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at sea, the first indication of its presence is the smoke from its funnel, which is seen long before the funnel itself is visible; likewise, when sailing ships are approaching each other at sea the upper sails and riggings are discerned some time before the hull and lower part of the vessel come into view. The same appearances are observed everywhere, in the Indian Ocean as well as in the Atlantic; hence, the rotundity is uniform and the earth globular. Again, the earth has been circumnavigated many times; and during eclipses of the moon the shadow projected by the earth on the moon's surface is invariably found to be circular. Many other proofs could be submitted but those already mentioned will perhaps suffice; they all tend to prove the same fact—the sphericity of the earth.
8. The dimensions of the earth, according to Col. A. R. Clarke, are as follows: . Equatorial radius = 3,963.307 miles, or 20,926,202 feet. Polar radius =3,949.871 miles, or 20,854,895 feet. From this it is evident that the flattening at the poles does not amount to much, the difference between the two radii being about 13.5 miles. By using this value of the radius the equatorial circumference of the earth may be approximately estimated at 25,000 miles, and the surface of the whole earth, land and water, at 49,243,000 square miles.
DEFINITIONS RELATING TO THE EARTH
9. The axis of the earth is that diameter around which the earth daily revolves with uniform motion from west to east; the revolution being completed in 24 hours.
10. The poles of the earth are the extremities of its axis, or the points in which the axis meets the surface. Thus, if the line PP, Fig. 1, represents the axis of the earth, the points P and P' are the poles; the pole to which we, in this country, are nearest is the north pole, while the opposite one is the south pole. Since the poles are the extremities of a diameter, they are 180° apart.
11. The equator is a great circle Ec E' on the earth's surface equidistant from the poles. It divides the earth into two equal parts, or hemispheres—the northern hemisphere and the southern hemisphere. The poles of the earth are the poles of the equator, every point of the latter being 90° from either pole.
12. The meridians are great circles that pass through the poles of the earth, and are therefore perpendicular to the equator. Thus, if / b d E’, Fig. 1, represents the earth's equator, the great circles Pa P", Pb P', etc., passing through and intersecting at the poles, are meridians.
Of all the innumerable meridians that may be imagined drawn on the surface of the globe, one is always, for the purpose of reference, selected as the prime, or first, meridian. Most of the maritime nations having a national observatory usually adopted the meridian that passed through the observatory as the first meridian; but at the conference held at Washington, D. C., in 1884, for the purpose of adopting a universal first meridian, the one passing through the Royal Observatory at Greenwich, England, was selected.
13. The latitude of any place on the earth's surface is the distance north or south from the equator measured on a meridian passing through the place. Thus, if a place is situated at A, Fig. 2, north of the equator /, /o/, the latitude of that place is the arc BA of the meridian P B P', which passes through the place, and is named north because A is situated in the northern hemisphere. Again, should a place be situated at C, in the southern hemisphere, its latitude is the arc DC of the meridian P/O P', and is named south. Latitude is reckoned from the equator toward the poles in degrees, minutes, and seconds, and since the distance from the equator to either pole is 90°, the latitude of any place can never exceed that amount. When a ship is on the equator, its latitude is 0°; if at the north pole, its latitude would be 90° N.; if at the south pole, 90° S. FIG. 2
14. Parallels of latitude, or latitude parallels, are small circles whose planes are parallel to the equator. Every point on the circumference of a latitude parallel is equidistant from the equator, and, consequently, all places situated on the same latitude parallel have the same latitude. Like meridians, latitude parallels can be drawn through any place on the earth.
15. The difference of latitude of any two places is the arc of a meridian contained between the two latitude parallels passing through those places. In Fig. 2, the difference of latitude between F and G is the arc Zo G of the meridian contained between the latitude parallels passing through the two places. All places situated on those latitude parallels have the same difference of latitude. Thus, the difference of latitude between A and F is the same as that between / and G.
For example, the difference of latitude between A and C, Fig. 2, is the arc G. C or A //. The difference of latitude between B and F, Fig. 2, is the arc A. D or J /3.
16. If the places are both on the same side of the equator, or, in other words, their latitudes have the same name, the difference of latitude is found by subtracting the smaller latitude from the greater; when the two places are on of posite sides of the equator, that is, when their latitudes have different names, the difference of latitude is obtained by adding the two latitudes.
ExAMPLE 1.-Find the difference of latitude between Rio Janeiro and the Cape of Good Hope, the latitude of the two places being 22° 55' S and 34° 22' S, respectively. Solution.—Lat. Cape of Good Hope=34° 22' S Lat. Rio Janeiro =22° 55' S(–) Diff. of lat. = 11°27. Ans.
ExAMPLE 2.-The latitude of Cape Verde is 14° 43' N; that of Cape St. Roque is 5°28' S. Find the difference of latitude.
ExAMPLE 3.-The latitude of Southampton, England, is 50°54' N; the latitude of New York is 40°42.7' N. Required the difference of latitude between the two places.
17. When a ship sails north in northern latitudes and south in southern latitudes, she increases her latitude; but when sailing south in northern latitudes and north in southern latitudes, she decreases her latitude. Hence, when the difference of latitude and one latitude is given, the other is readily found by addition or subtraction, as shown in the following examples.
NotE.-By lat. left is understood the latitude of the place the ship sailed from, and by lat. in the latitude of the place arrived at.
between the first meridian and the meridian passing through G is the longitude of that place, and is named west because it lies to the
west of the first
meridian. If the place G had been to the east of PAE P', its longitude would have been so many degrees east.
Longitude may also be defined as the
angle at the pole
subtended by the first meridian and the me ridian passing through the place. This angle is G. P/o, Fig. 3 (a); the arc G F of the parallel and the arc B D of the equator, we know, contain the same number of degrees, minutes, and seconds, although the //near distance of B /) is greater than G. F.
21. Longitude is reckoned from 0° to 180° east or west, but is never considered greater than 180° either way; if it exceeds 180° it is subtracted from 360° and given the contrary name. Thus, longitude 186° W is equal to 360°