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If we divide the whole space, contained between the northern and southern extremities of the axis of the earth, into two equal parts, and suppose a line to be drawn round the circumference of the globe, at that distance from the poles of the earth, such a line would divide the globe into two equal portions, or hemispheres, and thence it is termed the equator. It was already noticed that the circumference of every circle is by geometricians supposed to be divided into 360 equal parts, called degrees, (Vol. I. p. 383): if, therefore, we divide this surrounding line into 360 equal parts, each will be a degree on the earth's surface al the equator, or at its greatest distance from both poles : but supposing another great circle to be drawn on its surface passing through both poles, and crossing the equator at right angles, in two points diametrically opposite the one to the other, this circle, which is also to consist of 360 degrees, will be cut into 4 equal portions of 90 degrees each, extending from the north pole to the equator, from the equator to the south pole, from the south pole round to the opposite side of the equator, and from the equator back to the north pole; and the distance between the poles of the earth, or between any point of the equator, and another diametrically opposite to it, will be a semicircle, or 180 degrees.
All that part of the earth's surface which was known to the ancients, was of much greater extent from east to west than from north to south; for the temperate climates allowed them to penetrate much farther in the directions of the rising and setting sun, than towards the north, where eterna, Snows precluded all access, and to the south where intolerable heats, as they imagined, rendered it impossible for man to exist. Under such ideas it was natural for them to consider the easting and westing as the length of the earth, while the Borthing and southing was regarded as its breadth ; lience the distance of any place, east or west from a given station,
was called, agreeably to the Latin term which we have adopted, its longitude, and the distance of any place north or south from a given point they termed its latitude. Taking, therefore, the equator for the middle of the globe, equally distant from both poles, the latitude of any place is reckoned from it, being called north latitude when the place lies on the north side of the equator, and south latitude when the place lies on the south side of the equator. No place can have more than 90 degrees of latitude on either side, because 90 degrees is the whole distance between the equator and the poles: and the equator being a fixed, unalterable line surrounding the globe, latitudes may be calculated from any part of it.
The longitude of a place is calculated on the equator from a certain point chosen at pleasure, because in that circumference of the earth there is no natural limit whence it can be reckoned, as in the calculation of the latitude: it has therefore been the practice, in all times, ancient and modern, for geographers to assume the position of some remarkable island, cape, or town, from which all distances in easting or westing were to be counted: but as no proper situation may be conveniently found on the equatorial line of the earth, it becomes necessary to discover the point on that line which corresponds to some eligible spot on its north or south sides, from which the longitude may be reckoned.
Suppose we choose London as the spot from which longitude cast or west is to be computed: if a circle be drawn through the poles of the earth, and passing over London, it will intersect the equator at right angles, and consequently at the shortest distance from London, in a point corresponding to the position of that city, from which point if longitude be reckoned, it may be considered as computed from London itself; and if 180 degrees are set off on the equator, in both directions, east and west, the one semicircle will
indicate the longitude east, and the other the longitude west from London.
The circle just described as passing through the poles of the earıb, and over London, is termed the meridian of that place. This name is borrowed from a Latin term signifying what relates to noon or mid-day ; because when in the daily revolution of the earth this point comes to be turned directly towards the sun, he is then at his greatest apparent elevation, and consequently it is then mid-day or noon at London. As every spot on the face of the globe must have its own meridian, or point opposite to the sun at mid-day, the number of meridians must of course be indefinite : but having already made choice of London as the point from which to reckon the longitude, the great circle passing over it, and intersecting the equator, is termed the first meridian; and from this point of intersection the longitude is computed eastward and westward, with the same certainty as the latitude is counted northward and southward from the equator.
All great circles on the sphere being of equal magnitude, the meridian of London will contain the same number of degrees with the equator, and the arch intercepted between the pole and the equator being a quadrant, will contain 90 degrees: if then we measure the portion of the meridian intercepted between London and the equator, which will be found to contain 51 degrecs, 30 minutes, 49 seconds, we say that London is situated in north latitude 51 degrees, 30 minutes, 49 seconds. In the same way by measuring the arch of the meridian passing over Edinburgh, intercepted between that place and the equator, we find it contains 55 degrees, 57 minutes, 5 seconds ; we therefore say Edinburgh lies in north latitude 55, 57', 5" : thus also Dublin will be found to be situated in north latitude, 53°, 21', 11'.
Again, if the place over which a meridian passes be situated between the equator and the south pole, its latitude
will be termed southern ; as, for instance, the Cape of Good Hope, which lies in south latitude 33°, 55', 12"; thus also Cape Horn, the most southerly point of South America, lies in south latitude 55°, 58', 30", and Botany Bay, in south latitude 34', 6.
When we compare the arches of the meridians of two places intercepted between each place and the equator, we discover the difference of latitude between the given places : thus the latitude of Edinburgh being 55°, 57', 5", and that of London being 51°, 30, 49", and both lying on the same side of the equator, by subtracting the less quantity from the greater, we obtain 4., 26', 16", for the difference of latitude between London and Edinburgh ; that is, Edinburgh is situated 4°, 26', 16" farther to the northward of the equator than London. But if we wish to know the difference of latitude between two places on opposite sides of the equator, the one in north, and the other in south latitude, it is evident that as the two latitudes are calculated from the equator which lies between the two places, we must add the iwo latitudes together, when the sum will be the quantity of latitude between the given places ; thus, the latitude of London being 51°, 30', 49' northerly, and that of the Cape of Good Hope being 33°, 55', 12" southerly, by adding them together we obtain 85°, 26', 1", for the latitudinal difference between these two places.
If from either of the poles a number of concentric circles be drawn on the earth, parallel to the equator, such circlcs will serve to point out the latitudes of the several places through which they pass, from which property they are called parallels of latitude ; and being described with radii successively less than that of the equator, these parallels will gradually diminish until at the poles they finally disappear. But such parallels being described with radii less than that of the globe, their planes would not pass through
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 lo 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 castward 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 77o, 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 rougd 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 cquator : when, therefore, two places are situated on the same side of the first meridian, and that the most remote is