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GEOGRAPHY implies a description of the earth, being derived from the Greek words yn the earth, and ypáow to describe.
The form of the earth is very nearly spherical; the polar axis being only about 38 miles shorter than the equatorial, which, in a diameter of near 8000 miles, can produce no sensible difference.
The principal circles on the globe are the Equator, the Ecliptic, the Tropic of Cancer, the Tropic of Capricorn, the Arctic and Antarctic circles. Every circle, whether greater or less, is divided into 360 degrees; for the antients supposed that the Ecliptic, or circle which the sun appears annually to describe in the heavens, was completed in 360 days. Each day's advance in this circle they called a gradus, or step, or degree, and applied the same mode of division to circles in general. Each degree is subdivided into 60 minutes, and each minute into 60 seconds. Degrees, minutes, and seconds, are marked thus °, ', "; thus 23° 40' 52" means 23 degrees, 40 minutes, 52 seconds. The half of 360 is 180, and the half of 180, or the fourth part of 360, is 90. Hence if the whole circle contains 360°, a:
semicircle will contain 180°, and a quadrant, or quarter of a circle, will contain 90°, or an angle called a right angle. Hence it will be seen that the Equator dividing the earth equally, must divide it into two semicircles, containing 180° above and 180° below, or, reckoning by quadrants, into two quadrants of 90° each above, and two of 90° each below the Equator. *
The Ecliptic, or circle which the sun appears to describe in the heavens, sets out from the Equator, and continues to rise, through the first quadrant, to the Tropic,
* A straight line passing through the centre of any circle till it meets the circumference in two points, is called the diameter of the circle, because it drapstesi - measures through it. Half this diameter (or a line drawn from the centre to the circumference in one point) is called the radius of the circle. And it is a property of the circle to have all its radii, or diameters, of equal length. If a circle be supposed to turn round on its diameter, it will generate a solid figure called a sphere. Such is the figure of the earth very nearly. The diameter on which the circle revolves is called its axis. The extreme points of this diameter are called its poles, from podsiv - to
urn round. A great circle is any circle described on a sphere, whose diameter is equal to the diameter of the sphere, The Equator and Ecliptic are called primary great circles. A secondary is a great circle whose axis is at right angles to the axis of the primary: the poles, therefore, of the secondary will be 90° from the poles of the primary. An arc is any part of the circumference of a circle contained between two radii, and is denominated from the number of degrees it contains. Thus 30° of the circumference, contained between two radii, is called an arc of 30°; a quadrant is an arc of 90°; a semicircle is an arc of 180°. Parallels are lesser circles which every where keep at the same distance from the primary circle, and so run as it were paginańhes - by the side of each
other. The remaining greater and lesser circles of the globe are · omitted, as unnecessary to be described here.
of Cancer; it then * turns, or declines, towards the Equator, for the second quadrant, till it again meets the Equator 180° from the place at which it set out; it then descends, for the third quadrant, below the Equator to the Tropic of Capricorn, from whence it turns upwards towards the Equator, for the fourth quadrant, till it reaches the point from which it set out. Thus we see a change in the direction of the Ecliptic, with respect to the Equator, at every quadrant.
The Equator, or Equinoctial, is so called because on the two days on which the sun is in the Equator, in the signs of Aries, and Libra, noctes æquantur, or the time of day and night is exactly equal all over the world. .
The Ecliptic is so called because all éxasiyers, or eclipses of the sun or moon, can only take place when the moon is in or near that circle. + : :
The Tropics are two parallels to the Equator drawn through the Ecliptic, at those points where the Ecliptic is at the greatest distance from the Equator; this is found to be about 23° 30' from the Equator, on either side.
The Polar circles are those circles which are supposed to be described by the Poles of the Ecliptic revolving round the poles of the Equator. Hence they must be the same distance from the poles of the Equator, as the plane of the Ecliptic from the plane of the Equator, or 23° 30', which is the distance of the Tropics from the Equator. * Hence the name of Tropic, from spíru, to turn.
+ An eclipse of the Sun is caused by the moon intervening between the sun and earth, so that the moon's shadow falls on the earth. An eclipse of the Moon is caused by the earth intervening between the sun and moon, so that the earth's shadow falls on the moon.
The Zones are so called from (wval, belts or girdles, . being those spaces contained between the several principal circles we have described. Thus, between the Poles and Polar Circles are the two Frigid Zones, between the Frigid Zones and the Tropics are the two Temperate Zones, and between the two Tropics the Torrid Zone, deriving these appellations from the temperature of the atmosphere.
Longitude is the distance of any place from a given spot, generally the capital of the country, measured in a direction east or west, either along the equator or any circle parallel to it. *
Thus the English measure their longitude East and . West of London (or rather Greenwich), the French East and West of Paris, &c. &c. +
Latitude is the distance of any place from the Equator, north or south, and is measured along a secondary to the Equator, supposed to be drawn through the place. :
Meridians or circles of longitude are so called from
* The antients (who knew more of the earth from the Streights of Gibraltar to the Euphrates, and beyond it, i.e. from West to East, than from the Barbary Coast to the Baltic, i. e. from South to North,) called the greater dimensions the Longitude, or length, and the smaller the Latitude, or breadth. Hence the origin of the terms longitude and latitude, as applied to distances on the earth's surface; the former being measured in a direction East and West from a given point, the latter in a direction North or South.
+ The antients measured their longitude from one fixed meridian, which passed through the Fortunatæ Insulæ, or Canary Islands.
meridies, or mid-day; because, as the Earth makes one complete revolution round its own axis in 24 hours, every part of its surface must in the course of that time be directly opposite to the sun. The sun, therefore, will then appear at its greatest altitude to the inhabitants at that point; and will afterwards appear to descend for as long a time as he appeared before ascending: in other words, it will be mid-day or noon. It is, therefore, evident that there may be as many meridians drawn as there are points in the Earth's equator: for the sake of convenience, they are generally drawn at 10° distance from each other in maps of the world, and at 5°, or less, in maps containing a smaller portion of the Earth's surface.
Parallels of latitude are smaller circles drawn parallel to the Equator. As the circumference of the Earth's surface is greatest at the Equator, and decreases continually towards the Poles, it is evident that the circles of latitude, which are parallel to the Equator, must also continually decrease in like manner; therefore, the number of miles in each parallel of latitude must continually decrease. But the number of degrees in every circle, whether greater or less, is always 360°; therefore, the number of miles in each of these 360°, or in every degree of longitude, must continually decrease from the Equator to the Poles. We may, therefore, consider the Equator as the greatest of all the circles of latitude. *
* It is evident that the number of miles in the meridians, or circles of longitude, which are all great circles, is every where the same, therefore the number of miles in a degree of latitude is every where the same.