the meridian in equal times, the difference being always in proportion to the obliquity: and, as some parts of the ecliptic are much more obliquely situated, with respect to the equator, than others; these differences will, therefore, be unequal among themselves. If, for instance, two bodies, the Sun and a star, were to set out together from one of the equinoctial points, and to move through equal spaces in equal times, the Sun in the ecliptic, and the star in the equator; then the star moving in the equator would always return to the meridian exactly at the end of every twenty-four hours, as measured by a well-regulated clock; but the Sun moving in the ecliptic would come to the meridian sometimes sooner than the star, and sometimes not so soon, according to their relative situations; and they would never be found upon that circle exactly together, but on four days in the year, viz. on or about the 20th March and the 23d of September, when the Sun enters the equinoctial points; and on the 21st of June and the 21st of December, when that body is in the solstitial points.

This is easily shown on the globe, by making marks of chalk, or placing patches of black court-plaster, at equal distances, all round the globe, say 10 degrees, from each other ; beginning from the first of Aries, which answers to the 20th of March. Now, by turning the globe on its axis, it will be seen that all the patches in the first quadrant of the ecliptic, that is, from Aries to Cancer, come sooner to the brazen meridian than their corresponding marks on the equator. Hence, apparent time marked by the dial would be before equal or true time, and we should have to subtract to obtain the true equation. In the sécond quadrant, from Cancer to Libra, the patches in the ecliptic would come to the meridian later than those on the equator, and apparent time would be later than equal time, and we should have to add. In the other quadrants, the circumstances would be the same,

that is, from Libra to Capricorn, the Sun would be soonest, and from Capricorn to Aries, it would be latest.

If, however, the reader refers to the tables of equation of time, in each month, they will be found not to answer exactly to this: the apparent motion of the Sun, or apparent time, does not begin to get before time by the clock till about the 16th of April, instead of the 20th of March; and a similar change occurs about the 1st of September, instead of the 23d; and the times when the clock begins to surpass the Sun, are about the 16th of June and the 25th of December, instead of the 21st of June and the 21st of December. This is owing to the elliptical form of the Earth's orbit. If this orbit were circular, then the whole difference between equal time, as shown by the clock, and apparent, as shown by the dial, would arise entirely from the inclination of the Earth's axis; and the change from slow to fast, and fast to slow, would be, as we first mentioned, on the 20th of March, the 21st of June, the 23d of September, and the 21st of December.

This, however, is not the case; for the Earth travels when it is nearest the Sun, that is, in winter, more than a degree in twenty-four hours; and when it is farthest from the Sun, that is, in summer, less than a degree in the same time; consequently, from this cause, if it were to act alone, the natural day would be of the greatest length when the Earth was nearest the Sun; for it must continue turning the longest time after an entire rotation, in order to bring the meridian of any place to the Sun again, and the shortest day would be when the Earth moves the slowest in her orbit. Now these inequalities, combined with those arising from the inclination of the Earth's axis to the ecliptic, or orbit of the Earth, make up that difference which is shown by the equation table. In other words, the obliquity of the Earth's orbit to the equator on the Earth, which is the first-mentioned cause of difference between equal and apparent time, would

make the clock and dial agree when the Earth enters Libra, Capricorn, Aries, and Cancer; but the unequal motion of the Earth, in its orbit, would make them agree twice a year, when the Earth is in its aphelion and perihelion; and, consequently, when these two points fall in the beginning of Cancer and Capricorn, or of Aries and Libra, they will concur in making the Sun and clocks agree. But the aphelion is somewhere in the ninth degree of Cancer, and the perihelion in the ninth degree of Capricorn; and therefore the Sun and clocks cannot be equal about the beginning of those signs, nor at any time in the year, except when the swiftness or slowness of equation, resulting from one of these causes, just balances the slowness or swiftness arising from the other.

The times of Sun-rising and setting for the 1st, 11th, and 21st, will be found as follows, viz:—

Equation of Time.-[See the month of January.]

The following table will show what is to be added to the apparent time shown on the dial, to obtain equal or true time for every 5th day of March.

Tuesday, Mar. 1, to the time on the dial add 12m. 43s.

To obtain true time by the clock.

The Georgian planet will be stationary on the 6th. The Sun enters the sign Aries, 37m. after 5 in the morning of the 21st. The planet Mercury will be found stationary on the 23d.

The Moon will be at the full, at 15m. past 7 in the morning of the 6th, it enters the last quarter at half-past 1 in the morning of the 14th. The succeeding new Moon or change will happen at 6m. past 9 in the evening of the 21st, and this enters its first quarter at 21m. past 5 in the afternoon of the 28th.

The times of the Moon's rising from the first five days after she is in the full, will be as follow, viz. on the

7th of March, 5m. past 7 in the afternoon.

On the 1st day of March, the Moon will eclipse the star marked II.

The immersion will occur at 32m. past 7 in the evening, when the star will be 5' north of the Moon's centre: the emersion happens 43m. past 8, being at that time 3m. north of the Moon's centre..

Mercury's greatest elongation from the Sun happens on the 16th.

In the eclipses of the first satellite of Jupiter, that will be visible in London this month, the emersions at the following times, viz. on the

7th day at 29m. past 1 in the morning.

ACCOUNT OF THE COMET IN 1807.

THIS Comet which appeared in our hemisphere in the last three months of the year 1807, and until the 21st of February 1808, was seen by Dr. Herschel, who had very frequently observed it from the 4th of October to the latter period; but it is deficient in the usual elements of calculation, which have not yet been made public, and our limited time will not permit us to procure them soon enough for our publication, But in other respects the information we have to communicate to our readers will not be the less gratifying, from the particulars we are enabled to present to their notice. It was first observed by Mr. Figott, of Bath, on the 28th of September, 1807, who communicated the intelligence to Dr. Herschel, in consequence of which, the first opportunity that presented of seeing it was on the evening of October the 4th; but the doctor does act profess to state its motion and velocity, and our own observations upon it having been under great disadvantage, do not authorize us to draw conclusions sufficiently specific for elementary purposes. On that night, however, the doctor observed the nucleus of the Comet to be perfectly round, as he also did afterwards on the 18th and 19th; it did not amount to 2". 47" and was certainly less than the diameter of Jupiter's third satellite, with which he repeatedly compared it; and on the 20th of November it was only a mere point with a seven feet reflector, having a power of 75 only. The head of the Comet on the 19th of October appeared as large as Jupiter, but had become less brilliant on the 20th of November. The coma, on the former of the last-mentioned days was about six minutes in diameter, and on the 6th of December about 4" 54", with a mirror of 24 inches diaThe tail on the 18th of October extended over a space of 3° 45'; which was lessened to 2° 30' on the 20th of November, but had diminished to 23 minutes of a degree in

meter.