Thunderstorms underground: Giuseppe Saverio Poli and the electric earthquake

Abstract

This paper presents a case study of the “electric hypothesis” of the causes of earthquakes, which emerged in the second half of the eighteenth century as part of the first studies of seismology. This hypothesis was related to Franklin’s views on atmospheric electricity and developed in a period when electric phenomena were widely studied, and was essentially based on solid empirical evidence and confirmed by model experiments. Even though it resulted from scientific reasoning, the theory remained strongly empirical, and was supported by Italian scholars who were familiar with seismic events. Among these, Giuseppe Saverio Poli, a follower of Franklin, was able to provide a careful and comprehensive explanation of the disastrous earthquake of 1783, which occurred in Calabria, a region of southern Italy, and the St. Anne earthquake of 1805, by drawing not just upon the electric evidence, but all the relevant phenomenology available. We outline here the emergence, the development, and the later evolution (up to the beginning of the nineteenth century) of the “electric earthquake” paradigm by focusing on different works by Poli, including a previously unknown manuscript containing a thorough account of the Calabria earthquake prepared by the Neapolitan scholar for the Royal Society. The present case study therefore offers the opportunity to illustrate how electrical science shaped earthquake science to a degree not usually appreciated in the literature, and is also supported to some extent by the transition from Enlightenment scientific ideals to the Romantic conception of unity in the natural world, in search of common causes among phenomena belonging to different fields.

Introduction

In the second half of the eighteenth century, a relationship was hypothesized between seismic events and electricity, based on the traditional similarity between thunder in the atmosphere and quakes on Earth. ¹ Given such analogy, once thunderstorms were accepted to be electric phenomena (according to Benjamin Franklin) it necessarily followed that earthquakes too had to be electric, resulting from a sort of unbalance of the electric fluid between the Earth and the atmosphere. ² A model experiment by Joseph Priestley even served as a basis to demonstrate such explanation: he observed how wooden blocks would tumble on a wet plank floating in a basin of water after an electric discharge from a Leyden battery had been released through the plank. ³

However, the “electric hypothesis” was not the only suggested cause of earthquakes: a heated debate developed that later produced a division between firists and electricists, supporting different views on the main causes of seismic events. ⁴ According to an ancient theory set forth by Aristotle, firists admitted the existence of a sort of “underground wind” (later in the sixteenth century, it was believed to be produced by “fermentation”) heated by inextinguishable fires present in the bowels of the Earth. ⁵ When this wind blows freely through the vast underground cavities, nothing happens on the outer surface; but, if it gets trapped inside a crack or a tunnel, and is forced out through narrow channels, it causes the Earth’s surface to quake and shake, then eventually disperses into the atmosphere. According to this theory, volcanoes and earthquakes must have a common origin.

With the growing interest in electricity in the eighteenth century, however, a different view became popular: the main action of the underground wind was superseded (or, at least, accompanied) by that of the electric fluid. Now scientists looked for the cause of earthquakes over the ground, rather than under it, and the cause of the unbalance between terrestrial and atmospheric electricity was the cause of a friction between the atmosphere and the Earth’s surface, which set off the earthquake. As a matter of fact, Italy – and, especially, southern Italy – in the eighteenth century became the key place for such studies, the ideal place to both formulate and test hypotheses. That part of the continent represented a unique case in Europe for its articulated orographic system of volcanic origin and for the numerous events of earthquakes; such a wealth of available phenomenology offered incomparable opportunities to scholars to test their theories. Furthermore, that century was also the period when a tradition of studies on electricity developed in Italy, culminating (among other things) in the well-known findings of Alessandro Volta. Therefore, it is not surprising that the electric hypothesis on the origin of earthquakes was primarily developed in Italy, where the debate with the firists from across the Alps also became particularly heated.

Quite noteworthy was the work carried out by the Neapolitan scholar Giuseppe Saverio Poli, ⁶ a minor figure in the eighteenth-century scientific scene, who managed to collect different empirical evidence from diverse fields (not limited to electricity) into a consistent and comprehensive framework of the causes of (specific) earthquakes. He was very influential on other Italian (and non-Italian) scientists of the time, primarily due to his accuracy in experimentation and clarity in reasoning. His works were part of that tradition of studies that focuses on the effects of a given phenomenon rather than on its causes but does not go beyond the proposal of explanatory theories that are still strongly empirical, resulting in conclusions that today sound very peculiar, to say the least.

Nonetheless, such a case study offers the opportunity to explore the ever-boiling situation at the very origins of seismology. It highlights the importance of those debates through the eyes of a scholar who paid attention to all the different aspects of the science of his time, in a moment that marked the passage from Enlightenment ideals to the Romantic conception of the unity of the natural world, when scientists sought the common causes for phenomena that belonged to fundamentally different fields.

Poli’s contribution lies mostly in the spreading of Newtonian science and its scientific and technological applications in Italy, particularly as his network of scientific correspondence was quite remarkable. In addition, Poli worked in the Kingdom of Naples, a region in southern Italy that offered continuous geological inspiration, and he was even introduced to the Bourbon court: he had easy access to important documents and met important people. Therefore, his role in this scientific story is quite relevant and will be the object of our discussion.

To fully appreciate what happened between the late eighteenth and early nineteenth centuries in this specific field, in the following section we will first focus on atmospheric electricism, as debated at the time especially among the franklinist Neapolitan scientists, and highlight a few works by Poli that earned remarkable consideration in the academies, including the Royal Society of London. ⁷ We will then highlight the appearance and subsequent developments of the electric hypothesis on the origin of earthquakes, pointing out the key role played by some Italian scholars, to whom later researchers systematically conformed. The subsequent four sections are then devoted to experimental tests of the theories, including the phenomenology of relevant earthquakes studied in the considered period, among which are the Calabrian event of 1783 and the so-called St. Anne earthquake of 1805. An intriguing story is reported in the section “A mysteriously unknown account to the Royal Society,” where a previously unknown manuscript prepared by Poli for the Royal Society is analyzed in detail, with its “comprehensive” explanation of the seismic phenomenology exhibited by the Calabrian event. Given its relevance, the whole manuscript is reported in the Appendix to the present paper. Finally, our concluding remarks follow, highlighting a truly unifying approach by Poli that marked the transition to Romantic science and explaining, to some extent, the rapid decline of the electric hypothesis).

Air and electric fire: The field of atmospheric electricism

Franklin’s theory of electricity, according to which the most important atmospheric phenomena were thought to have a bearing on natural electric phenomena, practically underlay all the studies on atmospheric electricism. ⁸ The basic ingredient introduced by Franklin was the view that when we electrify a body, this involves the accumulation of a “charge” from elsewhere, rather than the excitation of matter already present in it, and this fact was particularly appealing in explaining atmospheric phenomena. ⁹ For example, as thunderstorms were the most dramatic aspect of atmospheric electricism, around the middle of the eighteenth century, it was determined that clouds were electrically charged, and once they reached a substantial charge, they would discharge in a lightning strike. However, while several illuminating examples of this kind can be found in the literature, here we focus on several less-known works by Giuseppe Saverio Poli – one of the prominent franklinists in Naples – that give us some useful information on how he approached the investigation of Nature. ¹⁰

A first paper by Poli, dated 1772, contained just a review of atmospheric electric phenomena explained according to Franklin’s theory, but his Riflessioni intorno agli effetti di alcuni fulmini of the following year went into more detail, and caused a heated debate with another Neapolitan franklinist, Gian Gaetano del Muscio. ¹¹ The main point of contention was the interpretation of some anomalies of the electrified glass, but, more generally, the theories of electric conductors and their explanation in terms of Franklin’s electric atmospheres were also debated. As usual, Poli performed a series of experiments of his own with the Leyden jar and the Franklin square, which apparently revealed both the transparency to light and opacity to heat of glass (and even the possibility of charging a jar with a fissure in it), in obvious conflict with Franklin’s dominant theory. From his experiments, Poli concluded that no substance seemed to exist in Nature that cannot spread electricity and, moreover, independently of its composition and form, glass could distribute electricity (when electrified) even inside its surface, contrary to what was purported by pure franklinists.

On December 3, 1777, an aurora borealis was observed in Naples, which allowed Poli to test “accurately” certain predictions made by Isaac L. Winn in a letter to Franklin (published in the Philosophical Transactions of the Royal Society). ¹² Following earlier suggestions, Winn had proposed to apply observations on northern lights to the weather forecast, since high winds would apparently follow an auroral display, and a storm would move in an opposite direction from the winds it had created. ¹³ Winn’s paper was followed by a somewhat skeptical comment by Franklin, but nevertheless it would attract – for more than a century – considerable scientific attention from many scholars, ¹⁴ Poli being among the first of them. He provided a detailed account of the same phenomenon observed in Naples, along with a series of precise measurements obtained with an accurate Ramsden’s barometer/thermometer owned by Giovanni Vivenzio. His observations broadly confirmed what Winn had suggested, but the most notable thing here is his own explanation of the observation, which assumed (as found in his previous papers) that the aurora borealis is caused by “electric fire.” ¹⁵ According to Poli, this hypothesis rested on four basic observations, including model experiments: (1) auroral displays can be produced by means of electric machines; (2) natural aurorae induce perceivable variations in magnetic needles; (3) electricity can be collected from aurorae through metallic sharp edges; (4) aurorae boreales produce a crackling like the one produced by electric sparks.

What was observed on the occasion of Naples’ northern lights was further confirmed by the same author, who had the chance to see an “extraordinary” aurora borealis in London on May 21, 1779, during his own philosophical journey throughout Europe. ¹⁶ Further meteorological details induced the Neapolitan scholar to conclude: “Is it not worthy of observation that the blowing of such a wind and the occurrence of the storm preceded the appearance of this Aurora, since after it the sky remained beautiful and serene for several days?” ¹⁷ It was his work on atmospheric electricism, presented before the Royal Society, that qualified Poli as a home member of that prestigious society. ¹⁸

Interestingly, however, the aurorae boreales were not the only extraordinary atmospheric phenomena related to electricity witnessed by Poli in England; in a later paper presented at the Royal Academy of Sciences in Naples in 1784, even stranger phenomena were reported, more closely concerning Franklin’s theory and lightning rods mounted on ships.

The lightning matter, drawn down in large numbers by the mighty strength of the sharp point of the conductor located at the top of the vessel’s mast [. . .], spread in the shape of a rapid torrent of very lively fire, along the continuation of that; and then, reaching the sea, it was consequently dispersed in the universal mass, in the same way that the electric fire developed by means of a powerful machine was seen rapidly flowing between the rings of a metal chain, which, hanging from the first conductor, and spread over the floor, go to communicate with the back of the pillow, whereby the globe, the cylinder, or the disk is wrinkled. ¹⁹

This is reminiscent of what is now known as “St. Elmo’s fire,” which was annotated along with several other impressive phenomena; here, however, it is interesting to note that several such phenomena were later reproduced in London by Poli in a series of model experiments. For example, the powerful charge of a battery of fifty-four Leyden jars was discharged on iron wires mounted on wooden sticks. No damage was observed when the wires were connected to the Earth, while striking “frightening failures” were produced at locations where that connection had been interrupted. Similarly, Poli was able to magnetize iron needles by discharging the mentioned battery several times on them. All these findings led to the conclusion that there was some sort of unity to the natural world.

I hold firm the opinion that if one wishes to reason sanely, the infinite number of natural effects and phenomena, which always adorns the great theatre of the Universe, must be looked at in the manner of as many streams, which, starting from a common centre, gradually move away from each other; from which it then happens that, mixed and confused the similar with the dissimilar, their immediate dependence on that immense and main source, from which they derive their origin, is totally lost. ²⁰

Balancing the Earth with the Heavens: The electric earthquake

During the eighteenth century, such “streams,” apparently branching off from a “common centre,” increased in number. Indeed, it became commonly understood that electricity was the primary cause of shocking phenomena not only in the air but also on Earth, according to what Pliny the Elder had written: neque aliud est in terra tremor quam in nube tonitruum (“there is nothing in an earthquake on the ground other than thunder in a cloud”). ²¹

At the turn of the seventeenth and eighteenth centuries, “naturalists are divided as to what the causes of earthquakes are. Some ascribe them to water, others to fire, and others to air; and all of them with some reason.” ²² For example, the overflowing of groundwater from subterranean caverns, veins, and canals was thought to directly induce a shaking of the Earth, because the generally accepted view was that conflagrations (or even explosions) in the cavities of the Earth might cause earthquakes. Also, some thought that water could be violently heated by subterranean fire, and the resulting fumes and blasts might give rise to concussions directly above. The air so furiously produced could be considered as another possible cause of earthquakes. In time, fire came to be seen as the principal cause of earthquakes, due either to the lighting of underground fires (whose existence was certainly proved by volcanic phenomena) or to chemical reactions. Indeed, the association between earthquakes and volcanoes was well established (although the necessity of such a link was not at all clear); the controversial model experiment of the artificial volcano performed by Nicolas Lémery showed that suitable chemical reactions could be the possible cause of volcanic activity, and hence earthquakes. ²³ With a mixture of iron filings and powdered sulfur – the two invariable components of pyrites – appropriately moistened to induce an “effervescent” reaction, he was able to produce a volcanic conflagration, resulting in an actual artificial earthquake. Even if this experiment did not always prove successful, still it convinced many that fire was the main agent in earthquakes, just like in several other natural phenomena.

Despite such substantial evidence, during the eighteenth century, as the study of electric phenomena increased, an alternative view began to stand out, which was centered around the simple observation that conflagrations and explosions in the cavities of the Earth could well also result from the “electric fire” generated by some unbalance in the “electric vapor” within the Earth. On the one hand, a new (and copious) phenomenology started to be considered, pointing out – for example – that during earthquakes the air could appear “charged with electricity”; fire balls, flashes of lightning, and several similar electric phenomena were reported in the records of earthquakes, along with curious electric shocks and related pains in people (for example, nervous pains and hysteric affections). ²⁴ Later in 1891, the Italian Mario Baratta managed to draft a schematic but extensive catalog of these electric phenomena reported in earthquakes records, starting from 114 C.E., which is an interesting historical document on this topic. ²⁵

On the other hand, thorough reflections also appeared in the literature as “firm” results were achieved as regards electricity, especially after Franklin had released his theory on the “electric atmospheres.” Although Franklin is often credited with developing an electric theory of earthquakes, he never developed such a theory. ²⁶ After some initial attempts in this direction by William Stukeley, Stephen Hales, and others, two influential works appeared in the second half of the century by two Italian scholars, Giambattista Beccaria and Tiberius Cavallo. ²⁷ Italy was a seismically active country, and investigations on electricity were particularly relevant, which explains why there was such an interest in this matter.

The similarity of the phenomenology, as in the atmospheric and the analogous electricism provided by earthquakes, induced Beccaria to assume that the role played by water and air in previous explanations might as well be interpreted through “electric vapour.” This was thought to exist in abundance in the Earth, and could move from one cavity where it overflows to another where there is shortage. Once the “electric fire” was ignited, it could evaporate water, explode air, crash towers, and so on, just like an earthquake. To complete the perfect analogy with the already existing fire-driven explanation mediated by chemical reactions, “it seems that sulphurs may be responsible for the effects that ensue, but also other substances have the same impact, such as the electric vapour released from them or electric bodies, while they change nature when fermenting.” ²⁸ Despite the similarities, however, what particularly struck Father Beccaria was that both lightning (or artificial electric discharges) and earthquakes were propagated at very high speeds. This was the crucial point that convinced him that earthquakes were caused by electric discharges. Indeed, one of the major difficulties encountered by firists was that of explaining the simultaneous effects of earthquakes observed at very large distances: it was hard to suppose that gases from inflammations could propagate so quickly over great distances, contrary to what they believed for the electric fluid. The striking of lightning during volcanic eruptions, along with the alleged observations of lightning and thunder during powerful earthquakes, and the way ships at sea felt those impacts, further confirmed Beccaria’s view, and subsequently led many scholars to follow his theory.

If Lémery’s model experiment was quite relevant in spreading the fire-driven explanation of earthquakes, a similar imitation of these in electric-based model experiments played an identical role in promoting the explanation refined by Beccaria. More than Priestley, who carried out the famous experiment recalled in the Introduction, for Italians the crucial figure was Tiberius Cavallo, a Neapolitan-born scholar who later moved to England. In his Complete Treatise on Electricity in Theory and Practice, he devised several experiments aimed at reproducing (on small scales) the destructive effects of earthquakes produced by electrically induced explosions and shocks. ²⁹ While illustrating earthquake propagation by means of the effects of an electric shock sent over the surface of some given substances, Cavallo realized that “an explosion will not pass over the same length of surface of all bodies, though they are equally good conductors.” ³⁰ He also realized that the explosion does not manifest only in the concussions on the surface but affects the whole body (and that the effect of the explosion is much louder in water than in air). “Nothing more is required than inserting part of the surface of said substances into the circuit of the two sides of a [Leyden] battery.” ³¹ And, even more strikingly, “the spark, that in this experiment passes over the surface of the water, seems to bear a great resemblance to the balls of fire that sometimes you see over the surface of the sea or land during an earthquake; and hence it seems very probable that those balls of fire are electrical phenomena.” ³²

The reproduction of such phenomenology inevitably brought many to accept the likeliness of the electric hypothesis. ³³ Furthermore, similarly to atmospheric electricism, some scientists suggested looking for possible warning signs in atmospheric phenomenology; they might even exploit what was known about lightning rods to provide protection against earthquakes – they could be deprived of their destructive electricity by planting in the ground some iron rods over large areas (thus enabling electric equilibrium to be restored between the atmosphere and the Earth). ³⁴ They might even expect to forecast earthquakes through a close observation of atmospheric phenomena (such as vapors, fogs, oddly shaped clouds, or lightnings) and sudden climatic or temperature changes. Not to mention that several authors, in agreement, also observed that earthquakes were more frequent in winter when the maximum intensity of atmospheric electricity occurs. Therefore, a special care in recording the occurrence of such phenomena was evidently mandatory in the supporters of the electric hypothesis.

A devastating opportunity: The Calabrian earthquakes of 1783

The eighteenth century gave scholars many opportunities to “test” the electric earthquake theory (and even infer new suggestions), and this was especially true in Italy, where many events occurred in the second half of the century. ³⁵ The vast literature of the time reveals a striking similarity between many distinct works, as they were all structured around the same angles: a precise account of where and when the earthquake occurred; a detailed record of the effects of the shocks on buildings, people, and the surroundings, with the extent of the area where they were observed; and, finally, the faithful description of the weather conditions, along with possible atmospheric phenomena that preceded or accompanied the earthquake event, which revealed the idea (or, at least, the suspicion) that atmospheric electricism was related to earthquakes.

The focus was on two major occurrences: the great Lisbon earthquake of 1755 and the long series of strong Calabrian earthquakes that started in 1783. The former was felt in most of Europe, in many places in North Africa, and even along the east coast of North America and in the Caribbean. It marked the beginning of serious earthquake studies, leading to a broad discussion on the causes of these natural catastrophes. ³⁶ However, a lack of systematic and accurate collection of the seismic data prevented the significant scientific advances that we will see with the Calabrian earthquakes, ³⁷ though it did attract the attention of many scholars, among them various Italian naturalists (such as Giovanni Vivenzio and Lazzaro Spallanzani), and travelers (including Johann Wolfgang von Goethe, Déodat de Dolomieu, and William Hamilton) to southern Italy. Noticeably, the Naples Reale Accademia delle Scienze e delle Belle Lettere soon set up the world’s first scientific commission to make a systematic investigation of the damage, the results of which flowed into a reference essay acknowledged by international academia. ³⁸

Before and after such reference works, many reports and memoirs – either written by direct witnesses or relying on indirect testimonies – appeared, containing varied information on the Calabrian earthquakes. An interesting and practically complete collection of such reports has been put together in recent years, and solid conclusions have been drawn. ³⁹ The devastating seismic sequence started on February 5, with an intensity of I₀=XI on the Mercalli-Cancani-Sieberg (MCS) scale (a Richter magnitude higher than M_W=7) and, although a major attack was formed by four further shocks of intensity IX–X between February 5 and 7, a long series of aftershocks followed, which went on for about ten years. These migrated from the Messina Strait heading north-northeast along the axis of the Calabrian Peninsula, and the total length of the huge epicenter zone exceeded 130km.

The first detailed report on the Calabrian earthquakes to be seen abroad was that sent on May 23, 1783, from Naples by the volcanologist William Hamilton to the Royal Society in London. ⁴⁰ In the first part of his Account, the British ambassador to the Kingdom of Naples referred to “the most authentic reports, and accounts received at the offices of his Sicilian Majesty’s Secretary of State,” ⁴¹ giving quite a clear idea of the natural disaster that had occurred in the south of Italy. Furthermore, “as I am particularly curious, [. . .] I took the sudden resolution to employ about twenty days [. . .] in making the tour of such parts of Calabria Ultra and Sicily as had been, and were still, most affected by the earthquakes, and examining with my own eyes the phenomena above mentioned.” ⁴² The journey started on May 2; Hamilton carried out a thorough “scientific” inspection of the areas affected by the earthquakes, and interviewed reliable witnesses and local experienced scholars. The second part of Hamilton’s Account was drawn directly from his own travel diary, without further analysis and appropriate meditation on what he was observing. ⁴³

Hamilton was a distinguished amateur naturalist. ⁴⁴ For instance, he observed that at the same distance from the center of the quake (the town of Oppido in Calabria), damage to buildings on higher ground was less than damage to those on lower ground, and he deduced that the earthquake focus was arguably beneath the Earth’s surface. This confirmed his idea that the disturbance was due to the action of confined vapors produced by some sort of fermentation process, although – as a volcanologist – he was rather inclined to ascribe the cause of the disaster to some subterranean volcanic activity, probably associated with the nearby Stromboli volcano. Even his correspondent Count Ippolito was persuaded that the earthquake was due to the action of an internal fire in the bowels of the Earth, as confirmed by the flames that they saw coming out of the ground at the occurrence of the earthquake. Interestingly, he also reported that before the disturbance, the ground appeared to be heated in the same place, while after the quake, a whitish flame manifested with the appearance of an electric fire.

Count Ippolito “kept a regular account from the day of the first shock of the fifth of February, not only of the convulsions suffered by earth, but likewise of all the meteors observed in the atmosphere,” so that it was not difficult for him to give a detailed account of the phenomena when Hamilton requested it. Many terrestrial and atmospheric phenomena that preceded or followed the earthquake of March 28 were reported, from drinkable water that turned into disgusting and sulfurous water (and the other way around), to extraordinary frosts in winter, followed by extraordinary drought and insufferable heat waves in spring and copious and incessant rains in fall. ⁴⁵

Even if he was not a supporter of the electric hypothesis, nevertheless, Hamilton did take into due consideration “the most authentic reports and accounts,” even when dealing with “curious facts” that were hard to believe. As a matter of fact, not only did he publish Count Ippolito’s letter, but he himself reported some facts, accurately checked on site, in his Account to the Royal Society. But, after producing empirical evidence of a strict electric connection between air and earth, Hamilton did not give up completely on his beliefs, but rather he highlighted a harmonization of the different views. He later (1786) changed his mind and fully adopted a volcanic hypothesis thanks to newly acquired knowledge, but this was quite the obvious conclusion of a careful non-Italian scientist. ⁴⁶

Similar, but more skeptical, conclusions were drawn by the French geologist Déodat de Dolomieu, who also traveled (one year later) through Italy and examined the devastating results of the 1783 Calabrian earthquakes. ⁴⁷ He saw no reason to support the electric hypothesis, because he believed that electricity could not accumulate in a region surrounded by water, but he rejected Hamilton’s volcanic hypothesis. On the contrary, he imagined a scenario where subterranean fire acted on air or water, producing hot vapors that were transmitted from Mount Etna to the mainland through underground passages, thus causing earthquakes. Quite intriguingly, and unconsciously, Dolomieu attacked fierily a

. . .species of half-scholars, who have added to their relations the most singular and contradictory circumstances; because they wanted to attribute to the current earthquakes all the phenomena of which they had some notions and which they knew to have happened during similar events. Besides, most of them had a small system to support, and they wanted to arrange the facts, to fit them into the framework they had prepared for them in advance. ⁴⁸

These were words that may well have described the Frenchman himself, according to many.

Many reports about the Calabrian earthquakes appeared in 1783 (see the CFTI5Med catalogue; https://storing.ingv.it/cfti/cfti5/), but the “Italian” point of view was fully acquired by foreign scholars mainly thanks to two academic memoirs, both published in Naples. The first one was written by Michele Sarconi, who led the special investigation commission of the Naples Reale Accademia (where Sarconi was Secretary), and documented very accurately the damage caused by the earthquakes. ⁴⁹ It provided a well-balanced technical report, showing no preference for any one hypothesis over another, although undoubtedly the Italian angle was (even unconsciously) present throughout the work. ⁵⁰ The second relevant Neapolitan work was quite different, since its author, Giovanni Vivenzio, explicitly supported the idea that earthquakes were due to subterranean electric discharges or thunderstorms. ⁵¹

However, the “definitive” proof that something like underground thunderstorms manifested during the Calabrian earthquakes came by another scholar:

D. Niccola Zupo from Cosenza, physician and a very good physicist, who was convinced that the cause of the earthquakes was to be electricity, dug a twelve-palm-long iron rod into the earth, and observed, at the occurrence of many shocks, a brush of electric fire from the pointed end protruding out of the earth. This true, and undisputed remark was entirely new in its kind, and had never been made by any other observer in similar circumstances. ⁵²

What Zupo devised was a pendulum-type seismoscope designed to detect both electric and seismic effects simultaneously. ⁵³ The overground part of the rod made two right-angled bends, so that one short section was parallel to the ground, with a suspended small lead ball and with a nail protruding from the lower part of the ball. The vertical rod served (according to its creator) as a lightning conductor, while the marks made by the nail in a layer of fine ashes, arranged in the ground below, recorded the possible movements of the system, both possibly caused by earthquakes. Such a system was not the first “seismoscope” invented, but the Calabrian earthquakes greatly stimulated the construction of these instruments based on pendulum devices. ⁵⁴

Later on, Vivenzio wrote another, more expanded version of his memoir, probably due to the success of his 1783 Istoria, and scholars both from Italy and abroad showed a positive but sometimes also critical interest. Vivenzio replied to Dolomieu’s arguments in his two-volume Istoria de’ tremuoti from 1788. ⁵⁵ He first replaced the previous Giornale tremuotico with a more detailed one. It was written by a learned scholar, Domenico Pignataro, and the whole second volume was dedicated to reporting the intensity, duration, and effects of the various quakes, as well as the relevant “atmospheric” information, for the events that took place from 1783 to 1787. All this phenomenology aimed at rebutting Dolomieu’s criticism, particularly in the first volume, by following the traditional lines of argument. Further observations that obviously supported the electric hypothesis against the Frenchman’s arguments were reported and widely discussed, but here Vivenzio appealed (without mentioning him) to the thorough work by another Neapolitan scholar, who had been left in the shadow.

A mysteriously unknown account to the Royal Society

Though foreign (non-Italian) scholars greatly benefited from the first published account that Hamilton sent to the Royal Society, it was not the first report produced for scholars abroad, nor even the first one sent to the Royal Society in London.

We have recently retrieved from the archives of the Society a manuscript by Giuseppe Saverio Poli that reports the Calabrian earthquakes in detail. ⁵⁶ It was sent from Naples on May 13, 1783 (ten days before Hamilton’s report), to a friend of Poli’s, Joseph Banks, President of the Royal Society, asking him to read it in front of the Society, where Poli was a home member. Nonetheless, for some reason, the paper was neither published nor read before the fellows, contrary to how Hamilton’s report was treated, so it has remained unknown until now. ⁵⁷

Poli’s paper, “A Short and Faithful Account of the Earthquakes Lately Happen’d in the Province of Calabria Ultra,” ⁵⁸ was written by taking “all the particulars not only from a great number of letters written from those who were present to the distressful misfortune, but also from the authentic papers sent to his Majesty by the Governors of that Province.” In particular, Poli, who was a regular attendant of the Royal court, got to have access to the detailed Report of the Vicar General Francesco Pignatelli, which he used as his primary source of information, though not the only one. ⁵⁹

As a matter of fact, the second part of “A Particular Prospect of the Most Remarkable Effects Occasion’d by the Said Earthquakes” contained more detailed descriptions of what had happened in the different Calabrian sites; in Poli’s report, we also find several scientific observations that proved to be very useful in the interpretation of these phenomena, and that only later appeared in Vivenzio’s memoirs.

The first, shorter part of Poli’s paper was more original and interesting, especially from a scientific point of view. It accounts for about a third of the whole manuscript and, given the way it begins, we might be missing the first pages. ⁶⁰ The second, longer one instead aimed at giving “a full and satisfactory account of the ruins, distressful consequences, and revolutions occasion’d by the horrid earthquakes abovemention’d,” just as in Hamilton’s account, but with a more circumstantial description of those effects. Although Poli made a reasoned selection of the facts he presented, this part was compiled by following quite closely Pignatelli’s official Report (and personal letters from a few other correspondents); it is, therefore, quite derivative.

Right from the very beginning of the manuscript, Poli was concerned with providing some connection between what happened in the “earth” with what appeared in the “air.” The “proven” sequence of events that was repeatedly observed during the different earthquake shocks, from February to April 1783, was the appearance of a thick fog, followed by heavy rain, right before the quake. This sequence, however, did not explicitly occur in all the events, but, as the author points out, in such cases the shock “constantly” produced a change in the atmospheric conditions that finally led to fog and rain.

Similar phenomenology was reported in Count Ippolito’s letter to Hamilton, but it is evident that the Englishman’s correspondent was not so accurate either in describing the effects observed or in appropriately interpreting them within a consistent scheme. For instance, he observed clusters of clouds and assumed that the atmosphere was somewhat negatively electrified, attracted by the positive electric state of the Earth. The shock that occurred right after rainfall, then, discharged the negative excess from the air to the Earth, by virtue of the water drops forming a sort of connecting conductor. Poli maintained that the effect was further increased by the fact that the previous summer had been very dry and had prevented electricity from flowing because of the absence of humidity; this delayed electric equilibrium between air and earth, and finally provoked disastrous shocks. All this was confirmed by a further piece of information, whose reporting was probably inspired by Beccaria’s suggestions: “Besides, it has been observed very frequently during these three months, a kind of a faint, and small aurora borealis over different points at the horizon. The same is to be understood regarding thunders, and lightnings, that have been very frequent during the said showers.” All this “electric” phenomenology acquired during the multiple Calabrian earthquakes was, of course, neither highlighted by Hamilton nor (unexpectedly) by Vivenzio in his 1783 Istoria, probably because he was unaware of it. ⁶¹

In contrast to “electricists” (like Vivenzio) and “firists” (like Dolomieu), Poli was convinced that all appropriate phenomenological observations had to be considered for a complete interpretation of what happened, including some “inflammation of the volcanic matter.” The second part of Poli’s account thoroughly reported several observations supporting a conclusion that was – at least in part – considered by Hamilton and others; he also explicitly considered the opposing thesis centered on the volcanic origin of the earthquakes. His conclusion on the subject was based solely on the results of empirical observations and their analysis, and this was the most “advanced” view at the end of the eighteenth century. ⁶² Indeed, in the absence of solid observational arguments regarding the origin of earthquakes that will emerge only much later, scholars continued to pursue specific theses, largely motivated by personal beliefs – albeit based on (partial) experimental evidence – even when the interest in the “electric hypothesis” (or in some other hypothesis) effectively waned in the early nineteenth century.

The ultimate St. Anne earthquake in Poli’s Memoria sul tremuoto

Preceded by some shocks on the day before, and followed by aftershocks that lasted until mid-1806, on July 26, 1805 – the day dedicated by Catholics to the devotion of St. Anne – a destructive earthquake hit the Molise and Campania regions, though it was felt by the whole of Southern Italy. The maximum damage was in the province of Isernia in Molise, where the intensity reached the value I₀=XI on the MCS scale (and an estimated Richter magnitude of M_W=6.7/6.8), causing considerable geological and hydrogeological devastation, with turbidity, variations of the water level, fractures, landslides, and soil liquefaction. ⁶³ Although some other original documents are now available, as reported in the CFTI5Med catalogue, the only source for such data was Poli’s Memoria sul tremuoto. ⁶⁴ Here we find, for example, the precise timing of the event (21h 1m 40s GMT) according to accurate astronomical observatory pendulums. Poli was a direct witness of what happened in Naples, but he acquired useful and detailed information both from the General Superintendent of Police, the Duke of Ascoli, and from data collected from various eyewitnesses (including, for example, the Duke della Torre).

By this time, Poli’s Elementi di Fisica Sperimentale was largely considered as the most authoritative and widespread general physics textbook in Italy. ⁶⁵ His own approach to the explanation of earthquakes was now widely accepted (although the influential works by Vivenzio and others were still taken into account), even before the appearance of Poli’s memoir. ⁶⁶ As a matter of fact, the Memoria sul tremuoto became virtually the only reference work on the subject, for better or for worse. It is interesting to note that even in the quite argumentative Ragguaglio Istorico-Fisico published by Gabriele Pepe (a man of letters and a revolutionary), the author conformed to Poli’s “scientific” approach and scheme of reasoning, despite his attempt at a meticulous rebuttal of Poli's arguments. ⁶⁷ Pepe’s on-site inspection produced “more certain” and detailed data than Poli, so that he was able to correct some inaccuracies and deductions drawn from these. However, the alleged different approach was just an analysis of different data carried out using the same method as Poli: “all Moderns have turned to the electric principle to recognize the true cause of [earthquakes] [. . .]. The position of such a principle was sealed by the exact correspondence of all the facts and by the unanimous explanation of all the effects that manifest themselves simultaneously with the quake.” ⁶⁸

It is quite notable that, while the (unpublished) Account of 1783 was essentially written by a meticulous researcher for other researchers, in Poli’s Memoria of 1806 he acts as an enthusiast who certainly wants to interpret what had happened but at the same time wants to explain it to the attentive reader (first of all to his former pupil, the Crown Prince, to whom the book was dedicated), both for education and for practical use. The honest style of the successful Elementi guided this work, which can be considered a treatise on the subject, providing different answers to different needs.

Poli’s Memoria started with a list of the general warning signs of an earthquake, which he originally divided into four distinct classes: remote signs (such as extraordinarily rainy or dry seasons, heavy rains after long dryness, etc.), proximate signs (northern lights, ignes fatui, etc., as well as turbidity of well water, its unusual taste, smell of sulfur, etc.), imminent signs (sudden lowering or elevation of the waters, their heating, igneous meteors peeling from earth, shivering or bellowing underground, escape of animals), and immediate signs (sudden wind and its cessation, a roar that resembles hissing, thunder, or drums). Only after that did the author report what happened in Naples on that fatal day. A large number of extraordinary effects were collected by thoroughly investigating among the population of the capital and its surroundings, which resulted in descriptions of momentary rays of light that shot up from the tops of the buildings, or an underground lightning bolt coming from the center of Lake Patria, as well as “a current of electric fire flowing through the iron hook to which a rope was attached, that carried two copper buckets to draw water from a well.” All this phenomenology was evidently reminiscent of what Poli himself had observed in quite different situations, as described earlier in this paper, which had nothing to do with earthquakes but only with genuine atmospheric electricism. Electric science was increasingly shaping earthquake science.

Here, Poli accurately describes the propagation of the seismic event and its related phenomenology in distant places (and different times), showing a different speed of propagation in air as compared to earth. This is particularly evident in the precise timing of the earthquake in different sites: in Rome it occurred 1 minute and 13 seconds later than in Naples, where it happened 4 minutes and 54 seconds later than in Molise, with the immediate conclusion that this earthquake was “progressive.” On the other hand, six days later, a dazzling light in the shape of a fire globe, which subsequently “cracked and fell down in innumerable sparks resembling a shower of gold,” was observed in Naples and along the Amalfi coast and even in Puglia, “on that same day, and at the same hour, from which we infer the great height at which it was generated.” Here, the striking features mentioned in the description served to highlight the simultaneity of the event, showing how far the electric connection between earth and sky could extend in height. ⁶⁹

The description of the more extended earthquake in Molise followed the same lines as that concerning Naples, with the same emphasis on the same kinds of electric and atmospheric effects. Though presenting some interesting considerations and descriptions, this part is certainly less original, as it is based on (selected) second-hand testimonies.

As soon as the news of the earthquake crossed the Italian borders, foreign scholars associated the event with a volcanic rather than electric cause, especially given the fact that the land of Molise was known to be full of extinct volcanoes. ⁷⁰ The most obvious sign, however, came several weeks after the seismic event, when the dormant Vesuvius awoke violently. For many, it was not easy to rule out some connection between an active volcano such as Vesuvius and the St. Anne earthquake, whose strong effects were also felt in Naples. Poli’s approach was just that, but without starting from prejudicial assumptions.

In his Memoria sul tremuoto, the Neapolitan scholar analyzed in detail the general facts supporting a volcanic origin of at least some earthquakes, concluding that they “can be produced by the violence of the volcanic fires.” Then, he considered whether and how water and/or underground air may cause severe quakes. Fire, water, and air were considered concurrent agents by the supporters of the firist hypothesis, because these general arguments were commonly thought to be related to volcanic ones. But here Poli provided distinctive empirical evidence that enough heat could generate strong pressures through the thermal expansion of subterranean vapors (air or evaporated water). With an evident didactic aim, he went as far as comparing underground explosions to mine explosions, because Nature could provide the required “ingredients” for analogous explosions (see “Balancing the Earth with the Heavens”). However, evaluating the relative weight of the different explanations, only one stood out: “in spite of such reflections, however, it is quite certain that most of the large and vast earthquakes are caused by electric fluid.”

Unlike what was commonly thought of the Calabrian disastrous event, however, here the electric cause was considered not to be one among the others – though the most powerful – but the one that gave way to the initial shock, which then propagated with the aid of concurrent agents, including combustible gases.

Inflammable air and a comprehensive explanation

Thanks to his empirical observations, Poli saw that, together with electricity, hydrogen gas was present during the Molise earthquake in many different regions. This reminded the author of the Elementi di Fisica Sperimentale, in which phenomena such as aurorae boreali, ignes fatui, shining sparks, and meteors were attributed to the ignition of the hydrogen gas in the atmosphere caused by the electric fluid; therefore, the idea that the earthquake could have been generated from the combined effects of these two agents was not that odd at all. This theory was confirmed by the more evident effect of what (before the introduction of Lavoisier’s nomenclature) was called inflammable air, that is, the explosion of hydrogen gas.

A thorough inspection of the available phenomenology, which the readers of the time could find in Poli’s Elementi, ⁷¹ revealed that hydrogen gas developed in large quantities in underground caves and in the deepest mines (where it could frequently ignite unexpectedly), but also in sewers and in the bowels of volcanoes, as annotated in the 1806 Memoria. A further indication in favor of this theory was the fact that the passage of the electric fluid could cause the decomposition of water into its components, as well as the inverse process, and that enabled the underground accumulation of hydrogen, or any other substance believed to cause earthquakes.

With all these data available, Poli developed his explanation of the origin of the Molise earthquake in even more detail.

Let us suppose, therefore, [. . .] the underground caves filled with hydrogen gas up to the convenient depth, and for a very long extension of the country [. . .]; and on the other hand, consider an enormous amount of electric fluid accumulated at a much greater depth in the womb of the Earth under the said districts, and hindered as it is between insulating materials. ⁷²

This was a traditional assumption for the supporters of the electric hypothesis, who obviously admitted the existence of a large amount of electricity in the Earth, the diffusion of which toward sites with a shortage of electric fluid could lead to a violent shock when “invincible obstacles” such as insulating bodies were met. Here, however, Poli pointed out that the same assumption was directly suggested by observations, that is, by the lack or scarcity of igneous meteors in the season immediately preceding the quake.

The intervention of a sudden, anomalous increase of the observed heat in the days right before July 26 induced – according to Poli – a remarkable thermal expansion, capable of releasing the hydrogen gas entrapped in the caves next to the Earth’s surface. Even small quantities of electric fluid nearby caused the ignition of that gas, allowing the formation of “fiery meteors” lifting high in the appearance of those strange phenomena such as northern lights or ignes fatui, as observed by eyewitnesses.

These explanations (or, rather, suggestions) concerning specific facts, empirically deduced, drove Poli to go even further in his arguments presented in the 1806 Memoria, far exceeding what he had argued in the 1783 Account for the Calabrian earthquake. Now, he focused first on other well-documented observations, such as the phenomena of the sea level rise in the Gulf of Naples and along the coast up to Gaeta, which was also associated with some “shaking” felt by the ships anchored there, like collisions against a rock. According to Poli, the electric fluid that began to emanate from the Earth spread into its waters as soon as it reached the bottom of the sea, thus electrifying the water particles. Due to the electric repulsion experienced by these, the sea water swelled at the same time as the earthquake developed, producing the observed effects, “until the electric fluid could make its way freely into the atmosphere.” ⁷³ Of course, the same argument applied to general changes in the water level in wells and cisterns.

Then he continued studying the dynamics resulting from the diffusion of the electric fluid, and this led to other predictions, which in his view were confirmed by the observations he made as an electricist scholar.

As it was not possible to overcome the resistance provided by the innermost and deeper mass of the Earth, the electric fluid had to push its way toward the Earth’s surface (which offered less resistance). First, it spread throughout underground caves, circulating for some time there, “shaking and shaking without restraint the natural vaults of the caves themselves, as well as the earth mass, the cities, the villas, the buildings, and the other places superimposed on them.” ⁷⁴ According to Poli, this explained the different manifestations of the earthquake, with an initial (vertical) jolting motion followed by a shaking one, produced by the said subterranean circulation, exactly as felt by people in the areas affected. Moreover, since those caves were supposed to be full of hydrogen gas, the approaching of the electric fluid caused its ignition, which on one hand caused an increase in the violence of the earthquake, and on the other produced “that internal bellow, that rumble, and that dark and deep thunder, which have been heard in many places.” ⁷⁵

There was hydrogen induced by thermal expansion, which diffused high into the atmosphere, and other gases which came out of fractures in the earth, caused by the attempt of the electric fluid to find its way to the surface. A part of this was ignited by the same electric fluid, which caused repeated phenomena of spontaneous ignitions, while the remaining part spread in the atmosphere, increasing the striking phenomenology of fireballs, meteors, and so forth. Also, it was probable that such further escaping of hydrogen could produce the thunderous roar that preceded the earthquake in several places, and which greatly impressed the unfortunate spectators.

Poli noted that these explanations could be applied not only to the area of “the central site of the formidable outbreak,” identified (according to the damage caused) in Molise, but also in other distant places where the seismic event was felt. Their direct or indirect application could be realized from the intensity of the phenomenology experienced. It was precisely his direct observation in Naples (and its surroundings) that allowed him to connect the St. Anne earthquake to the subsequent eruption of Vesuvius. Indeed, it was easy to assume that the powerful action of both the electric fluid and the hydrogen gas excited the quiet (but active) volcano, and then caused its sudden eruption. According to this perspective, the eruption of Mt. Vesuvius was induced by the Molise earthquake, and not the other way around, as indicated by the fact that the sites close to Vesuvius were less affected by the earthquake than Naples and other distant places.

Therefore, Poli finally succeeded in refining the whole picture by providing a consistent explanation of the complex and rich phenomenology observed in terms of the electric hypothesis, complemented by the action of the large amounts of hydrogen gas supposed to exist in the bowels of the Earth.

Discussion and conclusions

Strange as it may seem today, the electric hypothesis that developed in the second half of the eighteenth century enjoyed the same prestige as the competing firist hypothesis. On the one hand it had become fashionable to focus on ubiquitous electric phenomena, on the other hand all existing theories explaining the causes of earthquakes were based just on empirical reasoning. As a matter of fact, even in the following century, some perception of the connection between volcanism and seismic activity had been grasped, and only in the second half of the twentieth century was it possible to fully understand the elements of this complex puzzle.

Attempts to explain earthquakes through electricity were largely abandoned early in the nineteenth century, probably because of the loss of interest in the static electrical phenomena traditionally associated, in the laboratory, with the Leyden jar; a new interest had emerged in favor of Volta’s battery. Nevertheless, the study of the inspiring connections between electricity and earthquakes is still relevant today. ⁷⁶ For this reason, it has been of some interest to delve here into the various aspects of this historical case.

We have studied it by following a very particular guideline. In contrast to existing analyses, we have closely followed the development and subsequent applications of the electric hypothesis in Italy and, specifically, in the Kingdom of Naples, which was well known for its remarkable geological activity, and which hosted – among other things – the two major catastrophic events of the Calabrian earthquake of 1783 and the St. Anne earthquake of 1805. At that time, Italy was also the homeland of the major supporters of the electric hypothesis, thanks to a flourishing interest in studies on electricity and its applications such as those made by Father Beccaria, Tiberius Cavallo, and several others. These seminal works remained powerful in all subsequent studies on the electric origin of earthquakes, and they helped Italian scholars interpret the rich phenomenology of those seismic events. Most of these scholars, however, did not break away from this influence, and their derivative works well justify what is usually reported in the literature, that is, the rapid disappearance of the electric theory of earthquakes. ⁷⁷

Nevertheless, the situation was quite different with Giuseppe Saverio Poli, whose influence on his contemporaries was itself notable, with his work extending well beyond the beginning of the nineteenth century. Poli did so much more than just providing accurate data on certain phenomena; the development and evolution of Poli’s scientific reasoning on the origin of earthquakes, discussed here, has shown how his work was rather aimed at building a scientific theory – albeit still only an empirical one – that would explain the onset and development of earthquakes. Poli’s work is quite illustrative of a particularly relevant historical case study in Italy, where electrical science shaped earthquake science. It provided both an explanatory and interpretative framework and devised “appropriate” instruments to provide useful data about earthquakes, though with the final ambitious intent of preventing possible destructive effects. On the one hand, the Calabrian earthquakes of 1783 in particular boosted (for example) the development of pendulum-type seismoscopes, like the one devised by the Italian Nicola Zupo, which detected both electric and seismic effects. On the other hand, the collecting of precursory signs (suitably classified) inspired by atmospheric electricism, in addition to damage-related data, allowed a better definition of the investigative method of earthquakes’ origin and propagation, which established Poli’s Memoria sul tremuoto about the St. Anne earthquake of 1805 as a reference work.

In the light of this, what we have presented here, including the reconstruction of the whole path that led Poli from his initial studies on electricism (and atmospheric electricism), which made him well known to the international academic world of the second half of the eighteenth century, and his Memoria sul tremuoto, which can be considered as the climax of his geological studies, seems noteworthy. On such a path, a significant place is taken by a manuscript that for some reason has remained unpublished, written by Poli for the Royal Society on the 1783 Calabrian earthquake, where his distinctive method of scientific argumentation clearly emerged. While significantly influenced by Beccaria, Cavallo, and others, Poli revealed his original approach toward a comprehensive explanation of the given facts that would account for all the relevant phenomenology, even if not entirely connected to the electric hypothesis. This was, to a certain extent, a diversion from the attitudes of his contemporaries (whether they supported the electric hypothesis or not). His work was not merely conciliatory of the two opposing positions (firists and electricists), but also aimed at finding a “truer” theory. This emerged magnificently in his final Memoria sul tremuoto, where even the newly acquired phenomenology about inflammable air found its rightful place in Poli’s reasoning.

Unfortunately, Poli was not able to go beyond pure empiricism, since he stopped at a mere explanatory theory – though a beautiful one. Indeed, even if the subject hardly allowed the construction of a complete theory of earthquakes (which scientists would have to wait more than a century for), Poli still effectively held to the typical eighteenth-century style of studying electric phenomena, as is clear in his use of the term “electricism.” As clearly emerges from his most famous textbook, Elementi di Fisica Sperimentale, Poli did not acknowledge the lessons of people like Cavendish and Coulomb, according to which explanatory models were not intended as descriptions of reality, but rather as a means to assist with the mathematization of Nature. The shift from Enlightenment ideals to Romantic conceptions, which emphasized the unity of the natural world, settled this perspective on the electric hypothesis (but the same may be said also of the volcanic or the firist approach). This provides a possible explanation for its rapid decline, together with the incoming change of perspective regarding studies on electricity at the start of the new century.

Poli’s work (and especially his approach) has left a substantial mark on subsequent Italian science, as after almost a century the search for possible connections between earthquakes and electric (and magnetic) phenomena has not stopped. ⁷⁸ Above all, however, it is quite remarkable to note how the electric hypothesis about the origin of earthquakes could contribute to the history of geology to a degree not usually appreciated in the literature. This is a wonderful example of how even approaches and results that later turn out to be wrong lead to remarkable increases in scientific progress. The electric earthquake was certainly one of these historical cases.