ZONES.

7. The decrease of warmth from the equator to the poles has led to a division of the earth's surface, in respect to temperature, into separate belts or sections, called zones, extending east and west around the earth.

The term zone, from the Greek, wvn (zone), a girdle, is sometimes applied, in a more general sense, to any belt extending around the earth in this direction.

8. There are five zones; viz., the Torrid, North Temperate, South Temperate, North Frigid, and South Frigid zones.

9. The TORRID ZONE lies between the tropics; and being that section

Questions.-2. What part is constantly gaining heat? Constantly losing it? The degree of heat or cold depends on what? 3. Conclusion, from this fact, respecting extremes of heat and cold? 4. How else is the temperature of the different latitudes modified? 5. What rays produce most heat? Why? 6. Consequent condition of the intertropical regions? Of the extra-tropical regions? Meaning of the words intertropical and ex ratropical? (see notes at the bottom of the page.) 7. Division of the earth's surface into zones? 8. Number of zones, and their names? 9. Situation of the Torrid Zone?

* Intertropical-between the tropics.

+ Extra-tropical-outside the tropics.

of the earth's surface, to every part of which the sun is vertical twice a year, is characterized by excessive heat.'

10. The TEMPERATE Zones lie between the tropics and the polar circles. Receiving the sun's rays at all times somewhat obliquely, their temperature is generally mild.

11. The FRIGID2 ZONES are inclosed by the polar circle. As they receive the sun's rays most obliquely, and during several months of the year are mostly in darkness, they are characterized by intense cold. Yet they have a warm season, during that period when they are turned toward the sun, and the long nights give place to long days.

12. The Torrid Zone comprises less than two fifths of the earth's surface; the Temperate Zones, more than one half; and the Frigid Zones, less than one twelfth.

13. The annual changes of temperature, which occur at the same places on the earth, have led to a division of the year into equal parts, called seasons.

14. In the temperate zones four seasons are reckoned; viz., Winter, Spring, Summer, and Autumn or Fall.

15. WINTER is the season of greatest cold; SPRING, of increasing warmth; SUMMER, of greatest heat; and AUTUMN, of increasing cool

ness.

16. These different temperatures result from the various positions of the earth, in respect to the sun.

Referring to Figure 2, it will be seen that when the earth is at B-where the longest days occur, north of the equator-the sun shines most directly upon the northern hemisphere; and when at D-where the longest days occur, south of the equator-it shines most directly upon the southern hemisphere. Hence the warmest part of the year occurs in the former, when the earth is near B; and in the latter, when near D: or, in other words, Summer in the northern hemisphere is the season which includes the month of June; and in the southern, that which includes the month of December. But while the sun shines most directly upon one of these hemispheres, and gives it the longest daylight, it shines most obliquely upon the other, and gives it the shortest daylight. Hence the time of Summer in the northern

Questions. Its character? 10. Situation of the temperate zones? Their character? 11. Situation of the frigid zones? Their character? Their warm season? 12. Size of the several zones? 13. Division of the year into seasons? 14. Seasons of the temperate zones? 15. Characters of the seasons? 16. From what do these different temperatures result?

1 Torrid; from the Latin, torridus, hot.

Latin, frigidus; literally, stiff with cold.