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southern hemisphere. From the 21st of June to the 23d of September, the days and nights again approach to equality, and the excess of temperature in the northern hemisphere above the mean state, grows less, as also its defect in the southern; so that, when the Sun arrives at the autumnal equinox, the mean temperature is again restored. From the 23d of September until the 21st of December, our nights grow longer and the days shorter, and the cold increases as before it diminished, while we pass from autumn to midwinter, in the northern hemisphere, and the inhabitants of the southern hemisphere from spring to mid-summer. From the 21st of December to the 20th of March, the cold relaxes as the days grow longer, and we pass from the dreariness of winter to the mildness of spring, when the seasons are completed, and the mean temperature is again restored. The same vicissitudes transpire, at the same time, in the southern hemisphere, but in a contrary order. Thus are produced the four seasons of the year.

But I have stated not the only, nor, perhaps, the most efficient cause in producing the heat of summer and the cold of winter. If, to the inhabitants of the equator, the Sun were to remain 16 hours below their horizon, and only 8 hours above it, for every day of the year, it is certain they would never experience the rigours of our winter; since it can be demonstrated, that as much heat falls upon the same area from a vertical Sun in 8 hours, as would fall from hin. at an angle of 60°, in 16 hours.

Now as the Sun's rays fall most obliquely when the days are shortest, and most directly when the days are longest, these two causes, namely, the duration and intensity of the solar heat, together, produce the temperature of the different seasons. The reason why we have not the hottest temperature when the days are longest, and the coldest temperature when the days are shortest, but in each case about a month afterwards, appears to be, that a body once heated, does not grow cold instantaneously, but gradually, and so of the contrary. Hence, as long as more heat comes from the Sun by day than is lost by night, the heat will increase, and vice versa.

What, at the same time, takes place in regard to the temperature, in the southern hemisphere? During what portion of the year is the temperature decreasing? For what reason? During what portion of the year is the cold increasing? Why is it so? What change of seasons, then, takes place, in the northern and southern hemisphere? What other changes complete the seasons of the year? Whence is it evident that the unequal lengths of the days and nights are not the only, nor perhaps the most efficient cause of the heat of summer, and the cold of winter? What two causes produce the greatest vicissitudes of heat and cold? Why, then, do we not have the hottest weather wher the days are longest, and the contrary

BEGINNING AND LENGTH OF THE SEASONS.

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The north pole of the Earth is denominated the elevated pole, because it is always about 231 above a perpendicular to the plane of the equator, and the south pole is denominated the depressed pole, because it is about the same distance below such perpendicular.

As the Sun cannot shine on more than one half the Earth's surface at a time, it is plain, that when the Earth is moving through that portion of its orbit which lies above the Sun, the elevated pole is in the dark. This requires six months, that is, until the Earth arrives at the equinox, when the elevated pole emerges into the light, and the depressed pole is turned away from the Sun for the same period. Consequently, there are six months day and six months night, alternately, at the poles.

When the Sun appears to us to be in one part of the ecliptic, the Earth, as seen from the Sun, appears in the point diametrically opposite. Thus, when the Sun appears in the vernal equinox at the first point of Aries, the Earth is actually in the opposite equinox at Libra. The days and nights are then equal all over the world.

As the Sun appears to move up from the vernal equinox o the summer solstice, the Earth actually moves from the utumnal equinox down to the winter solstice. The days How lengthen in the northern hemisphere, and shorten in the outhern. The Sun is now over the north pole, where it is nid-day, and opposite the south pole, where it is mid-night.

Why is the north pole denominated the elevated pole? Why is the south pole denomiated the depressed pole? Why are there six months day and six month night, alternately, the poles? What is always the relative position of the Sun and Earth in the ecliptic? dive an example. When do the days lengthen in the northern hemisphere, and shorten in he southern? When is it mid-day at the north pole, and mid-night at the south?

As the Sun descends from the summer solstice towards the autumnal equinox, the Earth ascends from the winter solstice towards the vernal equinox. The summer days in the northern hemisphere having waxed shorter and shorter, now become again of equal length in both hemispheres.

While the Sun appears to move from the autumnal equinox down to the winter solstice, the Earth passes up from the vernal equinox to the summer solstice; the south pole comes into the light, the winter days continually shorten in the northern hemisphere, and the summer days as regularly increase in length in the southern hemisphere.

While the Sun appears again to ascend from its winter. solstice to the vernal equinox, the Earth descends from the summer solstice to the autumnal equinox. The summer days now shorten in the southern hemisphere, and the winter days lengthen in the northern hemisphere.

When the Sun passes the vernal equinox, it rises to the arctic or elevated pole, and sets to the antarctic pole. When the Sun arrives at the summer solstice, it is noon at the north pole, and midnight at the south pole. When the Sun passes the autumnal equinox, it sets to the north pole, and rises to the south pole. When the Sun arrives at the winter solstice, it is midnight at the north pole, and noon at the south pole; and when the Sun comes again to the vernal equinox, it closes the day at the south pole, and lights up the morning at the north pole.

There would, therefore, be 1864 days during which the Son would not set at the north pole, and an equal time during which he would not rise at the south pole; and 1781 days in which he would not set at the south pole, nor rise at the north pole.

At the arctic circle, 23° 27' from the pole, the longest day is 24 hours, and goes on increasing as you approach the pole. In latitude 67° 18' it is 30 days; in lat. 69° 30' it is 60 days, &c. (See Table XII.) The same takes place between the antarctic circle and the south pole, with the exception, that the day in the same latitude south is a little shorter, since the Sun is not so long south of the equator, as at the north of it. In this estimate no account is taken of the refraction of the atmosphere, which, as we shall

When do the summer days in the northern hemisphere grow shorter and shorter? When do they become of equal length in both hemispheres? When do the winter days shorten in the northern hemisphere, and the summer days lengthen in the southern? When do the summer days shorten in the southern hemisphere, and the winter days lengthen in the northern? When does the sun rise to the north pole, and set to the south? When is it noon at the north pole, and mid-night at the south pole? When does the Sun set at the north pole, and rise to the south? When is it mid-night at the north pole, and noon at the South? What is the length of the day at the north pole? What at the south pole? At the arctic circle? Between the antarctic circle and the pole

see hereafter, increases the length of the day, by making the Sun appear more elevated above the horizon than it really is.

THE SEASONS-UNEQUAL LENGTHS OF DAYS AND NIGHTS.

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The above cut represents the inclination of the Earth's axis to its orbit in every one of the twelve signs of the ecliptic, and consequently for each Imonth in the year. The Sun enters the sign Aries, or the vernal equinox, on the 20th of March, when the Earth's axis inclines neither towards the Sun, nor from it, but sideways to it; so that the Sun then shines equally upon the Earth from pole to pole, and the days and nights are every where equal. This is the beginning of the astronomical year; it is also the beginning of day at the north pole, which is just coming into light, and the end of day at the south pole, which is just going into darkness.

By the Earth's orbitual progress, the Sun appears to enter the second sign, Taurus, on the 20th of April, when the north pole, N, has sensibly advanced into the light, while the south pole, S. has been declining from it; whereby the days become longer than the nights in the Northern Ilemisphere, and shorter in the Southern.

On the 21st of May, the Sun appears to enter the sign Gemini, when the north pole, N, has advanced considerably further into the light, while the south pole, S, has proportionally declined from it; the summer days are now waxing longer in the Northern Hemisphere, and the nights shorter.

The 21st of June, when the Sun enters the sign Cancer, is the first day of summer, in the astronomical year, and the longest day in the Northern Hemisphere. The north pole now has its greatest inclination to the Sun, the light of which, as is shown by the boundary of light and darkness, in the figure, extends to the utmost verge of the Arctic Circle; the whole of which is included in the enlightened hemisphere of the Earth, and enjoys, at this season, constant day during the complete revolution of the Earth on its axis. The whole of the Northern Frigid Zone is now in the circle of perpetual illu.

mination.

On the 23d of July, the Sun enters the sign Leo, and as the line of the Earth's axis always continues parallel to itself, the boundary of light and darkness begins to approach nearer to the poles, and the length of the day,

in the Northern Hemisphere, which had arrived at its maximum, begins gradually to decrease. On the 23d of August, the Sun enters the sign Virgo, increasing the appearances mentioned in Leo.

On the 23d of September, the Sun enters Libra, the first of the autumnal signs, when the Earth's axis, having the same inclination as it had in the opposite sign, Aries, is turned neither from the Sun, nor towards it, but oblique ly to it, so that the Sun again now shines equally upon the whole of the Earth's surface from pole to pole. The days and nights are once more of equal length throughout the world.

On the 23d of October, the Sun enters the sign Scorpio; the days visibly decrease in length in the Northern Hemisphere, and increase in the South

ern.

On the 22d of November, the Sun enters the sign Sagittarius, the last of the autumnal signs, at which time the boundary of light and darkness is at a considerable distance from the north pole, while the south pole has proportionally advanced into the light; the length of the day continues to increase in the Southern Hemisphere, and to decrease in the Northern.

On the 21st of December, which is the period of the winter solstice, the Sun enters the sign Capricorn. At this time, the north pole of the Earth's axis is turned from the Sun, into perpetual darkness; while the south pole, in its turn, is brought into the light of the Sun, whereby the whole Antarctic region comes into the circle of perpetual illumination. It is now that the Southern Hemisphere enjoys all those advantages with which the Northern Hemisphere was favoured on the 21st of June; while the Northern Hemisphere, in its turn, undergoes the dreariness of winter, with short days and long nights.

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HARVEST MOON-HORIZONTAL MOON.

The daily progress of the Moon in her orbit, from west to east, causes her to rise, at a mean rate, 48 minutes and 44 seconds later every day than on the preceding. But in places of considerable latitude, a remarkable deviation from this rule takes place, especially about the time of harvest, when the full Moon rises to us for several nights together, only from 18 to 25 minutes later in one day, than on that immediately preceding. From the benefit which her light affords, in lengthening out the day, when the husbandmen are gathering in the fruits of the Earth, the full moon, under these circumstances, has acquired the name of Harvest Moon.

It is believed that this fact was observed by persons engaged in agriculture, at a much earlier period than that in which it was noticed by astronomers The former ascribed it to the goodness of the Deity; not doubting but that he had so ordered it for their advantage.

About the equator, the Moon rises with nearly the equal intervals of 48 the harvest moon is unknown.

throughout the year minutes; and there

What is the mean difference of time in the daily rising of the Moon? Under what circumstances is there a material deviation from this rule? Whence the name of Harvest Moon? By whom was this phenomenon first observed, and to what did they attribute ? Why is the Harvest Moon unknown at the equator}

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