so much of a degree sooner will he arrive at the same equinox, or solstice, than at any star, or fixed point on the heavens, in one annual revolution; so in respect to the fixed stars, will cause the equinoctial points, as well as itself, to recede about 30 degrees in 2160 years, and in the whole circle of the equinoctial, in about 25920 years. This period has been called the Grand Platonic Year.

The receding of the equinoctial points has thus occasioned an apparent advance of the fixed stars in longitude of about 50" per year, and from whence it follows that, since the time of Ptolemy, in the infant state of astronomy, the zodical figures, or constellations, have moved forward about one whole sign, and as shewn upon our New British Celestial Globes, the constellation of Aries, situate in that part of the ecliptic, named Taurus. Taurus, in the situation of Gemini, &c. Hence, the stars that rise and set at particular seasons of the year, in the times of Hesiod, Eudoxus, Virgil, Pliny, &c. at the present period, will have a manifest difference in respect to time. It is to the attractive influence of the sun and moon, on the redundant matter in the equatorial regions of the earth, that Sir Isaac Newton, and other astronomers, have asserted to occasion this peculiar slow motion. From the earth's motion on its axis, much more matter is accumulated about the equator, than at any other parts of the globe, and the power of the sun and moon's attraction, is judged to bring the equator quicker under them, than if there were no such accumulation of matter.

The phenomena of the precession, may be more familiarly understood, by the student placing before him a celestial globe : he must bring the pole of the ecliptic to the brazen meridian, and consider both the ecliptic, and its axis, to be immoveable, and the earth's axis, or poles of the equinoctial, to be in motion round the earth's centre, which he may conceive will form a double cone round the axis of the ecliptic, in the time the equinoctial points circumscribe the ecliptic, which is about 25,920 years; and, in that time, the earth's axis will describe a circle in the heavens, round the pole of the ecliptic, which is stationary on that circle, the earth's axis being inclined 234 degrees to that of the ecliptic: the circle described by the north pole, will be 47 degrees in diameter, twice that of the inclination of the earth's axis. Consequently, that point in the heavens, which is now the north pole, and very near to the polar star, as it is called, which is in the tail of Ursa Minor, will be receded from by the earth's axis, at the rate of about 1 degree in 72 years. And, in 12,960 years, will be directed to some other star in the heavens, diametrically opposite in the circle, on the other side of the pole of the ecliptic; and the north pole of the heavens will then be in a situation 8 degrees south of the zenith of London, which is at 51 North. The places, also, of the equator and two tropics, will be very materially changed, And the sun in the same part of the heavens, where he now covers the earthly tropics, and makes the shortest days, and longest nights, in the northern hemisphere;

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will then be over the earthly tropic of Cancer, and make the days longest, and the nights shortest. In 25,920 years, the north pole will have moved quite round, and returned to its present position, and then the apparent motions, and situation of the stars, will begin to have the same changes over again.

The motion of the apsis of the earth's orbit occasions the difference between the periodical and siderial years: the periodical year is the time of the earth's revolution in its orbit, and is 365 6 15 20", differing from the natural year, or period of the seasons, 26′ 21′′.

A distinction is made by astronomers between the precession of the equinoctial and solstitial points in the heavens, which only affect the apparent motions, places, and declination of the stars; and the anticipation of the equinoxes of the earth, which regard the seasons, the latter is owing to a difference between the civil and solar year, which is 11 minutes 3 seconds. This excess of the civil or Julian years, above the solar, amounts to 11 days in 1,433 years; and, consequently, so much have our seasons gone back, with respect to the days of the month, since the period of the Council of Nice, in A.D. 325; and, therefore, in order to restore all the fasts and festivals to the days then settled, it was requisite to suppress 11 nominal days, and that the same seasons, in future, might be returned to the same times of the year, to leave out the bissextile day in February, at the end of every century of years not divisible by 4, reckoning them only common years,

as the 17th, 18th, and 19th centuries, or the years 1700, 1800, 1900, and because a day intercolated every 4th year, was too much; and, retaining the bissextile day at the end of these centuries of years, which are divisible by 4, as the 16th, 20th, and 24th centuries, viz. 1600, 2000, 2400, &c. otherwise, in length of time, the seasons would be quite reversed, with regard to the months of the year, in the course of about 23,783 years.

OF THE NUTATION OF THE EARTH'S AXIS.

It is to the late Dr. Bradley that we owe the first observations of the libratory variation of the inclination of the equator to the ecliptic, and which is termed the nutation of the earth's axis. Sir Isaac Newton's theory of attraction had, for some time before, given grounds for the existence of such a property. Its whole effect is said scarcely to exceed 19 seconds.

OF ASTRONOMICAL ABERRATION.

Aberration, in astronomy, is the apparent change of place in the fixed stars, arising from the motion of the earth, combined with the motion of light. It is a discovery by the late Dr. Bradley, who, with an astronomical instrument, called a Zenith Sector, made a number of observations for three years upon the same stars, and found that their apparent places

differed from their true places about 20 seconds: and hence proving, that the velocity of light is about 10,000 times greater than the earth's velocity in its orbit. The velocity of the earth is computed at 58,000 an hour; and, therefore, light is propagated from the sun to the earth in 8 minutes and about 10 seconds of time, a distance of 95 millions of miles, or the distance of the earth at a mean from the sun. The velocity of light is thus proved to be uniform, whether originally from the stars, or reflected from the satellites of Jupiter, as precisely deduced by observations previously made on these satellites by Roemer.

OF THE TIDES.

The rising and falling, alternately, twice in a day, of the waters upon the earth, is one of the most remarkable of natural phenomena. To the vast penetration of Sir Isaac Newton, we are here, also, beholden for the demonstrations of the moon's action upon this globe, and the manner this admirable effect is produced. The tides are considered as affording a very obvious instance of the tnutual gravitations of the celestial bodies to each other.

The attractions of the sun and moon upon the fluid parts of the earth are now considered as the only agents of the tides, and from repeated observations the times of ebbing and flowing are found to be comparable to the periodical revolutions of the moon, particularly, and proportionate to them.