to be the First Meridian which passes through the Royal Observatory at Greenwich; and the French reckon for their First Meridian, that which passes through the Royal Observatory at Paris.-Formerly many French geographers reckoned the meridian of the island of Ferro to be their First Meridian; and others, that which was exactly 20 degrees to the west of the Paris Observatory. The Germans, again, considered the meridian of the Peak of Teneriffe to be the First Meridian. By this mode of reckoning, Europe, Asia, and Africa, are in east longitude; and North and South America, in west longitude. At present, the first meridian of any country is generally esteemed to be that which passes through the principal Observatory, or chief city of that country.

The Longitude of any place is that portion of the Equator which is contained between the first meridian, and the meridian of that place and is usually reckoned either east or west, according as the given place is on the east or west side of the first meridian; and, therefore, cannot exceed 180°.

The Difference of Longitude between any two places is the intercepted arch of the Equator be tween the meridians of those places, and cannot exceed 180°.

There are three different Horizons, the apparent, the sensible, and the true. The apparent or visible Horizon is the utmost apparent view of the sea or land. The sensible is a plane passing through the eye of an observer, perpendicular to a plumb line harging freely; And the true or rational Horizon is a plane passing through the centre of the... Earth, parallel to the sensible Horizon.

Altitudes observed at sea, are measured from the visible Horizon. At land, when an astronomiical quadrant is used, or when observations are taken with a Hadley's quadrant by the method of reflection, the altitude is measured from the sensible Horizon; and in either case, the altitude must be reduced to the true Horizon.

The Zenith of any given place is the point immediately above that place, and is, therefore, the elevated pole of the Horizon: The Nadir is the other pole, or point diametrically opposite.

A Vertical is a great circle passing through the Zenith and Nadir; and, therefore, intersecting the Horizon at right angles.

The Altitude of any celestial body in that portion of a Vertical, which is contained between its eentre and the true Horizon. The Meridian Altitude is the distance of the object from the true Horizon, when on the Meridian of the place of observation. When the observed Altitude is corrected for the depression of the Horizon, and the errors arising from the instrument, it is called the apparent Altitude; and when reduced to the true Horizon, by applying the parallax in Altitude, it is called the true Altitude. Altitudes are expressed in degrees, and parts of a degree.

The Zenith Distance of any object is its distance from the Zenith, or the complement of its Altitude.

The Declination of any object is that portion of its meridian which is contained between the equinoctial and the centre of the object; and is either north or south, according as the star is between the equinoctial and the north or south pole.

The Ecliptic is that great circle, in which the annual revolution of the Earth round the Sun is performed. It is so named, because Eclipses cannot happen but when the moon is in or near that circle. The inclination of the Ecliptic and Equinoctial is at present about 23° 28′; and by comparing ancient with modern observations, the obfiquity of the Ecliptic is found to be diminishing which diminution, in the present century, is about half a second yearly.

The Ecliptic, like all other great circles of the sphere, is divided into 360°; and is further divided into twelve equal parts, called Signs: each Sign, therefore, contains 30°. The names and charac ters of these Signs are as follows:

Since the Ecliptic and Equinoctial are great circles, they, therefore, bisect each other in two points, which are called the Equinoctial Points. The Sun is in one of these points in March, and in the other in September; hence, the first is called the Vernal, and the other the Autumnal Equinox-and that sign which begins at the Vernal Equinox is called Aries. Those points of the Ecliptic, which are equidistant from the equinoctial points, are called the Solstitial Points; the first the summer, and the second the winter solstice. That great circle which passes through the equinoctial points and the poles of the earth, is called the Equinoctial Colure: and the great circle which passes through the solstitial points and the poles of the earth, is called the Solstitial Colure

When the Sun enters Aries, it is in the Equinoctial; and, therefore, has no declination. From thence it moves forward in the Ecliptic, according to the order of the signs, and advances towards the north pole, by a kind of retarded motion, till it enters Cancer, and is then most distant from the Equinoctial; and moving forward in the Ecliptic, the Sun apparently recedes from the north pole with an accelerated motion till it enters Libra, and being again in the Equinoctial, has no declination; the Sun moving through the signs Libra, Scorpio, and Sagittarius, enters Capricorn; and then its south declination is greatest, and is, therefore, most distant from the north pole; and moving for ward through the signs Capricorn, Aquarius, and Pisces, again enters Aries: Hence, a period of the seasons is completed, and this period is called a Solar Year.

The signs Aries, Taurus, Gemini, Cancer, Leo, and Virgo, are called Northern Signs, because they are contained in that part of the Ecliptic which is between the Equinoctial and North Pole; and, therefore, while the Sun is in these signs, its declination is north: the other six signs are called Southern Signs. The signs in the first and fourth quarters of the Ecliptic are called Ascending Signs: because, while the Sun is in these signs, it approaches the north pole-and, therefore, in the northern, temperate, and frigid zones, the Sun's meridian altitude daily increases; or, which is the same, the Sun ascends to a greater height above the horizon every day. The signs in the second and third quarters of the Ecliptic are called Descending Signs.

The Tropics are circles parallel to the Equinoctial, whose distance therefrom, is equal to the

obliquity of the Ecliptic. The Northern Tropic touches the Ecliptic at the beginning of Cancer, and is, therefore, called the Tropic of Cancer; and the Southern Tropic touches the Ecliptic at the beginning of Capricorn, and is hence called the Tropic of Capricorn.

Circles about the poles of the Equinoctial, and passing through the poles of the Ecliptic, are called Polar Circles; the distance, therefore, of each Polar Circle from its respective Pole, is equal to the inclination of the Ecliptic and Equinoctial. That Circle which circumscribes the North Pole is called the Artic, or North Polar Circle; and that towards the South Pole, the Antartic, or South Polar Circle.

That semicircle which passes through a star, or any given point of the heavens, and the Poles of the Ecliptic, is called a Circle of Latitude.

The Reduced Place of a Star is that point of the Ecliptic, which is intersected by the circle of latitude passing through that star.

The Latitude of a Star is that portion of the circle of latitude contained between the Star and its reduced place-and is either north or south, according as the Star is between the Ecliptic and the north or south pole thereof.

The Longitude of a Star is that portion of the Ecliptic, contained between the Vernal Equinox and the reduced place of the Star.