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which border the European continent on the north and on the south.
2. In all the continents this line of greatest elevation in the summit of ascent is placed out of the centre, on one of the sides, at an unequal distance from the shores of the seas, from which result two slopes, unequal in length and in inclination. This is analogous to what, in mountains, is called the slope and the counter slope.
3. The law of increase of reliefs is common to the mass elevations, and to the linear elevations—that is, the height of the low lands and of the table lands increases at the same time with the absolute elevation of the mountains. There is a proportional gradation. The same law exists in the great peninsulas of Asia, whose basis is a table land, and which are almost small continents, as India and Arabia.
4. In the Old World, the long slopes are turned towards the north, and the short slopes towards the south. In the New World, the gentle slopes descend towards the east, and the short and rapid slopes towards the west. Thus, in this respect, each of the two worlds has a law peculiar to itself.
5. In each of the two worlds the two laws exert their influence. In the Old World, though the principal slope is towards the north, yet we may observe a gradual decrease of the reliefs from east to west; in the same manner, in the New World, the principal slope is from the west to the east, but it can be seen that the reliefs go on gradually increasing from north to south, as in the Old World. In these two secondary directions of the reliefs, we discover the law of the unequal slopes; in the Old World, the long slope descends to the west, the short slope to the east; in the New World, the long slope is to the north, the short to the south.
6. Generally speaking, the reliefs go on increasing from the poles to the tropical regions. The highest elevations, however, are not placed at the equator, but in the neighbourhood of the Tropic of Cancer, in the Old World, (Himalaya, 27° north lat.) and near the Tropic of Capricorn, in the New World, (Nevado de Sorata, 18° south lat.)
We notice here one of the great compensations, one of the great harmonies of nature. The effect of this law of arrangement is to temper the burning heats of these regions, and to give them a variety of climate, which seems not to belong to these countries of the globe. If this order were reversed, and the elevation of the lands went on increasing towards ·
the north, the most civilized half of the globe at the present day would be a frozen and uninhabited desert.
7. In fine, a common law combines in a single great fact all we have just said upon the general reliefs of the continent; it may be thus expressed :
All the long and gentle slopes descend towards the Atlantic and towards the Frozen Ocean, which is only a dependence of it; all the short and rapid slopes, or counter slopes, are directed towards the Pacific Ocean and towards the Indian Ocean, which is its continuation.
In this point of view, these two great oceans appear as two basins of different geological character. The Pacific Ocean seems an immense basin sunk in, the broken and elevated edges of which present on all sides the abrupt terminations of the continents.-GUYOT.
THE EARTH SELF-LUMINOUS.
When we apply the indefinite term polar light to the luminous phenomenon which we ascribe to a galvanic current—that is to say, to the motion of electricity in a closed circuit, we merely indicate the local direction in which the evolution of light is most frequently, although by no means invariably seen. This phenomenon derives the greater part of its importance from the fact that the Earth becomes self-luminous, and that as a planet, besides the light which it receives from the central body, the Sun, it shows itself capable in itself of developing light. The intensity of this innate terrestrial luminosity exceeds, in cases of the brightest coloured radiation towards the zenith, the light of the moon in its first quarter. Occasionally, as on the 7th of January 1831, printed characters could be read without difficulty. This almost uninterrupted development of light in the Earth leads us by analogy to the remarkable process exhibited in Venus. The portion of this planet which is not illumined by the Sun often shines with a phosphorescent brilliance of its own. It is not improbable that the Moon, Jupiter, and the comets, shine with an independent light, besides that of the reflected sunbeams visible through the polariscope. Without speaking of the problematical but yet ordinary mode in which the sky is illuminated, when a low cloud may be seen to exhibit an uninterrupted flickering light for many minutes together, we still meet with other instances of terrestrial development of light in our atmosphere. In this category we may reckon the celebrated luminous mists seen in 1783 and 1831 ; the steady luminous appearance exhibited without any flickering in great clouds observed by Rozier and Beccaria ; and lastly, as Arago well remarks, the faint diffused lumination which guides the steps of the traveller in cloudy, starless, and moonless nights in autumn and winter, even when there is no snow on the ground. As in the Aurora or the electro-magnetic storm, a current of brilliant and often coloured light streams through the atmosphere in high latitudes, so also in the torrid zones between the tropics, the ocean simultaneously develops light over a space of many thousand square miles. Here the magical effect of light is owing to the forces of organic nature. Foaming with brightness, the eddying waves flash in phosphorescent sparks over the wide expanse of waters, where every scintillation is the vital manifestation of an invisible animal world. So varied are the sources of terrestrial light !-HUMBOLDT.
Earthquakes manifest themselves by quick and successive vertical, or horizontal, or rotatory vibrations. In the very considerable number of earthquakes which I have experienced in both hemispheres, alike on land and at sea, the two first named kinds of motion have often appeared to me to occur simultaneously.
The propagation is most generally effected by undulations in a linear direction, with a velocity of from twenty to twenty-eight miles in a minute, but partly in circles of commotion or large ellipses, in which the vibrations are propagated with decreasing intensity from a centre towards the circumference. There are districts exposed to the action of two intersecting circles of commotion.
When the circles of commotion intersect one another—when, for instance, an elevated plain lies between two volcanoes simultaneously in a state of eruption, several wave systems may exist together as in fluids, and not mutually disturb one another. We may even suppose interference to exist here as in the intersecting waves of sound. The extent of the propagated waves of commotion will be increased on the upper surface of the earth, according to the
general law of mechanics, by which, on the transmission of motion in elastic bodies, the stratum lying free on the one side endeavours to separate itself from the other strata.
The phenomena of sound in earthquakes, when unattended by any perceptible shocks, produce a peculiarly deep impression even on persons who have lived in countries where the earth has been frequently exposed to shocks. A striking and unparalleled instance of uninterrupted subterranean noise, unaccompanied by any trace of an earthquake, is the phenomenon known in the Mexican elevated plateaus by the name of the 6
Roaring and the subterranean thunder” of Guanaxuato. This celebrated and rich mountain city lies far removed from any active volcano. The noise began about midnight on the 9th of January 1784, and continued for a month. I have been enabled to give a circumstantial description of it from the report of many witnesses, and from the documents of the municipality, of which I was allowed to make use. From the 13th to the 16th of January, it seemed to the inhabitants as if heavy clouds lay beneath their feet, from which issued alternate slow rolling sounds and short quick claps of thunder. The noise abated as gradually as it had begun. It was limited to a small space, and was not heard in a basaltic district at the distance of a few miles. Almost all the inhabitants in terror left the city, in which large masses of silver ingots were stored; but the most courageous, and those more accustomed to subterranean thunder, soon returned in order to drive off the bands of robbers who had attempted to possess themselves of the treasures of the city. Neither on the surface of the earth, nor in mines 1600 feet in depth, was the slightest shock to be perceived. No similar noise had ever before been heard on the elevated table land of Mexico, nor has this terrific phenomenon since occurred there. Thus clefts are opened or closed in the interior of the Earth, by which waves of sound penetrate to us or are impeded in their propagation.
The activity of an igneous mountain, however terrific and picturesque the spectacle may be which it presents to our contemplation, is always limited to a very small
It is far otherwise with earthquakes, which, although scarcely perceptible to the eye, nevertheless simultaneously propagate their waves to a distance of many thousand miles. The great earthquake which destroyed the city of Lisbon on the 1st of November 1755, and whose effects were so admirably investigated by the distinguished philosopher Emmanuel Kant, was felt in the Alps, on the coast of Sweden, in the Antilles, Antigua, Barbadoes, and Martinique; in the great Canadian Lakes, in Thuringia, in the flat country of Northern Germany, and in the small inland lakes on the shores of the Baltic. * * The hot springs of Töplitz dried up, and returned, inundating everything around, and having their waters coloured with iron ochre. In Cadiz the sea rose to an elevation of sixty-four feet, whilst in the Antilles, where the tide usually rises only from twenty-six to twentyeight inches, it suddenly rose above twenty feet, the water being of an inky blackness. It has been computed, that on the 1st of November 1755, a portion of the Earth's surface four times greater than that of Europe was simultaneously shaken. As yet there is no manifestation of force known to us, including even the murderous inventions of our own race, by which a greater number of people have been killed in the short space of a few minutes.
1 The capital of the richest mining district in Mexico ; its silver mines are inferior in value only to those of Potosi; it stands nearly 7000 feet above the sea level.
If we could obtain information regarding the daily condition of all the Earth's surface, we should probably discover that the Earth is almost always undergoing shocks at some point of its superficies, and is continually influenced by the reaction of the interior on the exterior. The frequency and general prevalence of a phenomenon, which is probably dependent on the raised temperature of the deepest molten strata, explain its independence of the nature of the universal masses in which it manifests itself. Earthquakes have been felt in the loose alluvial strata of Holland, as in the neighbourhood of Middleburg and Bliessingen, on the 23d of February 1828. Granite and mica slate are shaken as well as limestone and sandstone, or as trachyte and amygdaloid. It is not, therefore, the chemical nature of the constituents, but rather the mechanical structure of the rocks, which modifies the propagation of the motion, the wave of commotion. Where this wave proceeds along a coast, or at the foot and in the direction of a mountain-chain, interruptions at certain points have sometimes been remarked, which manifested themselves during the course of many centuries. The undulation advances in the depths below, but is never felt at the same points on the surface. The Peruvians say of these