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its centre, the parallels themselves would therefore not be termed great, but small circles of the sphere.

Having once established a first meridian from which the longitude of places on the earth is to be calculated, in order to discover the longitude of any place, or its longitudinal distance from that first meridian, we must suppose another meridian drawn through the poles and the given place, which must of course intersect the equator: and the distance of this point of intersection from that of the first meridian will point out the longitude of the given place, eastward or westward, agreeably to its position with regard to the first meridian.

Were it required, for instance, to know the longitude of Constantinople, reckoning from the first meridian passing through London, a great circle or meridian should be drawn through the poles, and passing over Constantinople, which would intersect the equator in a point 29°, 0′, 52", to the eastward of that where the first meridian intersects the equator: consequently, the longitude of Constantinople would be 29°, 0', 52" east from London.

Again, if we wished to ascertain the longitude of Kingston in Jamaica, we should find that the meridian of that town could cut the equator in a point 77°, 0', 28", to the westward of the intersection of the first meridian, and consequently reckon Kingston to be in longitude 77°, 0′, 28′′ west from London.

It has been the practice of many geographers to reckon the longitude from some first meridian eastward, quite round the equator, to the meridian where they began: but this practice is or ought now to be laid aside, as productive rather of confusion and labour, than of any utility; because, strictly speaking, no place can have from another more longitude than 180 degrees, or half the circumference of the equator: when, therefore, two places are situated on the same side of the first meridian, and that the most remote is

not

not more than 180° from it, their difference of longitude will be found by subtracting the less from the greater. Should it, for instance, be required to know the difference of longitude between Constantinople, situated in longitude 29°, 0, 52", and Pekin, lying in 116°, 30', 37' both east from London, we have only to take the less quantity from the greater, for the difference of longitude required, which will be 87°, 29', 45". But when the two places are on opposite sides of the first meridian, and neither of them 90 degrees from it, the difference of longitude between them will be found by adding their respective longitudes together : thus the difference of longitude between Constantinople, situated in 29°, 0', 52" east from London, and Kingston in Jamaica, lying in longitude 77°, 0', 28" west from London, will be 106°, 1', 20".

Besides the revolution round its axis, in that portion of time which we call a day, the earth has another motion, by which it revolves round the sun, in that portion of time which we call a year. The path or figure described by the earth, in this annual motion round the sun, approaches nearly to a circle, but is in fact an ellipse, and is named the ecliptic. Whatever may have originally been the case, the plane of the ecliptic does not now coincide with the plane of the equator of the earth; that is, the axis of the earth is not perpendicular to the plane of the ecliptic, being inclined to it in an angle which, in 1769, was ascertained, by accurate observations, to be 66°, 31′, 50"; and, consequently, the plane of the earth's equator must then have formed, with the plane of the ecliptic, an angle equal to the complement of the former, or 23°, 28, 10"; which angle is Had the plane of the

called the obliquity of the ecliptic. equator coincided with that of the ecliptic, the earth in its progress round the sun would have had its axis perpendicu lar to its path, the globe would have been illuminated by the sun's rays constantly in the same manner, all the way from

the

the north to the south pole, and the days and nights, produced by the diurnal revolution of the earth, by which all parts on its surface are successively exposed to his light, would have been constantly of the same length throughout the whole year. No sensible variation in the degrees of heat and cold, constituting that variety of seasons we now observe, especially in regions removed from the equator, would have been perceived: but from the oblique position of the earth's axis to the plane of its path in the ecliptic, proceeds that constant and regular variation we now experience, in the length of the day and night, and in the degree of heat and cold by which the different seasons of the year are distinguished:-in treating of astronomy this subject will be rendered more intelligible to the student.

As the earth in its annual course round the sun, moves in a plane not coinciding with that of its equator, these two planes must intersect each other in two points distant asunder one half of the earth's orbit, and when the earth is in these points, the earth's axis must of course be perpendicular to both planes, so that the sun's light is received equally on all parts of the globe, extending from pole to pole, and thereby causing the day and the night all over the world to be of equal length. From this circumstance it arises that these points of intersection are termed the equinoctial points, and simply the equinoxes.

In the middle point between these equinoctial points, when the earth's path is at its greatest distance from the plane of its equator, which is equal to the angle of the obliquity of the ecliptic, already stated to be 23°, 28', 10" the earth seems for some days to be stationary: and hence those intermediate points at equal distance from the equinoxes are called the solstices, a term signifying that the sun stands still, agreeably to the common notion, that all the phenomena of the seasons were occasioned not by the motion of the earth, but by that of the sun. When the earth is in

one

one of these solstitial points, we have the longest day of the year, and when it is in the opposite solstice, we have the shortest day.

If from the equator on the earth's surface a space equal to the obliquity of the ecliptic, or 23°, 28', 10', be set off towards cach pole, upon any meridian, and through that point circles be drawn from each pole, which must of course be parallel to the equator, these two circles will represent the earth's situation at the time of the solstices, where it begins to turn back towards the equator: and hence these circles are called tropics, from a Greek term signifying to return. That which lies on the north side of the equator is the tropic of Cancer, and that on the south side of the equator is the tropic of Capricorn, so named from certain clusters of stars, or constellations, to which they have a relation.

As the planes of the equator and the ecliptic intersect each other at an angle of 23°, 28', 10", it follows that the poles of a sphere, of which the plane of the ecliptic is a section perpendicular to its axis, must be situated at a corresponding distance from the poles of the equator, or of the earth if then from the poles of the equator as centres, with a radius equal to 23°, 28', 10", circles be described, these circles will represent the various positions in which the poles of the ecliptic must be placed, and the path which they will describe, in the course of an annual revolution of the earth round the sun. The circle described about the north pole, is called the arctic; and that described about the south pole, is called the antarctic circle; denominations borrowed from the name of a constellation in that part of the heavens to which the northern extremity of the earth's axis is always directed. This constellation was called by the Greeks arctos, or the bear, in which is a bright star, known by the name of the Pole star, from its situation very near to the point in the heavens, through which the earth's

axis, if produced northerly, would pass in the Latin language this constellation is called Ursa Miner, and by us the Little Bar.

By means of these circles, which are in general called the polar circles, and the tropics, the surface of the globe is divided into five portions, extending from pole to pole, termed zones, or girdles that which lies on both sides of the equator, and is bounded by the two tropics, is in breadth 23°, 28, 10"X2-46°, 56′, 20", and is called the torrid zone, on account of the intense heat which prevails in that middle region of the earth.

The spaces comprehended between the poles and the polar circles, each being in breadth 23°, 28', 10", are called the frigid zones, on account of the extreme cold experienced in those tracts of the globe.

The intermediate spaces lying between the tropics and the polar circles, each in breadth 43°, 03', 40", as extending from latitude 23°, 28', 10", to latitude 66°, 31', 50", are termed the temperate zones, on account of the moderate temperature of the atmosphere in those regions, equally removed from the heat of the torrid, and the cold of the frigid,

zones.

Thus the surface of the earth is divided into one torrid zoae, begirting its middle, two frigid zones surrounding the poles, and two temperate zones situated between the torrid and the frigid zones.

This division of the earth into zones being, however, too general for the purpose of ascertaining the positions of places respect to each other, or to the equator, the ancients thought it necessary to subdivide its surface into other por

with

tions more minute.

The principle on which this subdivision was made, was the respective length of the day and the night, in different situations on the globe, at the summer solstice, or at the longest day of the year. Supposing the day and the night

VOL. II.

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