DISTURBING FORCES-VOLCANOES AND EARTHQUAKES.

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In

Comrie, proceeding from south-west to north-east; and severe earthquakes in August. ravaged the towns of Arica, Irique, and other parts of South America, with the island of St Thomas. Towards the close of September, there was a great eruption of Etna. October, earthquake shocks were experienced at Frankfort, Darmstadt, Wiesbaden, and Mayence. They were renewed in Germany in November; and towards the close of the year, December 28, the town of Santa Maura, in the Greek island of that name, was entirely destroyed.

1870. In March, the sea was observed to rise suddenly around Malta two or three feet above its usual level, supposed to be occasioned by a submarine volcanic eruption; and a severe earthquake was felt at Comrie. On the night of the 11th of May, a succession of shocks shook the strongly-built city of Oaxaca in Mexico to its foundation, causing many deaths from the fall of houses, churches, convents, and other public buildings, while a great number of the inhabitants were variously maimed. The first movement of the ground was an oscillation from south to north, followed almost instantaneously by a vertical one. No dwelling or edifice escaped without some measure of injury, and nearly all the southern part of the city was reduced to heaps of rubbish. A few days afterwards, across a hemisphere of ocean, Japan was dreadfully disturbed. The houses of Yokahama rocked to and fro, many villages were laid in ruins; and the great volcano of Fusiyama renewed its activity. A strong concussion was felt on board a schooner coming up the Gulf of Yeddo, as if the ship had struck a rock; and immediately afterwards the vessel swung completely round, being entangled in a temporary vortex. Large fields of pumicestone and ashes were met with afloat in the neighbouring seas. In August, after many signs of restlessness through the previous months of the year, Vesuvius exhibited stronger excitement. A new mouth was opened in the centre of the crater, from which ashes and stones were ejected to a considerable height; and at the same time there were continued, and very severe shocks of earthquake. But they were far surpassed in intensity by a series of shocks in the early part of October, which lasted nearly a fortnight. No fewer than nineteen shocks occurred in a single day. The first and strongest, distinctly felt at Naples, had a distant part of Calabria for the centre of the movement. It lasted from ten to fifteen seconds, was at first undulating, then vertical, and to a certain extent rotary. Scarcely a single house was left standing in the towns of Mangone, Figline, Cellara, S. Stefano, and others, and entire families perished in the terrible catastrophe.

1871. On the night of the 19th of February, severe shocks of earthquake occurred in the Sandwich Islands, and were general throughout the group, though experienced in different places with varying intensity. At Honolulu, in Hawaii, the first shock was vertical. This was quickly followed by a series of horizontal jerks sufficiently powerful to cause people abroad to stagger in walking, and produce a feeling of nausea. While the motion of the earth continued, the thousands of dogs with which the town abounds were silent as the grave, but when it was over they howled fearfully with surprise and alarm. Every inhabitant of Lanai was roused from slumber. The entire island shook from peak to vale, and its bold rock-ribbed coast was rent in various directions. A great portion of the well-known bluff, Pali Kaholo, fell into the sea; enormous fragments were broken off the towering ocean walls between Mancle Bay and Kawaike Point; masses of the red basalt were torn from the beetling turrets of Pupehe, a lone sea-tower near the south-eastern end of the island; and some of the valleys were rendered permanently incapable of cultivation by the débris of rocks, earth, and trees dislodged from the sides of the mountains.

In March, on the night of Friday the 17th, a very decided shock of earthquake was felt in the northern counties of England, and was specially sensible over an extensive

area of the Cumberland lake district. It lasted fifteen or twenty seconds, and was accompanied by a noise as if a heavy goods train were passing at express speed. The vibration was so powerful that beds were lifted up, doors and windows rattled violently, flower-pots and light articles of furniture were thrown down. Numbers of people who had just retired to rest started up again, imagining that burglars had entered their dwellings, and then rushed half naked into the streets to ascertain the cause of the concussion. It was noticed that dogs gave unmistakable evidence of alarm for some time after the shock; cage birds also showed disturbance; and loud cries were heard from the pheasants in the woods. The movement of the earth was felt by the inhabitants of the valleys far more perceptibly than by those of the hills. There was a sudden and great rise of the barometer immediately after the occurrence, and the atmosphere became very mild. Again, on the night of the following Wednesday, the 22d, a heavy booming was heard in the north-west of Lancashire, and oscillations of the earth were felt which continued at intervals for a considerable time. These last extended to the mountainous district of Wales, and were distinctly experienced on its north-western coast. The wave of subterranean disturbance passed Pensarn in Denbighshire, shaking the windows of the houses with alarming violence. It was attended with a concentrated hissing sound, such as might be produced by jets of high-pressure steam escaping through a contracted orifice. At Llanduduo the shock was more severe; the ground shook, the houses rocked, and the accompanying sound was like the firing of artillery.

Thus experience in the present century, as in the last, points to the conclusion that the middle periods of centuries are times of unusual physical excitement, when the great agencies of fire and water are most frequently in conflict, and contend most violently with one another for the mastery. It remains for future seismologists to note the course of events, and accumulate evidence tending to sustain the opinion, or prove it to be fallacious. But the hand of nature is never really at rest. It is constantly in action, arranging and remodelling the constituents of the terrestrial system, thereby sustaining it in a condition to accommodate the living races on its surface, now acting by slow and noiseless processes in effecting this object, and now by sudden and violent catastrophes.

Lake-water corresponds to that of the rivers which form it, except in the lakes without an outlet, which are commonly saline or brackish. Marsh-water abounds with impregnations from animal and vegetable decomposition, members of both classes of the lowest grade flourishing in such sites. Sea-water is universally salt. The fresh-water formations, or those which contain no amount of foreign ingredients appreciable to the taste, and unfitting them for domestic use, include all the rivers, and by far the greater number of springs, lakes, and marshes. But their total extent is utterly insignificant when compared with the whole fluid surface of the earth.

I. SPRINGS.

The rains which descend from the clouds upon the land, with the melted snows, are partly drained from the surface by rills and brooks, which contribute to form the rivers discharging finally their contents into the sea. But only a very small proportion of the moisture precipitated is disposed of in this manner. By far the larger portion is returned again to the atmosphere by evaporation, or is devoted to the purposes of animal and vegetable life, or is received into the soil by minute absorption, percolating also through cracks and fissures of the surface. Arenaceous and cretaceous strata, with all loose soils and gravels, greedily absorb water, while argillaceous deposits and compact rocks are barriers to its passage. Hence, on meeting with clays and other impervious beds, the moisture is arrested in its downward course, and there is a resulting over-accumulation in the superincumbent water-bearing stratum. This is forced by hydrostatic pressure to find its way again to the surface, wherever there is an outlet; and it oozes from chinks and crevices on the sides of valleys and hills, occasioning the phenomena of natural springs. In level tracts of country where springs of water literally rise, or reach the surface by ascension, the supply is derived from distant elevated grounds, having travelled laterally to the point of discharge through intermediate pervious strata. Artificial springs, called Artesian Wells, from the province of Artois in France, the ancient Artesium, where they have been long in use, are constructed upon the principle involved in the natural. It has been applied with remarkable success by the French Algerian government to various parts of the Sahara, where districts, formerly dry, arid, and uninhabited, have been supplied with water by means of perpendicular borings, and now possess their date-palms, cultivated grounds, and Arab settlements. One of the most celebrated artesian wells is that of Grenelle, in the outskirts of Paris, where the water is brought from the depth of 1798 feet, and is raised with such force by hydrostatic pressure, as to be projected to the height of thirty-two feet above the surface. But such borings may be altogether unsuccessful, either because no water-bearing stratum is reached, or because its upturned edges do not rise sufficiently high above the point where the wells are sunk.

Many springs are inconstant in their discharge, flowing copiously, then feebly, or altogether failing for a time. These depend entirely for their supply upon the showers that fall upon a limited area of the surface, and are hence affected in this manner by the prevailing character of the season, whether rainy or dry. Other springs are perennial or constant, and pour fourth large volumes of water which show little or no diminution from the longest drought. These are obviously quite independent of the last showers that fall, receiving their supplies from vast subterranean reservoirs of water, formed by the rains and snows collected in interior cavities from extensive areas of the surface, and are not exhausted before they are replenished. St Winifred's Well, in Flintshire, the most copious spring in England, is estimated to discharge at the mean rate of about 4400 gallons per minute. It bursts forth with almost the force of a considerable river, through a fault in the coal measures, and is used to put in motion the machinery of eleven mills on its

passage to the sea, a distance of little more than a mile. It has never been known to fail, and is but slightly affected either by prolonged dry weather or excessive rains. Petrarch's Fountain, at Vaucluse, near Avignon, in the south of France, is a spring of dark blue water so abundant as to form at once the river Sorgue, capable at its source of moving machinery, and almost immediately navigable for boats. In some cases springs participate in the ebb and flow of the tide, as at Richmond, where they rise from the arenaceous strata on the banks of the Thames. In other instances, they are subject to intermittances at irregular intervals, apparently caused by the water collected in interior cavities discharging by a siphon-shaped channel, and varying in its level. To this class belongs the celebrated Pool of Siloam at Jerusalem. The fact of its ebb and flow was noticed in the fourth century; and after being generally doubted in modern times, except by the native Jews, it was satisfactorily substantiated by Dr Robertson in 1838.

Owing to the extremely solvent power of water, it becomes more or less charged with the materials in contact with it while percolating through the earth. Hence in the springs of limestone districts, the calcareous soil, or lime, combined with carbonic acid, is abundantly held in solution; and is frequently deposited in their basins and on their margins, or in the beds of the rills flowing from them, and upon the objects intentionally exposed to their influence. These are the petrifying wells and waters of popular speech, so called from their supposed power of turning into stone whatever substances may be subject to their action. But incrustations are not petrifactions. The leaves, twigs, and branches of trees submitted to experiment, are simply invested with a calcareous coating or crust, sometimes porous and friable, but often crystalline and compact. They are not permeated by the stony matter, but inclosed; and are either found upon the outer mass being removed, or the cavities remain which have been left by their decay. Thermal springs at Matlock, in Derbyshire, deposit lime so abundantly as to form thick beds of rock sufficiently firm in texture to be applied to building purposes. The Dropping Well, at Knaresborough, in Yorkshire, which trickles from its source over the surface of a rock, and falls from its edge in a number of tiny streamlets, with a pleasant tinkling sound, is highly charged with a gritty or sparry matter, and incrusts with it the grass, leaves, and shrubs, in contact with the water. Hot wells in the Azores, and the Geysers of Iceland, are surrounded with basins formed of siliceous sinter, which has been derived from their waters. Many springs hold in solution so large a quantity of iron, that their basins, with the channels of the rills flowing from them, are encased with a ferruginous deposit, while the mosses and grasses in contact with them are deprived of their natural green by a yellow incrustation. In all cases where the foreign ingredients have medicinal properties, the springs are known as Mineral Waters; and are variously saline, acidulous, chalybeate, and sulphurous, according to the nature of the substances in alliance with them. They annually attract thousands of visitors to them in search of health, and have caused naturally rugged and desolate sites to be crowded with fashionable buildings, while many, from the romantic beauty of the scenery around them, are inviting to those who merely require the refreshment of change of place.

The mean temperature of the springs which are exclusively associated with surfacedeposits, corresponds to that of the locality where they are found. But those which rise from considerable depths have a higher average temperature than that of the atmosphere at the place. This is owing to the internal heat of the earth gradually increasing with the depth below the superficial zone to which climate extends its influence. Hence the warm springs which occur in many countries indicate the general depth of the beds from which the water is derived, except in active volcanic districts, where fierce ignition is near the surface, while often visible above it, and hot or boiling springs are common. The