CHAPTER XXVI

THE GREAT LISBON EARTHQUAKE OF 1755

Lisbon, the capital of Portugal, on the Tagus River, is built along both banks for five miles, and on several small neighboring hills. It is supplied with water by means of an aqueduct, called the Alcantara, which brings the water from springs about nine miles to the northwest. For portions of its length the aqueduct is placed underground, but where it crosses the deep valley of the Alcantara it is supported, for a distance of 2,400 feet, by a number of marble arches, which in one place are 260 feet in height. This fact is put forward not merely for the sake of its artistic interest, but because, strange to relate, this part of the aqueduct remained uninjured during that great earthquake, the greatest of modern times.

On the 1st of November, 1755, this frightful catastrophe, according to Lyell, from whose excellent account much of the information contained in this chapter has been obtained, struck the beautiful city almost without any warning. Terrible sounds came suddenly from underground; almost instantly afterward a violent shock threw down the greater portion of the city; in less than six minutes 60,000 people were killed.

The place from which this earthquake started must have been situated on the bed of the ocean at some distance from the coast; for the great wave thus raised in the Atlantic Ocean did not reach the mouth of the Tagus River until about half an hour after the most severe shocks were over. The arrival of this wave at the mouth of the Tagus was announced by the sea retiring to such an extent as to leave the bar dry. Then a huge wave, sixty feet in height, rolled in from the ocean and completed the work of destruction that had been commenced by the earthquake.

So great was the shock that the mountains in the neighborhood were violently shaken and some of them split or fractured in a most wonderful manner.

Particularly large was the loss of life in the churches whither hundreds hastened for refuge when the shakings of the earth began, for most of these buildings fell and buried the worshippers. Another immense loss of life was caused by the destruction of a large marble quay or wharf that was suddenly swallowed up by the sea. While the buildings in the city were being overthrown by the violent shakings of the earth, a multitude sought the quay as a flat place where they could not be injured by the falling buildings. Suddenly, however, this structure sank into the water and not only were all the people drowned, but none of the bodies was ever afterwards found.

Failure to find any remnants of the pier or any of the people who perished on it has been attributed to the formation of eddies or whirls that were sufficiently strong to carry down vessels by suction similar to that of the famous maelstrom off the coast of Norway. Of course, in a time of boundless excitement like that of the Lisbon earthquake, accounts are apt to be highly exaggerated. For example, assertions are made in many books that the water left in the harbor after the sinking of the quay was unfathomable. Now, in point of fact, the depth of this place has been measured and found to be less than 100 fathoms.

When it is remembered that not one of the bodies of the people on that quay was ever again seen, it is possible, as Lyell suggests, that a deep fissure or chasm opened immediately on the ground on which the quay stood, so that it, together with all on it, were dropped into the chasm, which, closing, buried them deep in the earth.

The Lisbon earthquake was especially noted for the extent of country affected by it. Humboldt estimated this area as being more than four times the size of Europe. In parts of this area immense mountain ranges, such as the Pyrenees, Alps, etc., were violently shaken. When the size of these mountains is considered one realizes that it must have required a mighty force to shake them. These shakings were so severe that they produced a deep fissure in the ground in France. Continuing towards the north the solid earth was shaken as far as the shores of the Baltic and Norway and Sweden, generally. This, of course, included the flat country of Northern Germany. The hot springs of Toplitz disappeared for a time, but, breaking out afterwards, discharged such quantities of muddy water that the surrounding country was inundated.

The waves crossed the Atlantic, causing high tides on the island of Antigua, Barbadoes, and Martinique, of the Lesser Antilles, where, instead of the usual tides of two feet, tides of twenty feet high were observed. Further to the north the waves reached the eastern shores of North America, and shook the continent as far west as the Great Lakes, and spread in the North Atlantic as far as Iceland.

Toward the south the waves affected parts of northwestern Africa, where much loss of life occurred in the villages some eight leagues distant from the city of Morocco. Here from 8,000 to 10,000 people were killed, being swallowed up by deep fissures in the earth, which afterwards closed on their bodies.

Severe shocks were in many cases felt on vessels at sea. In one instance, although the vessels were at considerable distances from where the waves started, the captains reported that the shocks were so great that on several occasions it was believed the vessel had struck a rock, till, on heaving the lead, they found that they were in very deep water. In another instance, such was the shock to the vessel that the planks on the deck had their seams opened. In still another case several of the sailors were thrown into the air for a distance of about one and a half feet.

It has been frequently observed that when great earthquakes happen, curious changes take place in the level of the waters of lakes entirely disconnected with the ocean; for example, mountain lakes, far above the level of the sea, the water suddenly rising and then resuming its original level. Sometimes the waters of such lakes have suddenly disappeared, probably being drained off through a fissure formed in the bed of the lake. In such event the lake generally remains dry after the passage of the earthquake.

At the time of the Lisbon earthquake it was observed that the water of Loch Lomond in Scotland first rose above its ordinary, then sank again to its usual level. This difference of level is explained by Lyell as follows: when the earthquake waves reached the lake, the water being unable to take the sudden shove given to it by the earthquake waves, dashed over that side of the basin which first received the shock. Assuming this to be the case, since the rise of the level in the water of Loch Lomond was two feet and four inches, it is comparatively easy to calculate the speed of movement that the earthquake waves had, when they reached this body of water. Calculated in this way it would seem that the waves had a speed of about twenty miles a minute.

But what especially characterized the Lisbon earthquake were the great waves that were produced in the ocean. Besides the huge wave that entered the Tagus, a wave of the same height swept eastward along the southern coast of Spain, and the northwestern coast of Africa. At Tangier in Africa it swept the coast as a very high wave no less than eighteen times, or, in other words, eighteen huge waves rolled in from the ocean. At Funchal, on the Madeira Islands, this wave rose fifteen feet above the high water mark.

Many attempts have been made to explain the manner in which the great sea waves are started in earthquake movements. Some believe that they are due to the sudden raising or heaving up of the water, far above ordinary level. But, as Lyell points out, this explanation would not be satisfactory for the waves produced in the case of the Lisbon earthquake, since it would fail to account for the fact that both on the coasts of Portugal as well as on the island of Madeira the high wave was preceded by a movement of the water toward the point of origin; that is, the waters moved away from Lisbon and the Madeira Islands, so as to leave the water very low at those points, when shortly afterwards a huge wave rushed in from the sea and swept over the land.

Earthquake waves move much more rapidly through the solid rocks of the earth's crust than through the waters of the ocean. The shock transmitted through the solid earth from Lisbon to the Madeira Islands took only twenty-five minutes to reach the islands, while the waves in the ocean took about two and a half hours to cover the same distance.