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and is successively vertical to those who live under that circle.

About the 21st of June, when the earth is in Capricorn, a central solar ray terminates on the surface of the earth, in the northern tropic; and, for that day, the sun appears to be carried round in the tropic of Cancer, and is vertical to those who live under that circle. About the 22nd of September, the earth is in Aries, and the sun in Libra, and the central solar ray again terminates at the equator; consequently, the sun again appears in the celestial equator, and is vertical to those that live under it.

We have seen, that as the sun moves in the ecliptic, from the vernal equinox to the tropic of Cancer, it gets to the north of the equator; or its declination towards our pole increases. Therefore, from the vernal equinox, when the days and nights are equal, till the sun comes to the tropic of Cancer, our days lengthen and our nights shorten; but when the sun comes to the tropic of Cancer, it is then in its utmost northern limit, and returns in the ecliptic to the equator again. During this return of the sun, its declination towards our pole decreases; and, consequently, the days decrease and the nights increase, till the sun is arrived in the equator again, and is in the autumnal equinoctial point, when the days and nights will again be equal. As the sun moves from thence towards the tropic of Capricorn, it gets to the south of the equator; or its declination towards the south pole increases. Therefore, at that time of the year, our days shorten and our nights lengthen,

till the sun arrives at the tropic of Capricorn; but when the sun is arrived there, it is then at its utmost southern limit, and returns in the ecliptic to the equator again. During this return, its distance from our pole lessens; and, consequently, the days will lengthen as the nights shorten, till they become equal, when the sun is come round to the vernal equinoctial point.

Our summer is nearly eight days longer than the winter. By summer is meant here the time that passes between the vernal and autumnal equinoxes; by winter, the time between the autumnal and vernal equinoxes. The ecliptic is divided into six northern and six southern signs, and intersects the equa tor at the first of Aries and the first of Libra. In our summer, the sun's apparent motion is through the six northern, and in winter through the six southern signs; yet the sun is 186 days, 11 hours, 51 minutes, in passing through the six first; and only 178 days, 17 hours, 58 minutes, in passing through the six last. Their difference 7 days, 17 hours, 53 minutes, is the length of time by which our summer exceeds the winter.

In plate 6, fig. 1, ABCD represents the earth's orbit; S the sun in one of its foci; when the earth is at B, the sun appears at H, in the first point of Aries; and whilst the earth moves from B through C to D, the sun appears to run through the six northern signs, from Y through to at F. When the earth is at D, the sun appears at F, in the first point of Libra; and as the earth moves from D

through A to B, the sun appears to move through the six southern signs, from through v to Aries at H.

Hence the line FH, drawn from the first point of Aries through the sun at S, to the first point of ~, divides the ecliptic into two equal parts; but the same line divides the earth's elliptical orbit into two unequal parts. The greater part BCD is that which the earth describes in the summer, while the sun appears in the northern signs. The lesser part is DAB, which the earth describes in winter, while the sun appears in the southern signs. C, the earth's aphelion, where it moves slowest, is in the greater part; A, its perihelion, is in the lesser part, where the sun moves fastest.

There are, therefore, two reasons why our summer is longer than our winter; first, because the sun continues in the northern signs, while the earth is describing the greater part of its orbit; and, secondly, because the sun's apparent motion is slower while it appears in the northern signs, than whilst it appears in the southern ones.

The sun's apparent diameter is greater in our winter than in summer; because the earth is nearer to the sun when at A in the winter, than it is when at C in the summer. The sun's apparent diameter in winter, is 32 minutes, 47 seconds; in summer, 31 minutes, 40 seconds.

But if the earth is farther from the sun in summer than in winter, it may be asked, why our winters are so much colder than our summers? To this it

1

may be answered, that our summer is hotter than the winter; first, on account of the greater height

to which the sun rises above our horizon in the summer; secondly, the greater length of the days. The sun is much higher at noon in summer than in winter; and, consequently, as its rays in summer are less oblique than in winter, more of them will fall upon the surface of the earth. In the summer, the days are very long, and the nights very short; therefore, the earth and air are heated by the sun in the day-time, more than they are cooled in the night ; and upon this account, the heat will keep increasing in the summer; and for the same reason will decrease in winter, when the nights lengthen.

I should exceed the limits of this Essay, if I were to inquire into the several concurring causes of the temperatures that obtain in various climates; it may be sufficient, therefore, to observe what a remarkable provision is made in the world, and the several parts of it, to keep up a perpetual change in the degrees of heat and cold. These two are antagonists; or, as Lord Bacon calls them, "the very hands of Nature with which she chiefly worketh ;" the one expanding, the other contracting bodies, so as to maintain an oscillatory motion in all their parts; and so serviceable are these changes in the natural world, that they are promoted every year, every hour, every moment. From the oblique position of the ecliptic, the earth continually presents a different face to the sun, and never receives his rays two days together in the same direction. In the

day and night, the differences are so obvious, that they need not be mentioned; though they are most remarkable in those climates, where the sun at his setting makes the greatest angle with the horizon. Every hour of the day the heat varies with the sun's altitude, is altered by the interposition of clouds, and the action of winds; and there is little room to doubt, but what the various changes that thus take place, concur in producing many of the smaller and greater phenomena of nature. :

Be this however as it may, it is certain that the various irregularities and intemperature of the elements, which seem to destroy nature in one season, serve to revive it in another: the immoderate heats of summer, and the excessive cold of winter, prepare the beauties of the spring, and the rich fruits of autumn. These vicissitudes, which seem to superficial minds the effects of a fortuitous concourse of irregular causes, are regulated according to weight and measure by that SOVEREIGN WISDOM, who weighs the earth as a grain of sand, the sea as a drop of

water.

OF THE SOLAR AND SIDERIAL TIME.

I have already shewn, that the daily motion of the sun from east to west, is not a real but an apparent one; which is owing to the rotation of the earth round its axis. Now if the sun had no other mo tion but this apparent one, it would seem to go once

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