Earthquakes, terrae motus , are violent upheavals by which numerous parts of our globe are shaken in a way felt more or less strongly.

Of all the phenomena in nature there are none whose effects are so terrible and extensive as earthquakes . Due to them the face of our globe undergoes the most notable changes and the most disastrous upheavals. Due to them in an infinity of places a physicist [1] sees only a frightful pile of ruins and debris, the sea raised up from its immense bed, cities overturned, mountains split open, moved, collapsed, entire provinces swallowed, immense areas snatched from the continent, vast regions sunken under water, others uncovered and dried, islands rising suddenly from the depths of the seas, rivers that change course, etc. Such are the frightful spectacles that earthquakes present to us. Such disastrous events, to which the earth has always been exposed and that are felt everywhere, have frightened men and excited their curiosity, leading them to search for their cause. It did not take long to recognize fire as the author of these terrible phenomena and since the earth appears shaken to its very center, it has been suggested that our globe encloses within its breast an immense mass of fire that is always in action. That is what some physicists have designated by the name central fire . This notion was considered most appropriate to explain the unbelievable effects of earthquakes . There is no doubt that fire has the major part in these phenomena, but it is not necessary in order to find their cause to have recourse to chimerical hypotheses nor to suppose a mass of fire in the center of the earth where a human eye will not ever penetrate. If one observes nature at all and the structure of our globe, one will perceive that without descending to depths impenetrable for men, one encounters in several locations the accumulation of substances active enough to produce all the effects that we have indicated. These substances are fire, air, and water, that is to say, the most powerful agents in nature and whose existence none can deny.

The earth in an infinity of places is filled with combustible materials. One will be convinced of this truth if one pays attention to the immense layers of coal in the earth, masses of bitumen, peat, sulfur, alum, pyrites, etc., that are found buried in the interior of our globe. All these substances have the properties to produce blazes and to serve as fuel once ignited. In fact, experience teaches us that bituminous and aluminous substances, such as certain layered stones that accompany alum and coal mines, after having been piled up and exposed a while to the sun and rain, catch fire on their own, and spread a true flame. These phenomena are the same that chemistry presents to us in the burning of oils and acidic action and in pyrophoric substances.  [2] Furthermore, we know that subterranean mines, especially those of coal, are often filled with gaseous fumes that catch fire easily and then produce effects as violent as thunder. See Mineral coal. In order to ignite on their own some of these gaseous fumes need only encounter others or even to mix with pure air that they cause to expand; in this manner they are able to produce a kind of underground thunder. These fumes are produced especially by decomposing pyrites. It is known that these mineral substances are found abundantly throughout all areas of the earth. The gasses that emanate are sulfurous or of vitriolic acid. Upon encountering bituminous and greasy emanations they may easily burst into flame. To assure oneself of this truth one only has to make a mixture of one part coal and two parts pyrite forming vitriol. One has an accumulation that when placed in a pile will ignite at the end of a certain time and consume itself entirely. Umber earth has been seen to self-ignite after having been crushed with linseed oil. See Umber earth.

Several physicists have wanted to explain the formation of underground blazes by a famous experiment that is credited to M. Lemery. [3] It consists in mixing together sulfur and iron filings. One moistens this mixture and after burying it for a while it produces the phenomena of earthquakes and volcanoes. However ingenious this explanation may be M. Rouelle presents him with a major difficulty.  [4] This knowledgeable chemist observes that in M. Lemery’s experiment true iron was used and not iron stripped of its phlogiston, [5] or mineralized iron. From which one can see that in order to explain underground blazes from this substance it would be necessary for there to be within the earth a great quantity of pure iron, which is contrary to observations, since iron is found either mineralized or in the form of ochre, in other words, lacking its phlogiston within the earth. As for pure iron or native iron found in great masses, as in Senegal, one has reason to suspect that it was purified and melted by the fires of the earth.

By whatever manner the blazes are produced within the earth, they have an indispensable need of air; fire cannot burn without contact with air. Yet one cannot deny that the earth encloses a very considerable quantity of air. This fluid penetrates through cracks by which the earth is traversed. Air is contained in caves and cavities of which the earth is filled. Workers in mines while striking and piercing rocks with their tools sometimes hear the escape of a violent whistling noise that often extinguishes the lamps that provide light. One cannot doubt that the earth does contain a large enough quantity of air so that substances susceptible to bursting into flame can ignite. This same air that entered little by little expands; the collapsing of earth that occurs at the start of inflammation must undermine and excavate rocks bit by bit thus impeding the air from finding an escape. Then helped by the fire that it ignited, it makes an effort in all directions to open a passage; its efforts are in proportion to the quantity of substances on fire, to the volume of air that was made to expand, and the resistance that the surrounding rocks oppose to it. No one ignores the prodigious effects that air is able to produce when it is in this state. It does not take great effort to conceive that these reactions must necessarily operate in the interior of the earth.

With regard to water, all observations prove that the earth contains a lot of it. The more one descends into the depths of subterranean mines, the more of it one finds. Often one is forced for this reason to abandon works that held out great promise. The miners in piercing rocks are sometimes drowned or overwhelmed by it. See the article Mines. Water contained in the depths of the earth can contribute in several ways to earthquakes : 1. The action of fire reduces water to vapor, and however little knowledge one may have of physics → , one would know that nothing approaches the irresistible forces of these gasses that are caused to expand without an exit. Experiments done with Papin’s machine, [6] those of the eolipile, [7] etc., furnish convincing proofs for us. One may as a result conceive that water is reduced to vapor by heat in cavities in the earth; it makes an effort to escape and since it finds no passage to escape, it raises up the rocks that surround it and thereby produces violent shakings which are felt at unbelievable distances. 2. Water will probably produce still more prodigious effects when it comes to fall suddenly into a mass of burning substances. It is then that it will produce terrible explosions. To convince oneself of this truth, one only needs to pay attention to what happens when one allows imprudently a drop of water to fall onto a metal that is molten. One will see that it is capable of causing an entire workshop to explode and to place the lives of workers in the greatest danger. Thus waters contribute to earthquakes , increase the vivacity of underground fire, and cause it to spread. A common everyday experience can give us an idea of the manner in which these phenomena are able to operate. If in a kitchen melted grease in a saucepan catches fire, when one pours water on it to extinguish it fire spreads in all directions, the flame gets bigger, and one runs the risk of setting the house on fire. 3. Waters can also contribute to heighten subterranean fires since by their fall, they agitate the air and produce the effects of forge bellows. In this manner water can cause the spread of blazes. 4. Lastly, water can also contribute to the shaking of the earth, by the excavations it makes in the earth’s interior, by the layers it involves after they are soaked, and by the falls and collapses that it causes.

One can see throughout the preceding information that earthquakes, volcanoes or mountains that eject fire, are due to the same causes. In fact, volcanoes cannot be regarded as anything but the equivalent of cellar windows or hearths that produce earthquakes . See the article Volcano.

After having laid out the most probable causes of earthquakes, we are now going to describe the phenomena that precede and accompany them most often because in this manner as in all other operations of nature circumstances produce infinite varieties. It has often been noted that earthquakes occurred following very rainy years. One can conjecture from this that rain waters by soaking the earth block cracks and openings by which subterranean fire can circulate and find escape routes. Will-o’-the-wisps, gasses of a sulfurous odor, red and enflamed air, thick black clouds, humid and oppressive weather are the usual signs of those disastrous catastrophes. However, one has sometimes seen them preceded by a very great calm and a perfect serenity. Animals seem filled with terror expressed by their bellowing and howls; birds fly back and forth anxiously indicating the approach of large storms. One often hears noises similar to those of a subterranean thunder or a strong artillery discharge, or one hears tearing and violent whistling. In several places waters of springs and rivers cease flowing. After a while they begin to flow again, but they are troubled and mixed with mud, sand, and strange substances that change their color and quality. Earthquakes are almost always accompanied by violent agitations of seawater; on the shores of the sea vessels knock against each other in the ports and those that are on the high seas have often experienced extraordinary movements caused by the rising of the sea from the depths of its bed. These effects are due to the forces of air expanded by the fire in an effort to open a passage and to free itself. The shakings cause quakes that follow one another, sometimes at great distances from each other, sometimes they follow very promptly. The movement that they transmit to the earth is at times a kind of undulation similar to that of waves or at times a rocking motion is experienced similar to that of a ship battered by the billows of the sea. They are the source of nauseas and stomach aches that some people experience during earthquakes, especially in shakings that are slow and weak. These shakings ordinarily follow in a definite direction. For this reason, it sometimes happens that an earthquake will knock down buildings and walls that are not built following the direction it takes and will totally destroy those that are oriented in the opposite direction. The shakings are more or less frequent and strong according to how the substances that stimulate them are more or less abundant and according to how their explosions are more or less sharp. In America earthquakes have been seen to last for more than an entire year, causing several very violent shocks to be felt each day. In a word nothing is more terrible and more varied than the effects produced by earthquakes . At times the sea will recede several leagues and will leave vessels on dry seabed, then return to submerge the lands violently. Sometimes very sizeable parcels of land will be displaced, flowing like water and filling up lakes. Other times mountains will subside and lakes will come to take their place. Often one has seen the earth open and vomit from within its depths flames, charred sand, stones, sulfurous waters and an unbearable odor. These openings made in the earth sometimes close immediately, other times they remain in the same state.

One of the strangest phenomena of e arthquakes is their propagation, meaning the manner in which they spread great distances in a very short space of time. The most natural way to explain this propagation is to say that blazes underground communicate with one another through immense cavities that fill the interior of the earth. These cavities, being filled with the same substances, receive fire brought to them from those that were first lit. In this manner the blaze is transmitted from one side of the globe to the other. One may also suppose that the earth encloses several hotbeds which enflame themselves either in succession or simultaneously and which produce a series of explosions and shaking in the different parts of the earth where they occur. It has been noted that it is commonly by following the direction of great mountain chains that the propagation of earthquakes is felt, which makes one assume that mountains have for their base cavities by which they connect to one another. [8]

Earthquakes have often been confused with certain extraordinary movements that are felt sometimes in the air and that are strong enough to blow over houses producing considerable damage without anyone having perceived that the earth had shaken at all. These phenomena have been observed in Sicily and in the kingdom of Naples. They appear to be due to a sudden release of air enclosed within the earth freed up by subterranean fires, and which stimulate in the exterior air a commotion similar to that of a canon shot that often breaks windows in houses.

Such are the principal circumstances that accompany earthquakes . There are scarcely any parts of our globe that have not experienced them more or less strongly and at different times their destructive effects. The histories filled with frightening descriptions and the tragic changes that they have produced. Pliny informs us that under the consulate of Lucius Marcius [9] and Sextus Julius [10] an earthquake caused two mountains in the territory of Modena to collide violently with each other and to crush in their conflict edifices and farms that were located between them, a spectacle to which a large number of Roman horsemen and travelers were witnesses. Here are his own words: “Factum est semel,” he says, “quod equidem in etruscae disciplinae voluminibus invenio, ingens terrarum portentum. L. Marcio et Sexto Julio cos. in agro Mutinensi nunque montes duo inter se concurrerunt, crepitu maximo adsultantes recedentesque, inter eos flamma fumoque in caoelum exeunte interdiu, spectante e viâ Aemilia magna equitum Romanorum familiariumque et viatorum multitudine. Eo concursu villae omnes elisae, animalia permulta, quae intra fuerant, exanimata sunt, etc. ” [11]

Under the empire of Tiberius, thirteen sizeable cities in Asia were totally overturned and a countless number of people were buried under their ruins. [12] The famous city of Antioch experienced the same destiny in the year 115; the consul Pedo perished there and Emperor Trajan who found himself there at that time barely escaped the disaster of this famous city. [13]

In 742, there was a widespread earthquake in Egypt and throughout the Orient; in one night nearly six hundred cities were overturned and a prodigious number of men perished on this occasion. [14]

But why do we need to talk about ancient earthquakes ? A recent experience only proves to us only too well that the substances that produce these terrible events are not exhausted. Europe has barely gotten over the fear that was caused by the terrible catastrophe in the capital of Portugal. On the first of November in the year 1755, the city of Lisbon was almost totally knocked down by an earthquake that was felt the same day all the way to the ends of Europe. This awful disaster was accompanied by a prodigious rising of the sea that was carried with violence onto all the western coasts of our continent. The waters of the Tagus rose several times to inundate the buildings that the shakings had knocked down. At the same instant this frightening scene was taking place in Portugal, Africa was shaken in parallel manner; the cities of Fez and Mequinez in the kingdom of Morocco were almost completely overturned. Several ships, returning from the West Indies, felt violent and extraordinary shakings on the high seas. The islands of the Azores were sharply shaken at the same time. In the month of December of the same year almost all Europe was again shaken by an earthquake that was strongly felt in some regions. [15] America was not exempt from these sad ravages; this was around the same time that the city of Quito was completely overturned. [16]

All earthquakes are not felt with the same violence; there are some that only produce light shakes, and are sometimes imperceptible. Others bring destruction to the places where they exert their fury. It has been noticed that some countries are more subject to these convulsions than others. Above all, the hot countries seem to be most exposed, which is either because the heat of the climate is such that it causes the depths of the earth to release a greater number of gasses ready to ignite and to produce explosions, or because these lands contain a greater number of substances that are combustible and available to fuel and propagate underground fires. America and especially Peru seem subject to some very frequent shaking. According to Sir Hans Sloane, one expects to suffer an earthquake every year in Jamaica. [17] Asia and Africa are not exempt from these terrible accidents. In Europe, Sicily, the kingdom of Naples, and nearly all the Mediterranean are frequently the theatre of these fatal events. We see also that the northern countries although less often than the warm countries, have experienced at different times shakings caused by earthquakes . England, Iceland, Norway furnish us with convincing proofs of this. M. Gmelin informs us that he felt some in Siberia.  [18] He was even assured that a part of this area so far north underwent an earthquake annually and periodically. The southern provinces of France that are bordered by the Pyrenees also have sometimes felt very violent shakings. In 1660 an earthquake devastated the entire area between Bordeaux and Narbonne. Among other ravages it caused a mountain of the Bigorre to disappear and put a lake in its place. Due to this event a great number of hot springs were cooled, and lost their salutary qualities. [19] In the last earthquakes of 1755, it was also this part of France that experienced most strongly the shakings that were felt only very faintly in Paris and in the northernmost provinces. [20]

At the sight of the prodigious effects of earthquakes one feels that it is natural to consider them the principal causes of the continual changes that happen to our globe. History has transmitted to us some of the violent changes that the earth has undergone with influence from the underground fires, but the greatest number and the most significant among them are buried in the forgotten past of most remote antiquity. We can only speak of them by conjectures that seem, however, rather well founded. There is every reason to presume that Great Britain was torn from the European continent; Sicily was in like manner separated from the rest of Italy. Would it be such a bold opinion to consider the Mediterranean as a vast basin hewn by subterranean fires that often still exert their ravages? Plato and some other ancients transmitted to us the name of an immense island they called Atlantis , which tradition places between Africa and America. This vast area disappeared entirely. Cannot one conjecture that it was engulfed under the waters of the Ocean to which it gave its name and that the islands of Cape Verde, the Canaries and the Azores are the unfortunate vestiges of the terrible disastrous change that caused the disappearance of this area below the face of the earth? Perhaps the Black Sea, The Caspian Sea, the Baltic Sea, etc., are due to parallel disasters that occurred at a time when no historical document could preserve a memory of it.

From Peru to Japan, from Iceland to the Moluccas we see that the bowels of the earth are perpetually torn by blazes acting incessantly with more or less violence. Such powerful causes cannot fail to produce effects that influence the total mass of our globe. In the long term they should change its center of gravity, drying some of its parts while submerging others, contributing to the production of the cycle of violent changes in nature. Is it surprising that the astonished traveler no longer finds seas, lakes, rivers, famous cities described in works by ancient geographers, places of which today there remains no trace? How could the fury of the elements have respected the flimsy products of the hands of men, while it shakes and destroys the solid base that serves as their platform of support?

1. Experimental physics → in the eighteenth century included allied areas in natural philosophy or natural science, such as geology.

2. Pyrophoric substances ignite spontaneously in air below 130 degrees Fahrenheit. Examples include iron sulfide and other reactive metals when powdered or sliced in thin layers.

3. French pioneer chemist Nicolas Lémery (1645-1715), author of a chemistry textbook, Cours de Chymie (1687) that appeared in 13 editions; it became a standard work for 100 years. In his innovative lectures he treated chemistry as a demonstrative science rather than repeating timeworn theories.

4. Guillaume-François Rouelle (1703-1801) was a chemist and member of both the Collège de pharmacie and the Académie des sciences. Among those who took his private laboratory course were Diderot and Lavoisier.

5. What explains fire? In 1667 Johann Becher posited that all combustible material contained an element, phlogiston, released into the air during burning. Not until the 1770s, after this article was published, was the phlogiston theory successfully challenged when Antoine-Laurent Lavoisier demonstrated that combustion requires oxygen, recently discovered, and a specific high temperature for each material that possesses a potential to ignite.

6. Denis Papin (1647-1713) conducted experiments, some in collaboration with Christiaan Huygens and Robert Boyle, with air pumps and steam pressure. Papin invented a safety valve that was subsequently useful in the development of steam power.

7. The aeolipile or eolipile, also known as the Hero engine, a steam or turbine engine first described by Hero (10-70 AD).

8. Having just experienced the Lisbon earthquake in chapter five of Voltaire’s ironical Candide ou l’optimisme, Pangloss voices a similar conjecture concerning the interconnection of distant earthquakes: “Il y a une trainée de soufre sous terre depuis Lima jusqu’à Lisbonne... tout ceci est ce qu’il y a de mieux. Car s’il y a un volcan à Lisbonne, il ne pouvait être ailleurs ... car tout est bien.” (“There must be an underground vein of sulfur connecting Lima to Lisbon ... all this is for the best. Since, if there is a volcano in Lisbon, it could not be located elsewhere ... because all is for the best.”) Here, as in his Poème sur le désastre de Lisbonne (1756), Voltaire is satirizing Alexander Pope’s optimistic formula in The Essay on Man (1734), “Whatever IS, is RIGHT;” all is for the best, in other words. Voltaire’s source on earthquakes was Elie Bertrand’s Mémoire sur les tremblements de terre (1756). In the article “Lima” Jaucourt describes at length the earthquake that destroyed that city on October 28, 1746.

9. Lucius Marcius Philippis (c. 141-c. 73 B.C.), Roman Consul in 91 B.C.E.

10. Sextus Julius Caesar (c. 137- 90 B.C.E.), Roman Consul in 91 B.C.E., an uncle of Julius Caesar.

11. The passage quoted is from Pliny’s Naturalis Historia , II, 85. I have made a few modifications to the Latin text as it appears in d’Holbach’s article by following a version included in a recent article published by L.M. Jennarelli, “Demystifying Pliny the Elder’s Cosmology: The Earthquake in Modena, the God Vulcan and the Fluidization,” which appears on-line in XHAMUEL Papers, Classical Studies Series, 2013, viewed August 2, 2015. A translation of into English by H. Rackman (Pliny, Natural History , vol. I, 329 and 331, Harvard U. Press, 1958) of the passage reads as follows: “I find in the books of the lore of Tuscany that once a vast and portentous earthquake occurred in the district of Modena; this was during the consulship of Lucius Marcius and Sextus Julius. Two mountains ran together with a mighty crash, leaping forward and then retiring with flames and smoke rising between them to the sky; this took place in the daytime, and was watched from the Aemilian road by a large crowd of Knights of Rome with their retinues and passers by. The shock brought down all the country houses, and a great many animals in the buildings were killed.”

12. Tiberius was Roman Emperor from 14 to 37 A.D.; in 17 A.D. the Lydia earthquake destroyed thirteen cities in Asia Minor, as reported by historians Tacitus and Pliny the Elder.

13. On December 13, 115 A.D. a violent earthquake and tsunami devastated Antioch and its harbor. Greek historian Cassius Dio in Historia Roma described the earthquake and reported that Roman Emperor Trajan and his successor, Hadrian, survived the earthquake; Pedo Virgilianus, then consul in Antioch, perished.

14. This earthquake is mentioned in The Seismicity of Egypt, Arabia, and the Red Sea: A Historical Review by N. N. Ambraseys, et al (New York: Cambridge U. Press, 1994), 25.

15. There were many aftershocks following one of the most destructive earthquakes in all of recorded history that took place on November 1, 1755; it destroyed the city of Lisbon, Portugal, that some had proclaimed a “New Rome.” This terrible event called into question the contemporary religious and philosophical notions of Providence, Theodicy and Optimism. Voltaire’s Candide ou l’optimisme (1759) is an enduring example of the intellectual response to the Lisbon disaster. The Lisbon earthquake and tidal wave originated in the central Atlantic Ocean and were felt in North Africa, as far north as Finland and Greenland, and in eastern North America. The topic of these earthquakes comes up in several other articles in the Encyclopédie , including the article, “Lisbonne.” In « Feu central et feux soûterrains » Formey refers to 1755 as « a year that will be sadly famous in history.” As noted above (note 8), Jaucourt describes at length an earthquake that destroyed Lima on October 28, 1746 in an article devoted to that city. Since the year 2005 marked the 250 ^th anniversary of the Great Lisbon Earthquake, the event became the topic of numerous publications, including The 1755 Lisbon Earthquake Revisited, compiled by Luiz A. Mendes-Victor, Carlos Sousa Oliveira, Joāo Azevedo and António Ribeiro , in Geotechnical, Geological, and Earthquake Engineering , vol. 7 (2009, Springer Media). Other recent books that provide useful information on the Lisbon Earthquake include Nicholas Shrady, The Last Day: Wrath, Ruin, and Reason in the Great Earthquake of 1755 (New York: Viking, 2008); Edward Paice, Wrath of God: The Great Lisbon Earthquake of 1755 (London: Quercus, 2008); and Jean-Paul Poirier, Le Tremblement de terre de Lisbonne (Paris: Odile Jacob, 2005). UC Berkeley’s National information Service for Earthquake Engineering (NISEE) Kozak Collection of Images of Historical Earthquakes provides images, woodcuts and paintings of the 1755 Lisbon Earthquake.

16. The Pichincha, Quito, earthquake occurred on April 26, 1755.

17. British Royal Physician, pharmacist and naturalist, Sir Hans Sloane (1660-1753), began a fifteen-month stay in Jamaica in 1687 in the service of the governor. He recounted his experiences outside of Europe in two volumes: A Voyage to the Islands of Madera, Barbados, Nieves, S. Christophers and Jamaica , with the Natural History of the Herbs and Trees, Four-Footed Beasts, Fishes, Insects, Reptiles, etc. of the Last of those Islands, to which is prefix’d, an introduction, wherein is an account of the inhabitants, air, waters, diseases, trade, etc. of that place, with some relations concerning the neighbouring continent, and islands of America. Illustrated with figures of the things described, which have not been heretofore engraved (London, 1707) . Sloane’s extensive collections constituted the beginnings of the British Museum.

18. Chemist and naturalist Johann Georg Gmelin (1709-1755) joined Vitus Bering’s Second Kamchatka Expedition (1733-1743). Gmelin published his observations in Flora Sibirica (Petersburg, 1747-1769), and further described his journey in Reise durch Sibirien von dem Jahr 1733 bis 1743 (1751), translated into French as Voyage en Sibérie (Paris, 1767). Gmelin’s notes were recently revisited by A. A. Nikonov and L. D. Fleifel in “Forgotten Accounts of Historical Earthquakes in Siberia (17 ^th and 18 ^th Centuries) by J.G. Gmelin,” Russian Geology and Geophysics 55, no. 4, (2014), 522-29.

19. Bagnères-de-Bigorre was struck by an earthquake on June 21, 1660; it was followed by aftershocks lasting three weeks. The city’s hot springs were affected temporarily.

20. See note 15.