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the source of endless beatitude, as it lighted the features and gleamed from the eyes, which were now dimmed, and shrouded, and closing, on the cross.-R. S. McAn.

LESSON XI.-MONDAY.

OROGRAPHY. The oxidized crust of the earth has been variously raised and depressed by the operation of powerful forces. The unevennesses so formed are to a large extent withdrawn from observation by the water, and maps only represent those which rise above the level of the sea. Although the figure of the earth, its rotation, the diversified condition of the coasts, the particular direction of winds and currents, and other local circumstances, modify the regular curve-surface of the great mass of waters on the globe; these variations are too insignificant to prevent the employment of the sea level as the standard for the measurement of elevations and depressions.

Heat, on which organic life depends, decreases in a two-fold direction, horizontally from the equator to the poles, and vertically from the bed of the sea to the external limit of the atmosphere. A knowledge of the height and extension of the land is therefore essential to a correct view of animated nature.

The known interior of Australia bears a very small proportion to the unknown. The hilly portions appear to be merely littoral elevations, or to extend only a short distance inland, while the interior spreads out into low and flat plains. The most distinguishing feature of the eastern side is a long chain of mountains, which, with the exception of some short deviations in its southern part, maintains a meridional direction through 35 degrees of latitude. It is continued at one extremity from Torres Straits, at the north of the Gulf of Carpentaria, far into the interior of New Guinea, but at the other it traverses the whole of Van Diemen’s Land. It is low in the northern parts of New Holland, being in some places merely a high land; but about the 30th degree of south latitude it assumes the form of regular mountain chain, and running in a very tortuous line from N.E. to S. W., terminates its visible course at Wilson's Promontory, the southern extremity of the con

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tinent. It is continued, however, by a chain of mountainous islands across Bass's Straits to Cape Portland, in Van Diemen's Land; from thence the range proceeds in a ziz-zag line of high and picturesque mountains to South Cape, where it ends, having in its course of 1,500 miles separated the drainage of both countries into eastern and western waters. The distance of the chain from the sea in New South Wales is from 50 to 100 miles, but at the 32nd parallel it recedes to 150, yet soon returns, and forms the Liverpool range, from whence, under the name of the Blue Mountains and Australian Alps, or Warragong Mountains, its highest part, it proceeds in a general westerly direction to the land's end. The average height of these mountains is only from 2,400 to 4,700 feet above the level of the sea; and even Mount Kosciusko, the loftiest of the Australian Alps, is not more than 6,500 feet high.

The distribution of elevations and depressions in the Old and New Worlds presents many striking peculiarities. In both, the arctic shores are the northern boundaries of very extensive lowlands, and the sharp-pointed southern extremities of the continents are, without exception, outlying spurs of greater or smaller mountain ranges. The northern lowlands near the pole are only in a few places interrupted or enclosed by mountains running for the most part north and south. In the eastern hemisphere are the Scandivian, Uralian, and Kamtschatkan mountains ; and in the western, the Cordilleras of North America, and probably a mountain district in Greenland.

The nearer we approach to the poles, the more is the capability of civilisation confined to low situations; the nearer we approach to the equator, the more is it confined to high situations : hence the beneficial influence of the low lands in the north, and of the high lands never wanting in the torrid zone. Each of the four great divisions of the globe presents a juxta-position of high and low land, inasmuch as an extensive lowland adjoins a great mountain system. Thus, by the side of the great highland of Eastern and Western Asia, lies to the north the lowland of Siberia and Turan ; in Europe, north-east of the Alps, and of the central mountain districts of Germany and the Carpathian

mountains, are the Germanic and Sarmatian plains, or northeastern European lowland. The south of Africa is occupied by a continuous table-land, the north by the lowland of Sahara. In America, the lofty system of the Cordilleras de los Andes lies along the western coast, and low level plains spread out widely from its eastern bases. Mostly, members detached from the mountain systems to which they belong rise again in the great lowlands before they sink beneath the sea; such are Scandinavia in Europe, the highlands of Barbary and Barca, in Africa ; the Alleghanies, in North America; and in South America, the mountainous districts of Guiana and Brazil. On the side opposite to that on which the mountain system slopes to the great lowland in Asia and Europe, smaller lowlands adjoin the lofty mainstem of the mountain systems, with mountain groups, partially or wholly detached, rising beyond them. These are not found in Africa or South America, nor in North America, with the exception of the peninsula of California. Europe presents the most completely developed structure—the profile, in the direction of the 12th meridian, displays in succession, from south to north, the Italian offshoot, the lowland of Lombardy-the Alpine mainstem, the Germanic lowland, and the isolated highland of Scandinavia. In the other divisions of the world, one member or another of this complete arrangement is wanting.

As only a very small part of the space occupied by islands has the lowland form, the ratio of elevation to depression,

inclusive of islands, in the four great divisions of the world, might be represented as 11 to 1. The statements in the uccompanying table may be compared in various ways with one another, and with the areas of the continents. If, for example, the high and low lands are compared, the ratio in Africa is as 2 to 1; in North America, as iš to 1; in South America, as 3 to 1; in Asia, as 13 to 1; and in Europe, as to 1. The lowland form therefore predominates in Europe and South America, the highland form in the other divisions; and to how great an extent is this the case in Africa! The bare numbers, however, appear more or less abstract, and are valuable only as connected with other geographical elements, especially that of vertical elevation.

AREA IN ENGLISH SQUARE MILES.

1. AFRICA. 2. N.AMERICA. 3. S. AMERICA.

4. ASIA.

5. EUROPE.

a. MOUNTAIN STEM

South African
Highland.
7075000

Cordilleras.

3665000

Cordilleras.

938000

Alps.
95300

Central High-
land, from the

Pacific to
the Mediter-

ranean.
8665000
2350000
Arabian
Highland.
1016900

6. SEPARATE MOUNTAIN MEMBERS
C. GREATEST OF THE MOUNTAIN MEMBERS

487000
Barbary.

445000

170000
Alleghanies.

170000

858000
Iberian
Highland.

212000

d. SMALLEST OF THE MOUNTAIN MEMBERS

Alleghanies.

170000

650000
Brazilian
Mountain
Tract.

381000
Sr. N. de
St. Marta.

2100
Amazon.
3072000
Guiana.

46600

Kamtschatka. Mt. Taurus.
63500

2500

Plateau of
Barca.

42000
Sahara.

2330000
Valley of the

Nile.
424000

e. GREATEST LOWLAND ..............

Arctic.
2120000
Atlantic.

360000

Siberia.
3940000
Chinese.
211800

North Eastern.

2076000
Middle Rhine.

3400

f. SMALLEST DETACHED LOWLAND

g. TOTAL HIGH LAND h. TOTAL LOW LAND

7563000 3750000

3834000 3410000

1589000 5211000

11016000 6567000

953000 2436000

TOTAL OF ALL THE HIGH LAND 27000000. TOTAL OF ALL THE LOW LAND

RATIO OF HIGH TO LOW As 1 $ To 1.

21400000.

LESSON XII.-TUESDAY.

ELASTIC FLUIDS.

Some reasonings apply to all fluids, whether liquids or gases; others have reference only to gases. Of the latter are those which investigate the effects of a repulsive force between the particles of a fluid body, no repulsion existing between the particles of a liquid. Common air is a perfectly elastic fluid, and the deductions from experiments on this gas are true for all other elastic fluids.

If a cylinder filled with air be fitted with a piston pressing vertically downwards, it is found that as often as the piston is pressed down it springs up to its original position. This is caused by the elastic force of the contained air. Suppose, when the weight of the piston is one pound, the air in the cylinder occupies the space of a pint measure; then, if an additional pound weight be laid on the piston, the air will be compressed to half-a-pint, two pounds more will reduce it to a quarter of a pint; or, in other words, to diminish the space by one half we must double the pressure. This is called Mariotte's law, from the name of its discoverer, and is expressed thus :—The elastic force of air is inversely as the space it occupies. Now, by reducing the space which a given quantity of air occupies by one half, we double its density; hence, The elastic force of air is proportional to its density.

Air' has been found rigidly to correspond with this law when it was expanded to 300 volumes, and also when compressed to 1-25th of its primary volume. But there is reason to doubt whether the law holds with absolute accuracy, in the case of a gas either in the state of extreme rarefaction, or of the greatest density. Thus the atmosphere does not appear to be indefinitely expansible, as the law of Mariotte would require; for there is certainly a limit to the earth's gaseous atmosphere, and it does not expand into all space. Dr. Wollaston supposed that the material particles of air are not indefinitely minute, but have a certain magnitude and weight. These particles are under the influence

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