BOOK VI

This is, perhaps, the most valuable part of the volume, for it contains more of the author’s own observations than the rest of the work. It deals more particularly with the great earthquake of 5th February A.D. 63, which occurred in his own country, and about which he could collect information at first hand. As already mentioned, the subject of earthquakes had long fascinated him, and he had published, in his youth, a volume about it. The calamity which brought so much injury to the towns of Campania was more especially likely to enlist his vivid interest, for the region that had been convulsed was with him a well-known and favourite part of Italy, where he often came to spend, on the shores of the Bay of Naples, such leisure as the life in Rome allowed him. Besides, it was the native district of Lucilius, to whom the volume was addressed, and whose town of Pompeii had suffered from the shock.[118] Hence he here plunges at once into details of the damage caused by this particular earthquake. As a prelude to his inquiry into the whole question of the origin of such catastrophes, he indulges in reflections on their appalling nature. Some of the unfortunate residents in the convulsed district had fled from it, vowing never to return. But where, the writer asks, can they be sure of safety, seeing that no quarter of the world is exempt from this form of danger? He urges that it is at least some consolation to be assured that such calamities are not the work of angry gods, as was popularly believed, but are traceable to their own special causes in the processes of nature (228).

[118] In Seneca’s letters, frequent reference is made to his visits to the district. He seems generally to have taken a villa at Baiae, or some adjacent place on that western part of the coast. He appears to have been a poor sailor, glad to make for the nearest landing-place between Baiae and Naples, so as to escape from the pangs of sea-sickness. On one of his excursions he revisited Pompeii, and was set into a reverie of his youth there. See his _Letters_, 49, 51, 53, 55, 57, 70, 77.

He then considers the various opinions entertained on this subject by earlier writers, which, on the whole, he regards as crude and inexact. The cause of earthquakes had been found in water, fire, air, and the earth itself, or in a combination of several of these agencies, or even in the co-operation of the whole of them. As regards the action of water, he dismisses the opinion of Thales (231), but in the statements of other authors, who maintain the power of internal water in causing earthquakes, he sees a greater probability of truth. He fully admits the existence of large rivers and extensive lakes inside the earth, and that in these dark uninhabited regions flooded rivers undermining their banks, and a swollen sea lashed into fury by the subterranean winds, may communicate shocks to the surface of the earth (234).

That fire is the origin of earthquakes had been held by various philosophers, who, however, differed as to the manner in which the fire acts. Anaxagoras thought it was by explosions caused from the collision of underground clouds (236); others held that the immense mass of vapour produced by the subterranean conflagrations as it accumulates may exert such a pressure as to disrupt all obstructions; or when the pressure is less may cause no more than a heaving of the surface. The idea that the shock of an earthquake results from the removal of material underneath, whereby the stability of the overlying portion is undermined, and a collapse of the ground ensues, was held in various forms. Some thought that this destruction arose from extensive combustion within the earth. Anaximenes supposed that just as at the surface, rocks and old buildings yield to the ravages of time and fall down, so in the interior of the earth similar landslips may occur and cause shocks to the districts above them (237).[119]

[119] The collapse of the roofs or sides of underground caverns may undoubtedly be in some instances the cause of local earthquakes. This origin is enforced by Lucretius:

terra superne tremit magnis concussa ruinis, subter ubi ingentes speluncas subruit aetas. --_De Rer. Nat._ vi. 544.

But the favourite opinion of antiquity regarded earthquakes as primarily due to the violent commotion of air. Seneca comments on the views of various philosophers, and more especially Aristotle’s, as to the way in which the air acts, and he then proceeds to deliver his own judgment. He has no doubt that, though some of the other agencies may co-operate, the chief motive force in earthquakes is air. By no part of nature, he affirms, is such violent energy displayed as by air; it kindles fire, tosses the surface of the waters into waves, destroys large tracts of the earth, uplifts new mountains, and raises in the midst of the sea islands never seen before. Not only does air exist above ground, but it also fills the hollows and interstices of the interior of the earth, into which it freely enters from the surface. Nothing in nature is so restless as air, and the earth cannot but be affected by the movements of the air included in its inside. The author agrees with the general opinion that when the air begins to be agitated in a subterranean cavern which it has filled, pressed by that which is still entering, it struggles to escape, and, when it does so, emerges with a violence proportionate to the narrowness of the passage for its exit. But if unable to make its way out, it becomes furious, acts like a swollen impetuous river, and that overthrows everything in its path.[120]

[120] Lucretius gives a picturesque recital of these views (_De Rer. Nat._ vi. 535–607).

It is not difficult to realise how this explanation should have been accepted in antiquity, and should have held its ground down even into modern times. The violence of the commotions of the atmosphere was a familiar feature on the surface of the earth, although its physical causes, variously guessed at, were utterly unknown. To minds that had no conception of the very rudiments of meteorology, there seemed to be no reason why air inside the earth should not be affected by as violent hurricanes as the air outside. And as such hurricanes were the most powerful natural agencies known, their action was not unreasonably invoked to account for the phenomena of earthquakes. Assuming that the air in a large subterranean cavern would behave as the free open atmosphere does, the old philosophers did not find themselves under the necessity of explaining what was to set the air in motion within the subterranean recesses and lash it into fury there, any more than they had to account for tempests above ground.

Obviously, if the air found its way from the outside into the internal parts of the earth, it must have had equal facilities for egress. And in the convulsions of an earthquake it might be supposed to issue with violence through some of the previous openings or from the rents made at the time. In corroboration of the truth of the prevalent opinion, it was asserted that after an earthquake air was found to issue from the ground, but no account appears to have been preserved of any violent outrush of air. As a further evidence that it is to the force of air that all these internal disturbances are due, the author remarks that after a violent earthquake another shock of equal violence cannot occur, because the first has opened a passage for the struggling winds.

The progress of investigation has, in modern times, thrown a flood of light on the phenomena of earthquakes, though there still remain many problems in the subject which await solution. It is needless to say that no foundation whatever has been found for the ancient faith that the air plays the chief part in these subterranean commotions.

Seneca discusses the nature of earthquake motion. He recognises three kinds of movement--quaking (_succussio_), tilting (_inclinatio_), and trembling (_vibratio_)--and he gives illustrations of the kind of causes to which they may be referred (252). He believes that the extent of country convulsed by an earthquake depends upon the area of the subterranean cavern in which the wind performs its exploits, and as these internal cavities do not continuously underlie vast tracts of the earth’s surface, no large spaces of that surface are simultaneously shaken. In his day there appears to have been no record of a shock affecting the whole basin of the Mediterranean Sea. He thinks that no earthquake ever extends as much as two hundred miles. He cites the recent calamity in Campania, which did not pass beyond that district, though marvellous tales about it had spread far and near, and he gives other examples of the markedly local character of the phenomena, so far as then known. He affirms that maritime districts are those most frequently shaken (255, 257), in proof of which he gives various instances, including the late disaster to Pompeii and Herculaneum in a region which had never been known to be shaken before.

He had received information about the Campanian shock, and the narrative in which he embodies it has the interest of being the most detailed account of an earthquake that has come down to us from antiquity. First of all, as already mentioned, he states that the movement was confined to the district of Campania, no mention being made of its having been felt even so near as Rome. He notices the injury done to Herculaneum and to Naples by the damage of public and private buildings; bronze statues were split open and some people were driven out of their minds. He records that Campania continued to tremble for some days after the great shock. He had heard that a flock of six hundred sheep was said to have been killed near Pompeii. Accepting the report as true, he sees no reason to suppose that the animals died of fright, but thinks it not unlikely that they were poisoned by the ascent of pestilential vapours from the ground. This conjecture of his receives perhaps some support from the fact that in this volcanic district, after an eruption of Vesuvius, so much carbonic acid gas has been said to escape from the ground as to suffocate hundreds of hares, pheasants, and partridges. But the most vivid experience of the earthquake which he narrates is that of a grave philosophic friend who, when in his bath, saw the tiles of the floor separate from each other, allowing the water to sink through the opened joints, while the next moment, as the pavement closed again, the water was forced out all bubbling. A better illustration of the transit of a wave of shock could not be desired.

Seneca was prepared to believe that great changes had been wrought by earthquakes on the face of the land. He cites in support of this view some remarkable examples which had occurred within the times of human history, such as the sinking of the towns of Buris and Helice, the disappearance wholly or partially of the island of Atalanta, and the subsidence of Sidon (256). He refers also to various striking features of landscape in different regions which had been popularly assigned to the work of earthquakes, such as the separation of Ossa and Olympus, the disruption of Sicily from the Italian shore, and the severance of Spain from the continent of Africa (263).

That the phenomena of earthquakes are closely connected with those of volcanoes was the general belief in antiquity, and continued to be accepted up to the middle of last century. It was believed in early days that just as the collision of clouds during storms produces the fire seen in lightning, so during the tempestuous agitation of the air within the earth, such heat is generated as to set fire to beds of sulphur or other combustible materials, and thus that rocks are melted and are forced up to the surface by the vast energy of the escaping air.[121] It is to be regretted that Seneca has not left an account of his own opinions on this subject, but from the allusions in the present treatise he may be inferred to have held the prevalent opinion. He alludes in various passages to volcanic eruptions that had taken place in his own time, or not long before, in the Mediterranean basin. An eruption of Etna is briefly noticed, when the mountain was in violent eruption, ejecting such a quantity of fine burning sand and dust as to turn day into night, accompanied with much thunder and lightning (77). This may have been the eruption alluded to in similar language by Cicero, who adds that for two days nobody could see his neighbour.[122] Seneca further cites two eruptions in the Aegean Sea, one of which had taken place in his own time, when a new island was upraised “by the force of air.” He alludes to Thera and Therasia, and the interesting account given by Posidonius of the uprise of an island in the same sea, with attendant circumstances closely resembling those of the eruptions at Santorin in modern times (73, 252). According to Asclepiodotus, the fire, after overcoming the resistance of the thick mass of sea, shot up above sea-level to a height of two hundred paces.

[121] This view of the nature of volcanic energy is graphically expressed by Lucretius (_op. cit._ vi. 639–702).

[122] _De Nat. Deor._ ii. 38. See also Lucretius (_ib._ vi. 641), who describes the more conspicuous features of an eruption, and concludes with the line

ne dubites quin haec animai turbida sit vis (693).

From a consideration of the causes of earthquakes the author is led by his accustomed train of thought to draw the ethical lessons which the subject suggested to him. He repeats his belief that against the perils of earthquakes, as against all the other dangers and fears of life, the only assurance is to be obtained from elevating studies and a contemplation of nature (265). It matters not when or in what form we shall quit life, whether from some trifle or from a world-wide catastrophe. To be happy without fear of anything that may befall us, we must carry our life in our hands, steeling ourselves against fear, and prepared even to welcome death as the advent of a friend.