LESSONS IN GEOGRAPHY. 



Iff 



To draw tho meridians in tho Eastern Hemisphere, describe 



arcs of circles through tin- north and south poles as two points, 



ami through each of tho degrees marked 0, 10, 20, 30, etc., of 



. hothor east or west, aa tho third or middle point, and 



tin* will tfivo the meridian of each point so marked, at every ten 



degrees from 0" to 180, east or { ; thefto meridians will servo 



aa a guide to the determination of other meridians, and enable 



the geographer to approximate to tho true position of those 



which he may wish to lay down on the map, of which 



in- has thus ilr.isvn the skeleton. 



ity tho combined help of the parallels of latitude and tho 

 rurri'lians, tho draughtsman may now set to work to fill up this 

 skrl.-tnu map from a table of latitudes and longitudes, with tho 

 11:11111--' of ull the most important places in the world ; he may 

 :i! -. ' ilraw a pretty correct outline of the coast of each continent 

 by laying down the latitudea and longitudes of as many coasting 

 p. iints as possible from such a table, and drawing a curve 

 through them, as like other maps of the world as he can ; the J 

 accuracy of the map increasing with tho number of point* laid 

 down according to their latitudes and longitudes. In Norie's 

 Navigation, Table 56, are given the latitudes and longitudes of 

 tho principal ports, harbours, capes, shoals, rocks, etc., in the 

 world, founded on thousands of observations made by the most 

 eminent astronomers and navigators ; and this table will enable 

 a true student of geography to lay down the outline of tho coasts 

 of almost all the continents, islands, and peninsulas in the Map 

 of the World, to any scale or size which he chooses to adopt. 

 He may then fill up the interior of these with the positions of 

 the most important places of the world, from the tables of lati- 

 tudes and longitudes usually attached to the ordinary atlases 

 used in colleges and schools. 



Before concluding this lesson, it may be proper to remark 

 that there are four small circles on the globe, placed among the 

 parallels of latitude, which serve to divide the earth into five 

 zones (from tho Greek favri, zo'-ne, a belt) between the two poles. 

 The two smaller circles, which are of the same size, are called 

 the Polar Circles ; tho one, the Arctic, or North Polar Circle ; 

 and the other, tho Antarctic, or South Polar Circle. The two 

 larger circles, which are also of the same size, are called the 

 Tropics ; the one, the Tropic of Cancer ; and the other, the Tropic 

 of Capricorn. The polar circles are each 23 28' distant from 

 the poles, when that distance is measured on a meridian : and, 

 consequently, the one, the Arctic Circle, is the parallel of latitude 

 66 32' N. ; and the other, the An' arctic Circle, is the parallel of 

 latitude at 66 32' S. ; because tho poles being 90 distant from 

 the equator, we have 90 23 28' = 66 32'. 



The Tropics are each 23 28' distant from the equator when 

 that distance is measured on a meridian : and, consequently, the 

 one, the Tropic of Cancer, is the parallel of latitude at 23 28' 

 N. ; and the other, tho Tropic of Capricorn, is the parallel of 

 latitude at 23 P 28' S. ; each being at the distance of 66 32' from 

 the poles, because, as before, 90 66 32' = 23 28'. The 

 origin of these circles was explained in a former lesson (page 80), 

 and by referring to the diagram (Fig. 4) on that page you will 

 see that they arise from the different positions of the earth in 

 her orbit or path which she describes in a year in her motion 

 round the sun. The constant inclination of the earth's axis to tho 

 plane of the orbit, or the parallelism of that axis to itself in all 

 positions, occasions all the space around the poles to the extent 

 of 23 28' from each to be alternately illuminated by the oblique 

 rays of the sun for six months of the year, and alternately 

 darkened by the absence of those rays for the same period. It 

 also occasions all the space between the tropics and the equator, 

 to the extent of 23" 28' on each side of the equator, to receive 

 the direct rays of the sun in succession, that is, to have the sun 

 successively vertical to the inhabitants in every latitude, from 

 to 23 28' N., and from to 23 28' S., for a period of six 

 months alternately. 



It is plain, therefore, that the spaces between tho tropics and 

 the polar circles can never have the rays of the sun vertical to 

 them ; but these rays will be more or less oblique to them in tho 

 course of a year in the former case constituting winter with its 

 preceding autumn ; and in the latter summer, with its preceding 

 spring. 



The mathematical notion of the manner in which these circles 

 are generated is the following : Suppose the plane of the 

 ecliptic (the real path of the earth in the heavens, and the appa 

 rent path of the sun in the heavens) to cut the globe, it must 



pass through the centre, o (MO Jig. 4, p. 80), aa the ecliptic ia 

 the path of the centre, and forma the circle whose radio* La 

 O E. This circle interact* the equator, q, at aa angle, E o o, 

 of 23 28', called tho obliquity of the ecliptic, and iU two oppo- 

 site pointa remoteat from tho equator (called toltiitial point*), 

 generate, by the revolution of tho earth on iU axia, the twr 

 tropics aeen on oppoaite aidea of B Q, the equator, the one being 

 p E, 23 28' north of it, and the other at the aame dutanee 

 Houth of it. Tho extromitiea of the diameter of the globe at 

 right angles to the circle of the ecliptic whoae radios is o B, 

 generate, by the same revolution, the two polar ciralea aeea at 

 equal distances, 23 28', from N. and B., tho north and Booth 

 polos, and touching the dotted perpendicular which ia the said 

 diameter produced. 



Tho space or belt between the two tropics (from the Greek 

 rpotros, trop'-os, turning) ia called the Torrid Zone. The word 

 torrid, which means burning, ia derived from the Latin torreo, 

 to burn or roast, and the zone is so called because it ia parched 

 by the direct raya of the *un falling on every latitude in suc- 

 cession during the year ; its breadth ia twice 23 28', that ia, 

 46 56', measured on a meridian. The apace between the Tropic 

 of Cancer (so called, because when the sun appears to enter this 

 constellation in the heavens, at midsummer, he seems to tun 

 again and move towards the equator) and the Arctic Circle is 

 called the North Temperate Zone, because tho sun's raya fall 

 neither so directly as to produce great heat, nor BO obliquely aa 

 to produce great cold, although on the limits of the zone both 

 will be felt in a very considerable degree ; its breadth ia 43 4', 

 measured on a meridian. The space between the Tropic of Capri- 

 corn (so called, because, when the sun appears to enter thia con- 

 stellation in the heavens, at midwinUr, he seems to turn again 

 and move towards the equator) and the Antarctic Circle is called 

 the South Temperate Zone, for the eame reasons as stated 

 respecting the North Temperate Zone, and its breadth ia the 

 same, being 43 4', measured on a meridian. The space betwei: 

 the Arctic (from the Greek apxros, ark'-tos, a bear, and thenca 

 taken to mean the north, because the constellation in the heavers 

 called the Oreat Bear always points to the north) Circle and tae 

 North Pole is called the North Frigid Zone, because it is aiwiys 

 frigid or cold in this space or portion of the globe, owing to the 

 great obliquity of the sun's rays ; its breadth is 23 28', measured 

 on a meridian. Lastly, the space between the Antarctic (from 

 the Greek ami, an'-ti, over against, or opposite to, and ofxrroj) 

 Circle and the South Pole is called the South Frigid Zone; 

 and its breadth is the same, being 23 28', measured on a 

 meridian. 



Tho following table contains the breadth of each of the tone* 

 in degrees and British miles, their surfaces in square mile*, the 

 middle latitude of each zone, and the length of the parallel ia 

 that latitude in miles : 



, Breadths ; 



in in ' 



Degrees. 'Miles. I 



Surface* 



in Square 



Miles. 



North Frigid . .j 23 



North Temperate . 43 



Torrid I 46 



South Temperate . 43 



South Frigid 



23 



2,974 

 3.240 

 2.974 

 1.820 



8.132,807 

 51.041.534 

 78.3U.214 

 51.041.534 



8.132,807 



i TS'-ie'N 

 1 45 N| 

 1 | 

 ' 45 08! 

 78 168] 



5.055 

 17,57* 



ffJN 



5.055 



1180 112.428 



The determination of the numbers in the second and third 

 columns of the preceding table depends on the length of the 

 mean diameter of the earth, which, as we have aeen before, ia 

 about 7,913 British miles. Hence, the circuntferete of the earth 

 is about 24,856 miles, and the mean length of *> degree on ita 

 surface about 69 045 miles. Consequently, we find that the 

 extent of the surface of the globe, including both land and water, 

 and taking no account of the elevations and depressions of 

 either, is about 196,662,896 square miles ; and that its capacity, 

 or solid content, is about 259,332,805,054 cubic miles. 



The term oUfifial means literally w*-la*d9 (from Latin vol. tfe 

 un. and to, I .land) ; it i* applied to the .pparent motion of the tun 

 t thoee poinU, which seems to be so very slow that this Inmiuary naj 

 be said to all appearance, for a few days, to be stationary. 



