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LESSON XXXI.-MONDAY.

OROGRAPHY. Of the detached mountain-ranges of Asia, only two are completely separated from the main stem. One is the meridional chain of the Ural, separated by the scantilywatered and partly desert lowland of Turan; the other is the plateau of the Deccan, leaning against the meridional chain of the Western Ghauts, and detached by the lowland of Hindostan. As the Ural, with a truly continental position, has an insular prolongation in the icy regions of Nova Zembla, so the Deccan, which, like Mexico, enjoys a perpetual spring, rises once more towards the equator, in the island of Ceylon.

In the west, the valley of the Kur is as little able to separate the Caucasus from the elevated plains of Armenia, as the valley of the Orontes and Euphrates to separate the Anatolian from the Syro-Arabian highland. The Caucasus is a nearly longitudinal chain in the southern background of the isthmus, over which the waves of the Caspian and Black Seas once flowed together, and thus forms a solid boundary-wall between Asia and Europe. Syro-Arabian highland likewise rises in the eastern background of the Isthmus of Suez, which is indubitably the filling up of a communication which formerly existed between the Red and Mediterranean Seas. But a deep chasm, the greatest depression in the earth's surface yet known, divides this highland into two very unequal parts. In the northern continuation of the Gulf of Akabah, the Wady el Arabah cuts deep into the rocky and desert plateau of Arabia, once called Arabia Petrea; running parallel with the Levantine coast, it sinks down to the deep yawning ravine filled by the Dead Sea and the Jordan, and after slight interruptions has a northern continuation in the upper valleys of the Leontes and Orontes. According to repeated measurements made in recent times, the depression of the level of the Dead Sea amounts to 1,312 feet. What contrasts on a small area: Jerusalem on a plateau 2,000 above, the Dead Sea 1,312 feet below the level of the Mediterranean. The more particular division of the Syro-Arabian highland is threefold; for we must distinguish the highland of Syria Proper, surrounding the ravine just spoken of, and having its culmi. nating points in the peaks of Lebanon and Anti-Lebanon; further, the mountain mass of the peninsula of Sinai; and lastly, the plateaus of the Arabian peninsula. Neither the Isthmus of Suez, nor the Red Sea, can prevent us from assigning an African character to these south-western members of Asia ; for the wide-spread deserts, as also the simplicity of the horizontal and vertical profile of Arabia, impress upon these countries, in the most various ways, a truly African stamp.

In the east of Asia likewise, we find two mountainous districts not completely divided from the main stem, but still having an independent character; and here, again, one is in the north and the other in the south. In the north are the mountains stretching through the length of the peninsula of Kamtschatka, which are connected with the mountain chains of eastern Siberia, and are distinguished on the eastern coast by a multitude of active volcanoes. In the south is the system of chains in further India and Malacca. Between the great river valleys which run towards the south, partitions are formed by high mountain chains, which probably unite in the north in an inaccessible mountain knot, and from which the dividing ridge of West Siam spreads over the greater part of the peninsula, with repeated depressions, as far as the extreme southern point of the Asiatic continent.

If we compare the low plains of Asia with one another, the subdivision into small detached districts on the southeastern and southern sides of the high central stem contrasts with the continuity of the great plain which lies on the northern and north-western sides of it. In the south-east, the fertile plains of China are embosomed in Alpine tracts; in the south, the broad flat valleys of the rivers of Further India penetrate a long way northward, the lowland of Hindostan carries tropical nature to the entrances of the Himalayan valleys, and the grassy pastures of Mesopotamia join the sandy wastes which ascend gradually to the neighbouring highlands. In the north, the boundless flats of Siberia follow the inhospitable shores of the Icy Sea, running south

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wards along the valleys of the gigantic rivers far into the mountainous countries, which, rising continually higher, skirt the northern border of higher Asia, but which, on the south-west, join the low plains of Turan. On the watershed between the basins of the Obi and the Sea of Aral there are, indeed, a few inountain ranges running in the direction of the parallels, in continuation of the chains of the Altai, among which the Ulutau is usually distinguished ; but their connexion is frequently interrupted, and the height gradually decreases to a few hundred feet in the west, until north of the Sea of Aral the level is nearly unbroken ; so that the representation of a high watershed, uniting the Ural with the elevations of Inner Asia, must be set down as altogether

There is, therefore, no obstacle to the entrance of Siberian nature into the low steppes and wastes of Turan; and we may trace from the shores of the Caspian Sea to the country of the Chukchees a continuous lowland, which covers 5,255,000 square miles; that is, one quarter of the total area of the lowlands of the earth. Here, in the basin of the Caspian Sea, the largest inland lake in the world, we meet with a second depression, for this lake is 83 feet below the Black Sea, and we cannot wonder therefore that the Oxus (the present Amoo) once flowed into it. The ground, indeed, between the Aral and Caspian Seas, rises into the steppes and desert plains of Ust-Urt, which are a few hundred feet high; but there is on that account no reason to suppose the Ural chains to be prolonged in this isthmus of Turkman ; a due appreciation of this important region requires us rather to recognise the southern extremity of the Uralian Mountains in the low group of Airk, near the sources of the Emba, and to view in the open communication between the low steppes of Turan with those on the north of the Caspian, that wide gate through which Asiatic hordes rushed to inundate Europe.

LESSON XXXII.-TUESDAY.

SPECIFIC GRAVITY.

If equal bulks of matter of different kinds be compared together, they will be found to differ very greatly in weight.

Platinum, the heaviest body with which we are acquainted, is upwards of 200,000 times as dense, bulk for bulk, as hydrogen, which is the lightest material known.

The comparison of the weights of equal bulks of different bodies, when referred to a uniform standard, constitutes their specific gravity, or relative weight, i. e., the weight which is specific or peculiar to each kind of matter. The specific gravity of a body forms one of its most important and distinguishing physical characters. The mineral iron pyrites, for instance, is in colour almost exactly like gold ; but it is at once distinguished from the precious metal by the difference in specific gravity, an equal bulk of gold being nearly four times as heavy. The numbers used to represent the specific gravity of solids or liquids are obtained by comparing a known weight of the body under experiment with the weight of an equal bulk of distilled water, which has been selected as the standard of reference. In this country the experiment is made at a temperature of 60° F. For gases and vapours, atmospheric air at 60° F., and the barometer standing at thirty inches, is employed as the standard. For the purpose of calculating the specific gravity of any substance, solid or liquid, it is therefore simply necessary to ascertain, first, the weight of the body in question, then that of an equal bulk of water. When this is done, we obtain by simple proportion the specific gravity of the body under examination, that of water being assumed as 1. If, as is the case with a large number of solids, they are heavier than water, the specific gravity merely tells how many times heavier they are than their own bulk of that fluid.

The determination of the weights of equal bulks of liquids and of water is easily made in the following manner:Take a light bottle furnished with a stopper, and weigh it when empty ; fill it with water and weigh it again; the difference, of course, will be the weight of the water which it contains. Empty the bottle, then fill it with the liquid for

trial, and weigh. On deducting the weight of the bottle, we . obtain the weight of a bulk of liquid exactly equal to that of the water.

In principle, the determination of the specific gravity of

gases is the same: a flask, or globe, is weighed when empty, again when filled with air, and a third time when the gas under trial has been substituted for atmospheric air. Gases, however, are liable to considerable changes of bulk from slight variations of external circumstances; hence, in taking their specific gravity, certain precautions are necessary.

With solids, a different, but not less simple method, is adopted, though resting on a principle by no means so obvious. This principle was one of the great discoveries of Archimedes; it may be thus explained :- When a body is plunged beneath the surface of a liquid it obviously displaces a bulk of such liquid equal to itself, and consequently it is pressed upon or supported in the liquid, with a force exactly equal to that with which the particles of the liquid were supported, when they previously occupied its place; the solid will, therefore, appear to have lost weight exactly equivalent to that of the bulk of the liquid which it occupies. The operation required for ascertaining the specific gravity consists, therefore, in weighing the solid in air, then having suspended it by a horse-hair from the scale-pan, placing it in distilled water at 60°, and again weighing; the difference of the two weights will be that of its own bulk of water. A piece of lead, for instance, weighs in air 820 grains, in water 749 grains ; consequently, the loss is 71 grains, which is the weight of an equal bulk of water. The specific gravity of the lead is obtained from these data by dividing the weight in air by the loss in water, thus:-820 = 71 =

(11:54 sp. gr. of lead). The rule for obtaining the specific gravity of a solid may, therefore, be expressed in the following terms :-Divide the weight of the body in air by the loss which it experiences when weighed in water; the quotient is the required specific gravity.

Occasionally it happens that a knowledge of the specific gravity of a body in the form of a powder is required; in such a case, the method of taking the specific gravity requires to be slightly modified. Suppose it be desired to find the specific gravity of a species of sand, we may proceed as follows:- Take a bottle which contains, when full, a known weight of distilled water-1,000 grains, for example; weigh

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