these of scarcely perceptible effect, would take place, and then would come a series of marked shocks, extending over a good many months. Thus, after a comparative immunity of several years' duration, a succession of tremors, some of them somewhat alarming, lasted through the greater part of the years 1816, 1817, and 1818. In 1819 there was not much disturbance, but there was a good deal of subterranean uneasiness during the next two years. Again, in the winter of 1833-4, the ground in the county of Dorset began to suffer a succession of shocks, which lasted until the early part of 1835 —almost every month in that interval having been signalised by a slight earthquake. In the year 1840, at least fifty shocks were felt at Comrie and the surrounding parts of Perthshire. During the forty years which have just been referred to, the most widely felt subterranean movement was probably that which occurred on the 23d October 1839. The weather had been dull and wet, and the barometer, already low, had fallen still farther during the evening of that day, when, at half-past ten o'clock P.M., the ground beneath Comrie, in Perthshire, seemed to be suddenly jerked upward to the height of several inches. From that central point the movement appeared to spread itself outward; and it was felt over two-thirds of Scotland, reaching north-westward to the line of the Great Glen, and southward to the Solway Firth. The nature of the shock is variously described. In the centre of the disturbed region it consisted of several distinct undulations, which passed into a kind of tremulous motion. At other points the only motion perceived was tremulous. At the time of the shock a loud noise was heard it has been compared by different observers to the firing of cannon, the sound of the explosion of a powder-work, very_loud thunder, the roar of a tempest or of wind through a forest. Furniture was displaced in houses, walls were shaken, plaster was dislodged, doors and windows quivered, and bells jingled. The river Earn ceased to flow in some parts of its course for a few seconds, and in other places was seen in violent agitation. This earthquake shock is a good type of the form which underground movements take at present in the British Islands. The shock is often preceded by a low rumbling sound like that of a loaded wagon drawn over a causeway, then comes the vibration of the movable articles indoors, and in a few seconds all is over. Nevertheless, in not a few instances, more permanent tokens of the event have been left in chimneys overturned, walls rent, and even in houses and churches destroyed. On the 21st of April 1841, a smart earthquake shock was felt over the south-west Highlands of Scotland, and the Connal ferry-house was rent. On the 13th of August 1816, a still more severe and extended shock was experienced in Invernessshire. An octagonal spire in the town of Inverness had its upper part so twisted round that the angles of the octagon projected over the smooth faces of the unmoved part underneath. The Masonic Lodge was rent from top to bottom, and chimneys thrown down. Nearly half a century before, the same town had suffered from an earthquake which ruined several houses. In earlier centuries, too, we find that similar and even more marked effects were produced by earthquakes in this country. Thus, on the evening of the 26th February 1574, a rather violent shock affected the south-western counties of England. People on their knees in chapel were thrown down, and a part of Ruthin Castle was destroyed. In 1275, an earthquake, according to Matthew of Westminster, threw down or otherwise injured many of the most famous churches in England, among others that of St Michel-du-Mont, near Glaston. The year before that, an earthquake, accompanied with thunder, lightning, a comet, and 'a fiery dragon,' had spread terror through the country. In 1185, a violent shock extended over the eastern counties, throwing down the cathedral of Lincoln and many other buildings. Nor have the effects of such subterranean movements been confined to human constructions, though these have naturally suffered most. The solid rocks have been shattered, and the ground has been rent open. Thus, in the spring of 1755, a smart shock affected the north of England. In Yorkshire, large masses of rock were detached from the hill-sides, and sent in fragments into the valleys below, while the ground was thrown into undulations. On the 9th of February 1827, along with a tremendous explosion, the ground at Ripon opened, and formed a rent twenty-four yards deep and nearly twenty yards wide. A fissure had been cleft there by another earthquake thirty-one years previously. Early in the year 1838, an earthquake shock was felt in Northumberland, and after it had passed, some fields were found to be traversed by a fissure more than a mile in length. Water, whether in the form of the sea, or of rivers, lakes, ponds, canals, &c., is peculiarly sensitive to tremors of the earth. Sheets of water have sometimes been seen in commotion when no wind was blowing, and when no one experienced any sensation of an earthquake. Some of the deep fresh-water lochs of Scotland have shewn these movements in a singular manner, more particularly at the time when any severe earthquake has affected some other part of Europe, as we shall see a few pages farther on. Even small and shallow pieces of water afford a sensitive indication of the tremors, as, for instance, when, in the winter of 1789, during an earthquake at Comrie, the ice on a pond was shattered to pieces. The sea exhibits like sympathetic movements. When, on the 27th August 1834, a marked shock passed under the south of England, a sloop of war lying in Chichester harbour was shaken, and bent over to one side so violently, that the crew supposed she had been struck by another vessel. At the time of the Lisbon earthquake, the sea rose to a great height along the coast-line of Britain. The subterranean movements experienced in this country are, however, of very trifling moment when compared with those of the first magnitude which have happened within human experience in other regions. We shall first consider briefly the nature of the results produced by these violent movements, and then look for a little at the details of one or two great earthquakes, by way of illustration. 1. The heaving or undulatory movement of the ground causes perpendicular objects to sway to and fro. If we watch the vessels in a harbour when a strong swell is coming in from the sea outside, we see the masts rocking uneasily backwards and forwards, as each undulation passes under them. The motion produced by an earthquake wave is of the same kind. Trees are bent over, now to the one side and now to the other; sometimes their upper branches touch the ground. In a wooded country, the crashing of boughs is heard far and wide as the trees are thrown against each other; after the calamity has passed, the ground is found strewed with broken branches and prostrated trunks. Among the still erect trees, some are found locked into each other, the boughs of one having been inextricably twisted into those of its neighbour as they were swung to and fro by the rocking ground. Tall pinnacles of rock, in like manner, after reeling backwards and forwards, sometimes fall in headlong ruin into the valley below, hurling down woods and hamlets, and spreading desolation over cultivated fields. It is natural, however, that these effects should be more noticeable among human works than in nature. Accordingly, it is the results of earthquake shocks on buildings which have been chiefly chronicled, and which have enabled us to arrive at some knowledge of the nature of the movements by which these shocks are produced. When a very gentle earthquake wave, such as those of the British Islands, passes under a building, its progress is marked by the reverberation of such loose objects as bells, pictures, windows, &c. A little more intensity suffices to displace chairs and tables, while a person lying in bed is conscious that first the one end of the room, and then the other, is lifted up and let down again. By a still sharper shock, the walls are made to vibrate, chairs and loose objects indoors, and chimneys, slates, and plaster out-of-doors, are thrown down. If this should happen at night, the terrified inhabitants escape for shelter to the nearest open space, and wait anxiously for the cessation of the earthquake. But when a shock of full violence passes under an inhabited country, the houses and other buildings rock to and fro like the ship-masts in the harbour. The result is, that in a few seconds the walls give way, and the buildings sink in ruins to the ground, burying such of the luckless inmates as have been unable to escape. Sometimes it is the buildings solidly constructed of stone which suffer most, and lamentable instances are on record of thousands of people having been crushed under the ruins of churches, into which they had gone either for the purposes of devotion, or for greater security. At other times, the well-built houses escape, whilst those more slimly formed of wood or of brick tumble down as if they had been built of cards. In such cases, the different modes in which the houses suffer appear to depend upon the nature of the ground on which they are built, as well as of the materials of their construction. There seems something almost capricious in the earthquake shock. A whole street will sometimes be levelled to the ground, except one house, which may be but little injured; half of a house will be thrown down, while the other half remains not much the worse; a pillar or obelisk will have its stones twisted round upon each other, and yet the whole remain still standing. Instances are also recorded of buildings having been cracked in two, the one half sinking down several feet below the level of the other. But it is not only perpendicular objects which suffer, for besides the undulatory movement of the earthquake, which causes these to sway to and fro, there is often in the centre of the disturbed area an upward jerking motion which affects even horizontal bodies. For instance, cases are known where the paving-stones of a street have been pitched out of their sockets, and have been found after the earthquake lying with their under surfaces uppermost. This motion, combined with the wave-like one, produces sad havoc in a town. On sloping surfaces, stupendous results are often brought about. Thus, along river-courses, banks of loose earth, sand, or gravel are shattered, and masses are launched down towards the river. On mountain slopes also, large areas of soil and débris have been shaken loose from the rock on which they rested, and hurled into the valleys. It was a case of this kind occurring last century in Calabria, which was described at the time thus: Two mountains on the opposite sides of a valley, walked from their original position until they met in the middle of the plain, and there joining together, they intercepted the course of a river.' Similar results take place along the margin of the sea. Earth, soil, and stones are thrown from steep slopes to the beach; and cattle browsing on these declivities are likewise swept down—even solid cliffs are shaken, and large fragments of them detached to fall into the waves below. 2. Besides the effects produced by the undulatory or jerking motion of the earthquake on objects at the surface, another highly important feature is the actual rending open of the ground. This does not necessarily take place in all earthquakes; but it is one of their frequent and terrible accompaniments. Cracks of the soil are formed, and these vary in size from only a foot or two in length, and an inch or two in breadth, up to rents ninety miles long, and sometimes several yards in diameter. During the progress of an earthquake such cracks are observed to open and close again sometimes in rapid succession. Trees, houses, men, cattle, anything, in short, which may happen to be on the surface at the time, fall into the chasm opening beneath them, and may be there engulfed for ever. Yet cases are known where men have fallen into the cracks, and though the walls have closed upon them, they have been thrown out again alive when the chasm reopened. Quantities of mud and sand along with water are sometimes ejected from the rents, or from curious funnel-shaped cavities formed in the ground at the time. The fissures either close again permanently, and, after the earthquake, cease to be visible, or they remain open, and may continue so for many years, until, as their sides crumble down, they become filled up, and in the end gradually obliterated, or in some cases they may give rise to new minor valleys. 3. It is evident that no shaking of the solid land could take place without affecting more or less the waters of the ocean. This would be the case if the seat of the earthquake shock, or the place where it first reached the surface from below, lay beneath the inland parts of a country, and the earthquake wave undulated outwards to the sea-margin. But it often happens that the point of origin of an earthquake lies somewhere beneath the bed of the ocean, and though the actual earthquake wave may never be propagated through the solid crust to reach the land, the commotion it produces in the waters gives rise to an ocean-wave which rolls landward until it breaks upon the coast. In truth, in all maritime districts subject to earthquakes, the amount of disaster achieved by the shaking of the ground is often far less than that which is worked by the inroad of the sea. The inhabitants have perhaps been terrified by the first shock of the earthquake, when before they have recoyered from their surprise, they see the sea in front of them retire for several hundred yards, laying bare the bottom of the harbour or the beach. By-and-by, when it has reached its farthest limit of retreat, they watch it surge and foam, and gathering itself into a broad breast of water, rush furiously towards the shore. Then arises the cry to flee to the heights; but the wave is instantly upon them. In a few minutes hundreds or thousands of the inhabitants are drowned or dashed against houses, or transfixed with broken wreck. What of their city has been left unprostrated by the earthquake is now inundated, and in great part levelled by the torrent of sea-water. Ships riding in the roadstead, even heavily armed men-of-war, are swept inland, and left high and dry half a mile, it may be, from the shore. In short, wherever the sea-wave reaches, it carries with it indescribable desolation. Property of every kind is destroyed, and in a few moments a busy seaport town is actually blotted out of existence. 4. The effects which we have hitherto been considering, although of terrible import in relation to man and his works, are not those features of earthquake phenomena which leave the most permanent marks on the surface of the earth. A city may be shaken to pieces or No. 42. 9 |
