situated toward the enlightened pole, will have a greater part of their circumference in the enlightened than in the dark hemisphere; while similar parallels towards the other pole, will have the greater part of their circumference in the dark hemisphere. Whence it follows, that the first-mentioned parallels will enjoy longer days than nights: and the contrary will happen to the latter, where the days will be the shortest, and the nights the longest; while, at the equator, the days will always be of the same length.

Having shewn, that the vicissitudes in the length of the days and nights are occasioned by the position of the terminator with respect to the axis of the earth, I have now only to explain what occasions those various positions; which is the more important, as on these depend the diversity in the seasons of the year.

OF THE SEASONS OF THE YEAR.

In considering this subject, you will find further proofs of that DIVINE WISDOM which pervades all the works of GOD; and see that no other conforma tion of the system could have given such commodious distributions of light and heat, or imparted fertility and pleasure, to so great a part of the revolving globe.

The changes in the position of the terminator are occasioned-1. By the inclination of the earth's axis to the plane of the ecliptic, or orbit in which it

moves.-2. Because, through the whole of its annual course, the axis of the earth preserves its position, or continues parallel to itself; that is, if a line be conceived as drawn parallel to the axis, while the earth is in any one point of its orbit, the axis will, in every other position of the earth, be parallel to the said line.

It must be evident to you, that the parallelism of the axis must occasion considerable differences. By a bare inspection of the globes, A, B, plate 5, fig. 1, you will see that, when the earth is in one position of it's orbit, the north pole will be turned towards the sun; but, in the opposite part, will be turned from him. But the absence of the sun's light produces a proportionable degree of cold; hence the seasons are, in the northern and southern parts of the globe, distinctly marked by different degrees of heat and cold. It is this annual turning of the poles towards the sun, that occasions the very long days in the northern and southern parts. It is owing to the same cause, that the sun seems to rise higher in the heavens during summer than in winter; and this alternate sinking and rising is perceptible over the whole globe,

If the axis of the earth were perpendicular to the plane of its orbit, the equator and the orbit (or ecliptic), would coincide; and as the sun is always in the plane of the ecliptic, it would, in this case, be always over the equator, as at C, fig. 1, and the two poles would be in the terminator; and there would be no diversity in the days and nights, and but one

season of the year; but, as this is not the case, we may fairly infer, that the axis of the earth is not per pendicular to the plane of its orbit.

But if the earth's axis be inclined to the plane of the ecliptic, when the earth is in the situation re presented at A, plate 5, fig. 1, the pole N will be towards the sun, and the pole S will be turned from it; but just the contrary will happen, when the earth, by going half round the sun, has arrived at B, the opposite point in its orbit. Hence the sun will not always be in the equator, but at one time of the year it will appear nearer to one of the poles, and at the opposite season it will appear nearer to the other. Here, then, you find a cause for the change of the seasons; for, when the sun leaves the equator and approaches to one of the poles, it will be summer on that side of the equator; and when the sun departs from thence, and approaches to the other pole, it will be winter.

These ideas may be strengthened, and a clearer notion obtained of the effect produced by the inclination of the earth's axis, by considering fig. 2, plate 5, in which the ellipsis is supposed to represent the earth's orbit, the eye somewhat elevated above the plane thereof. The earth is here represented in the. first point of each of the twelve signs of the ecliptic, as marked in the figure, with the twelve corresponding months annexed; P, the north pole of the globe; Pm, its axis, round which the earth performs its diurnal revolution from west to east; this axis is exhibited as parallel to itself in every part of the orbit;

PCE shews the angle of its inclination; e, the pole ; ed, the axis of the ecliptic, perpendicular to the plane of the orbit.

In March, when the earth is in the first point of Libra, the sun appears in the opposite point of the ecliptic at Aries. In September, when the earth is in the first point of Aries, the sun will be in Libra. At these times, the terminator passes through the poles of the world, and divides every parallel into two equal parts (see C, fig. 1); consequently, the nocturnal and diurnal arcs, or the length of day and night, will be equal in all places over the world.

Conceive the earth to have moved from Libra to Capricorn in June, the axis, Pd, preserving its parallelism by this motion, the north pole will have gradually advanced into the enlightened hemisphere; so that the whole northern polar circle will be therein, while the southern pole is immerged in obscurity; the northern parts of the world will enjoy long days, while they are short in the southern parts. While the earth is moving from Libra through Capricorn to Aries, the north pole remains in the illuminated hemisphere, and will therefore have six months continual day.

But in the other half-year, while the earth is moving from Aries through Cancer to Libra, the north pole is turned from the sun, and therefore in darkness; but the south pole is in the illuminated hemisphere. When the earth is at Cancer, the sun is at Capricorn; at this season the nights to us will as much exceed the days, as the days exceeded the

nights, when the earth was in the opposite point of her orbit.

From the foregoing explanation, it is easy to perceive that the inhabitants of the southern hemisphere have the same vicissitudes with those of the northern, though not at the same time; it being winter in one hemisphere, when it is summer in the other.

From what has been said you must have perceived that, during the course of the earth through her orbit, there are four days particularly to be remarked; these astronomers have distinguished by the names of the solstitial and equinoctial days. The solstitial days are those on which the sun appears most to the northward and southward; the equinoctial days arè those on which he appears in the equator, and the days are equal to the nights.

The annual motion of the earth occasions a daily apparent change in the declination of the sun. Thus, about the 22nd of December, when the earth is in Cancer, the sun will be over the tropic of Ca pricorn; and, consequently, by the earth's rotation on its axis, the inhabitants of every part of this circle will successively have the sun in their zenith; or, in other words, he will be vertical to them that day

at noon.

About the 21st of March, the earth is at Libra, and the sun will then appear in Aries; a central solar ray will terminate upon the surface of the earth in the equator; and, therefore, the sun appears to be carried round the celestial equator,