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The longitude of any place, therefore, being its distance from a given point, measured along the Equator in a direction East or West, and the latitude of the same place being its distance from the Equator, measured in a direction North or South, if we know both the latitude and longitude of a place we know its exact situation on the Globe. To exemplify this in the Map of the World (see also the Map of Europe), on the right hand, or Eastern Hemisphere: every place from the top to the bottom of the map, lying under the meridian marked 30 on the equator, is in the thirtieth degree of longitude East of London: I should find Alexandria, in Egypt, and Petersburg, in Russia, both in this degree of longitude, but I know not their relative distance from each other; they may lie one under the Equator, the other under the North or South Pole, or in any other possible position on the meridian marked 30 from the top of the map to the bottom of it; but when I find the latitude also, I have their exact position, both with respect to each other and all other known places on the globe : thus I find also, that the sixtieth degree of North latitude passes through St. Petersburg, and about the thirtyfirst degree of North latitude through Alexandria, or that Petersburg is 60°, Alexandria 31°, North of the Equator; consequently I now know their exact situation with regard to each other, to London, and to every place on the Earth's surface. We say, therefore, that Petersburg is in 60° N. lat. (North latitude), and 30° E. long. (East longitude), and that Alexandria is in 31° N. lat. and 30 E. long.
Every degree contains 60 geographical, or 69} English miles: when other miles are not expressed, geographical miles are to be understood as spoken of. In the Map of the World the divisions of latitude and longitude are made at 10° distance, as the most convenient: but it is evident, that the more minute the divisions can conveniently be made, the more accurately willthe situations of places be ascertained. In the Map of Europe the divisions are at every 5° of lat. and long.; in the Map of England, at every degree, and each degree is subdivided. into six equal parts; each, therefore, contains ten miles.. Hence, although it is not easy in a small Map of the World to ascertain the situation of places within perhaps: a degree, in the Map of Europe we may do it with tolerable accuracy to the half or quarter of a degree, and in the Map of England almost to the precision of a single mile. It may be necessary here to caution the young student, that the exact situation of a place is to be taken from the small spot, or circle, which is made near, and generally at the beginning of the name. *
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of the globe des cin value, wholly latitude, the figur side of the
* It may be remarked, that the latitude is to be reckoned on either side of the map; and the longitude at the top or bottom, according as the reference may be made with the greatest convenience. The learner may farther observe, that when the figures increase in value from the bottom to the top of the map, the portion of the globe described in the map is wholly in North latitude; when they decrease in value, wholly in South. latitude; when it is partly in North and partly in South latitude, the figures increase upwards on the North and downwards on the South side of the cypher. - See the Map of the World, of Europe, and of Africa. There is no map in common collections used at schools comprising a part of the globe wholly in Southern latitude: South America contains the greatest portion of itr Also when the figures continually increase to the right at the top or bottom of the map, all the places in it are in East longitude; when they continually increase to the left, in West longitude; when they partly increase to
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It may be an useful Exercise to describe the latitude
. . Canterbury
Of turning Degrees of Longitude into Zime. As the Earth makes one complete revolution on its : axis, in a direction from West to East, in 24 hours, any
the right and partly to the left of the cypher, the places are situated partly in East and partly in West longitude. - See the Maps of Germany, Ireland, and Europe. In most maps, unless the contrary be expressed, the top of the map is the North, the bottom the South, the right side the East, and the left side the West. 1'.
one point of the Earth's surface must have been carried through 360° in that time; or if we suppose à fixed meridian above the Earth, like the brazen meridian of a globe, every part of the Equator, containing 360°, must have passed under that meridian in 24 hours. Hence, if we divide 360 by 24, we shall find how many degrees pass under this meridian in one hour, which will be 15. Now, as the Earth really revolves in a direction from West to East, it is continually as it were falling below the heavenly bodies on the Eastern, and rising above those on the Western horizon; hence, those bodies appear to rise in the East, and set in the West, because they come into sight as the Eastern edge of our horizon falls below them, and are hid from view as the Western edge rises above them. If the sun, therefore, appears to rise at six o'clock at London, it will be yet an hour before it appears to rise at a place 159 West of London, two hours before it appears to rise at a place 30° West of it, and so on. In like manner, it will have already appeared to have risen one hour at a place 150 East of London, two hours at a place 30° East, and so on. Therefore, when it is twelve o'clock at noon at London, it is one o'clock in the afternoon at all places 15° East of London, and only eleven o'clock in the forenoon to all places 15o West of London. The following questions may serve to exercise the young student:
It is eleven o'clock in the morning at Vienna – Where is it noon, where is it one in the afternoon, and where is it ten in the morning ?
When it is mid-day at London-Where is it midnight?
When it is two o'clock at Jamaica (see the Map of the West Indies) - What o'clock is it at Shrewsbury (see the Map of England) ?
Suppose an eclipse of the sun takes place at three in the afternoon at the place where I am, and I see by the almanack that it took place at half past twelve in London - In what longitude am I?