• Measuring a potato planet, by Ian Hembrow

    Research grant recipient Ian Hembrow reports on a fascinating study trip to learn about 18th century surveying methods from SIS’s Nicolàs de Hilster.

    At the start of 2022 I was thrilled to receive a grant from the Society to support my project to write a biography of the Swedish scientist Anders Celsius (1701-44). Best known of course for his centigrade temperature scale, Celsius made an array of other discoveries and breakthroughs in a stellar career cut tragically short by his death from tuberculosis aged just forty two.

    Figure 1. Measuring one degree of latitude in the Arctic Circle, 1736-37.

    In particular, he played a vital role in settling the great scientific debate of his age about the shape of the Earth, taking part in a perilous winter expedition to the Arctic Circle in 1736-37 to measure the exact distance of one degree of latitude (see figure 1). Comparing the results with a parallel expedition to the equator proved Newton’s theory that the planet is an oblate ellipsoid, slightly flattened at its poles by the centripetal force of its spin. The Earth, they showed, is shaped like an orange – roughly 43km broader than it is tall.

    Keen to find out how Celsius and his companions made their measurements I reached out to the SIS network for help to understand the methods and equipment they used in the frozen north. Surveyor and collector Nicolàs de Hilster answered my call, inviting me to visit him in the Netherlands to do some hands-on study. It turned out to be an enthralling and memorable trip.

    Figure 2: Nicolàs lines up for a viewing of the sun.

    When I entered Nicolàs’ home, a short train ride west from Amsterdam, I noticed a laptop with what appeared to be an image of the sun surrounded by tables of flickering data. Some sort of astronomical website I thought. But no, this was actually the live-feed from Nicolàs’s own observatory perched on top of the house! I followed my host up the spiral stairs to discover his amazing creation: a cluster of powerful telescopes mounted within a beautiful wooden dome – all hand-crafted from scratch by Nicolàs (see figure 2). The instruments included a custom-made Galilean-type refractor intentionally made to replicate the imperfections of 17th century lenses.

    Nicolàs attached a special filter to one of telescopes and invited me to look directly at the sun. Unsurprisingly, I saw a bright, fiery orange ball. “Now adjust that knob and you’ll be able to see some solar flares,” he advised. And sure enough, as I fine-tuned the focus, little looping swirls appeared around the edge of the image, clearly moving in real time.

    “Wow!” I said. “How big are they?” “About five times the size of Earth,” replied Nicolàs. I was stunned and nearly moved to tears by the magnificence of what I was seeing.

    The observatory though was simply the tip of a very big iceberg. I went on to admire an astounding collection of surveying instruments and devices – from the simplest sighting sticks to massive, high-specification theodolites, an enormous library and two gorgeous bespoke ‘de Hilster’ sun-dials mounted on the walls outside.

    Figure 3: ‘Touring the horizon’ with a modern theodolite.

    After a fine lunch kindly prepared and served by Nicolàs’s wife Ria, we loaded up the truck with a collection of sextants and measuring rods and headed out into the polder nearby – flat countryside criss-crossed by drainage ditches giving way to sand dunes up towards the nearby North Sea coast. Ever prepared, Nicolàs had found a spot where we could ‘tour the horizon’, first using a modern ‘total station’ instrument (see figure 3) and then a handheld, brass sextant.

    Figure 4: Back to older technology with a sextant.

    My heart sank when it was explained to me that geodetic sextants use the 400-point Gradian system, rather than the more familiar (to me at least) 360 degrees of geometry. But I needn’t have worried, because once I’d got the hang of how to use the heavy brass device – merging the direct and mirrored images of adjacent sighting points like church spires and telephone masts – I managed to take and call out full-circle readings from our location (see figure 4). Nicolàs noted down the numbers at each point and then pulled out his phone to check their accuracy against the digital results.

    Figure 5: Student and teacher: Ian (L) and Nicolàs (R).

    Thanks to the expert tuition and supervision, my readings added up to 399.49 grad – just 0.51 of a grad out. Nicolàs smiled and declared this was “acceptable for a first attempt,” before going on to explain how even a small difference like this soon multiplies up to a huge margin of error when applied over the sorts of distances Celsius had to measure.

    Back at the house, Nicolàs then showed me the inherent difficulties of using measuring rods to gauge accurate distances. The 1736-37 expedition members used thirty-foot-long wooden poles laid continuously end-to-end on a frozen river surface. But even on the flat floor of a modern building the kinks and dips of a twelve-foot sectioned rod were obvious to the naked eye. Our planet is of course not entirely smooth, so surveyors have to take account of all the slumps and bumps in their way. “Remember Ian,” said Nicolàs, “the Earth is not a true ellipsoid – it’s a potato!”

    The key to surveying accuracy, I learned, is to measure, measure and measure again – using multiple base lines and repeated calculations to obtain the best possible answer. And, Nicolàs reminded me, measuring the linear distance was only the start of the job – Celsius then had to do some complex astronomical sightings to determine their precise latitude.

    That will have to wait for another time and visit…

    My sincere thanks to Nicolàs and Ria for making me so welcome and treating me to such an extraordinary day.


    In early May 2022 Ian is retracing the route of the 1736-37 expedition in Swedish and Finnish Lapland. Watch out for his report of that trip.



  • A Measurement of the Weight of the World: Then and Now, by Carolyn Kennett, FRAS

    Descending ladders into the mine to seek a suitable sub-surface location to run the experiment to measure the density of the Earth. Photograph credit: © Carolyn Kennett, 2021

    How did we weigh the Earth (and why did this go beyond simple curiosity)? This may be a question people asked themselves during childhood, and have not considered since. Yet it is a question a small group of scientists, including myself, have returned to as we research experiments conducted in the 1820s in a Cornish mine to measure the acceleration due to gravity of the Earth.

    In 2022 it is our intention to re-create the mine experiments by building a replica Kater invariable pendulum and taking it down a Victorian mine in west Cornwall to make measurements of gravity. We will set the pendulum in two locations, one overground and one underground, and time the swing of the pendulum in both locations. The difference in the rate allows us to calculate the amount of gravitational pull on the pendulum, as the underground pendulum will swing at a slower rate. The original experiment was conducted by George Biddell Airy and William Whewell in the deepest mine in England, Dolcoath. This has unfortunately closed and the lower recesses are flooded, so we are using a mine named Rosevale, which gives us a difference of 250 metres between the overground and underground stations. Although Rosevale is not as deep as Dolcoath (700 metres at the time of the original experiment), it gives the opportunity to explore how the experiment was conducted in what can only be described as less than ideal conditions. Mines are dirty places which can be excessively damp and hot. During the original experiment the scientists would have had to contend with vibrations and noise from the working environment, making their achievements all the more significant.

    Why is this all important now you may ask? Yes, simple curiosity does play into this but we find ourselves in a time when the power of gravity is something we have learnt to manipulate and overcome. There are frequent launches into space and discussions of journeys to far-flung destinations such as Mars. Without the arduous and at times dangerous early experimentations into measuring the gravity of the Earth untaken by Airy, Whewell and others we could still be stuck without the knowledge to reach beyond our own planet. Therefore we think it is the perfect time to highlight the work they undertook and their achievements in what was an important building block for us to travel into space. Details of the actual experiment itself, and the results obtained, will be published in a forthcoming issue of the Society’s Bulletin.

    For more information see: https://archaeoastronomycornwall.com/

    This experiment has been part-funded by a SIS grant. For more information on the grants we offer, click here.



  • Abbot and Inventor: Giovanni Caselli, the Pantelegraph, and Catholic Science in the 19th Century, by Carlo Bovolo, Università di Torino

    Figure 1. Giovanni Caselli’s Pantelegraph (image courtesy of Biblioteca di Fisica e Astronomia of the University of Padua)

    In 1860 the famous composer Gioacchino Rossini sent an autographed text from Paris to Amiens, a distance of about 140 km, in only a few minutes. To celebrate such an important event, the musician wrote a piece for piano, the ‘Allegretto del pantelegrafo’. What made the incredibly fast transmission possible was the invention of an Italian priest: in the middle of the nineteenth century, the abbot Giovanni Caselli (1815–1891) had created the pantelegraph (a portmanteau of the words pantograph and telegraph – Figure 1), which, anticipating the facsimile machine, was able to send handwritten texts and pictures by telegraph.

    Caselli, a priest from Siena devoted to physics, realized a prototype of the pantelegraph in Florence in 1856, a technology which mixed telegraphy and electrochemistry and whose operation was based on a regulating clock. Searching for funding, he then moved to Paris, where he improved his instrument thanks to Paule-Gustave Froment and attracted the attention of the public and of Napoleon III.

    The French government funded further research and experiments, including Rossini’s transmission in 1860. The experiments were successful: in 1864 the French government officially adopted the pantelegraph, establishing lines between Paris, Lyon, and Marseille. Other short lines were built in England, Russia, and China.

    At the Italian national exhibition in Florence in 1861 and, above all, at the Universal Exhibition in Paris in 1867, the pantelegraph reached the pinnacle of its success: public demonstrations were made, newspapers described its functioning and extolled its advantages, and Caselli was celebrated as a brilliant inventor. However, its success – though intense – was short. Due to the complexity of the technology, its costs, and an incomplete understanding of its potential, the pantelegraphic network remained very limited and was dismantled in subsequent years. Caselli (inventor also of a cinemograph for measuring the speed of trains, a nautical electric torpedo, and a hydromagnetic rudder) returned to Italy, where he died in Florence in 1891.

    Despite the parable of the pantelegraph, Caselli became a hero of the Catholic apologetics of science: in the apologetical and propagandistic discourse about science and technology proposed by the Catholic press in the second half of nineteenth century, the inventor-priest was a symbol of the Catholic contribution to scientific and technological progress, represented a rebuttal to accusations of obscurantism against the Church, and laid claim to a public role for Catholicism in science.

    Carlo’s research was funded by a 2020 SIS Grant. A longer piece on this subject will feature in a future SIS Bulletin.



  • Poleni’s geometrical instruments to trace transcendental curves, by Pietro Milici, University of Insubria (Italy)

    Figure 1. Poleni’s machines for the tractrix (left) and the logarithmic curve (right) in the Letter to Hermann.

    During the 17th century, mathematicians radically modified their idea of a curve, transitioning from the trace of a geometrical mechanism to the solution of an equation. With that in mind, Descartes and Leibniz introduced theoretical machines (i.e., sketches to be considered mentally but not practically realized) to legitimate the geometrical status of algebraic and transcendental curves obtained as solutions of analytical equations. In the first half of the 18th century, a few scholars converted some theoretical machines for transcendental curves into material instruments. We focus on the geometric instruments designed by Giovanni Poleni (1683-1761), a polymath and Professor at the University of Padua in Italy. These instruments had been described with an engineer’s precision both in textual and graphical representations (as visible in Figure 1) in a letter to Jacob Hermann who was Poleni’s predecessor in the chair of Mathematics. The letter was included in Poleni’s Epistolarum Mathematicarum Fasciculus of 1729.

    In 1739, Poleni inaugurated a unanimously praised laboratory of experimental physics (“Teatro di Filosofia Sperimentale“) that grew also with his successors. Such a Paduan museum still exists and is dedicated to its founder (today it’s named “Giovanni Poleni museum”). Between the machines present in the collection, Poleni listed the ones for the tractrix and the logarithmic curve: also the modern catalogue (Gian Antonio Salandin & Maria Pancino, Il “teatro” di filosofia sperimentale di Giovanni Poleni, LINT, 1987) confirms that there is a sample of a geometrical machine for our transcendental curves. Thanks to an SIS grant, I got the opportunity to observe, manipulate and film such a machine (see Figure 2).

    Figure 2. The machine of the Giovanni Poleni Museum classified as the one for the tractrix and the logarithmic curve.

    Indeed, differences between the artefact in the collection and the designs of the Letter are visible at the first glance. Furthermore, reading the description in Poleni’s index, the machine also differs in materials. In any case, also because of missing pieces, we have no idea of the purposes of such an artefact.

    To our knowledge, no copy of Poleni’s geometrical machines is available: therefore, together with Frédérique Plantevin (University of Brest, France), we decided to reconstruct the machines meticulously described in the Letter to Hermann. After a 3D digital modelling of the machine, we contacted the artisan Uri Tuchman to realize a working model in the materials described by Poleni. We are still working to complete an informative video and a paper on these topics.



  • The use of astronomical instruments in Colonial Chile, by Virginia Iommi Echeverría, Instituto de Historia, Pontificia Universidad Católica de Valparaíso (Chile)

    The use of astronomical instruments in Chile during the colonial period (1540-1810) remains an obscure subject. Despite a few references in contemporary sources to glasses and armillary spheres, very little is known about the instruments themselves and how they were used. Although the introduction of the printing press in Chile has been dated to near the end of the eighteenth century, the existence of numerous libraries and an active book market by then show the importance of considering bookish astronomy as a fundamental part in this historical task, for not only did books provide theoretical insights into the discipline but also astronomical tables, models and visual instructions for the fabricating instruments. The purpose of this project was to examine these paper instruments as surviving testimonies of astronomical study and practice.

    Figure 1: The second volume of Tabulae Primi Mobilis by Andrea Argoli (1570-1657)

    Thanks to an SIS grant, I was able to examine astronomical books printed between the sixteenth and eighteenth centuries preserved at Biblioteca Nacional de Chile in Santiago. One of the founding collections of this library was the books owned by the Jesuits at the moment of their expulsion from the territories of the Spanish crown in 1767, when their possessions were requisitioned and inventoried in detail. The catalogues of belongings found in churches, houses, missions, and colleges show that the members of the order owned several books on astronomy and mathematics. The largest book collection in eighteenth-century Chile was located at the Colegio de San Miguel in Santiago, where more than 6,000 volumes were preserved. By comparing the titles mentioned in the inventories with the present holdings at the Biblioteca Nacional, it was possible to conclude that most of their astronomical books are now lost. One of the few exceptions is the copy of the second volume of Tabulae Primi Mobilis by Andrea Argoli (1570-1657), which is clearly identified as a possession of the Colegio de San Miguel with a handwritten inscription in the frontispiece (see figure 1).

    Figure 2: Giovanni Paolo Galluci’s Theatro del mundo y del tiempo

    In the inventory, it is described as “Argoli Tabula” (Archivo Nacional de Chile, Fondo Jesuitas, 1767 Vol. 7, fol. 311r). Although not mentioned in the catalogue, a copy of Giovanni Paolo Galluci’s Theatro del mundo y del tiempo contains signs of use that may be helpful for studying astronomical practice in the period (see figure 2). The copy in Biblioteca Nacional belonged to the Jesuit house of San Juan and has numerous handwritten annotations and marks. This research has confirmed that despite the scarcity of sources and objects preserved, a study of astronomical activity in Colonial Chile should give books a central role in the reconstruction of observation, computation and instrument fabrication.


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