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 ta 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, à 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 mutual 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.

From the greater distance that the sun is from the earth, his attractive power in raising the water, must be very considerably less than the power of the In general, the sun counteracts the effects of the moon, more than it assists in augmenting the flux and reflux of the sea.

moon

In the course of 24 hours, 48 minutes, the sea is observed to ebb and flow twice: the time of rising being about 6 hours, and the time of falling the same.

From the motion of the moon in her orbit, the lunar day exceeds the solar day; the former being about 24 hours, 48 minutes, and the latter 24 hours: hence, the effect of the sun and moon on the tides, are not the same every day. The influence of the sun and moon on the equatoreal parts of the earth, is greater than towards the poles, from the attractive powers acting less obliquely at the equator, than the poles; and also the accumulated matter about the equator. The time of high water is about 48 minutes later every day, on account of the moon being about that time in her arrival later daily at the meridian of that place; it being high water at the time that the moon is nearly in the meridian of the place, and the sea having flowed to its highest state; and it is low water at the interval of time, 6 hours before or after this period, when the moon is in, or near to, either the east or west points of the horizon. Or, when at high water at any place, on the meridian of 90 degrees, east or west of that place on the earth, is low water. But, a remarkable circumstance

is, that when it is high water at any place, it is also high water at the place where its meridian is diametrically opposite, the cause of which is derived from another principle, different from the moon's attractions in the other case, and which shall be explained hereafter.

The power of the sun is sufficient to raise the waters, somewhat when on our meridian, and an effect oppositely, also, then takes place. When it acts in conjunction with the moon, it produces a more powerful effect on the waters, and it is then called Spring Tides: when it acts in a direct perpendicular to the moon, it tends to diminish her power, and produce a diminished tide, called Neap Tides : hence, at the time of new and full-moon, there must be high tides, and low tides at the two quarters, first and third of the moon. When the sun and moon are in the Syxgies, or in opposition and conjunction at the time of the equinoxes, there is the highest tides of any, on account of the sun being then in the equator, and the moon in the same situation, or very near to it.

The action of the sun is judged to raise the waters of our seas about two feet, and the moon about nine feet; therefore, in joint action, the elevation will be about eleven feet at the spring tides, and when act¬ ing at right angles, be as the other at neap tides, about 7 feet, the difference between the two.

The earth and the moon, according to the laws of gravitation and motion, move round their centre of gravity, which centre is as much nearer to the