Measuring instruments – The state-of-the-art instruments used to measure the degrees in the Torne Valley

After meeting Maupertuis, Clairaut and other expedition members in Paris, Anders Celsius continued his study trip from France to England. Celsius purchased a zenith sector, an astronomical pendulum clock, a pendulum measuring instrument and a telescope from London.

In Tornio, on the 6 July 1736, the instruments and the French were loaded into boats and the journey towards the hills of the Torne Valley began. There were seven boats in total, each with three rowers.

Quadrant – measuring angles

The quadrant is made up of a quarter-circle arc and a sighting rod attached to the centre of the circle. On the arc of the circle is a scale of angles. The quadrant was used to measure angles.
The expedition had four quadrants at its disposal.

The most important of these was the quadrant, which had a radius of two feet, or about 65 cm. It was made by the Parisian Claude Langlois (1700–1756). Langlois took French scientific instrument making to a higher level than it had previously ever reached.

Maupertuis expert Osmo Pekonen found the quadrant used by Maupertuis in Potsdam, Germany, in the corridor of the observatory of the Leibniz Institute. Photo: Veli-Markku Korteniemi.

The quadrant was always carefully aimed at the centre of the marker. All the researchers who were at the measurement site made their own observations and wrote them down, after which the average of the observations was calculated.

Differences in height between hills were also taken into account in the measurements and the angles were “reduced to horizontal”, i.e. the triangulation network was mathematically transformed into the same plane.

The drawing illustrates the size of a two-foot quadrant.

Sector – measuring latitude

The sector, or zenith sector, arrived from England by ship directly to Tornio after the expedition, at the end of August 1736.

It was made by the distinguished English maker of astronomical measuring instruments and member of the Royal Society, George Graham (1673–1751). Maupertuis had become familiar with Graham’s instruments in London on his study trip ten years earlier.

George Graham.

Le Monnier described the sector as “one of the most beautiful astronomical instruments ever seen.”

The radius of the sector was about nine feet, or almost three metres (about 292 cm). It was large and heavy, and four porters were needed to move it. The sector had a heavy pyramid-shaped base 12 feet (about 390 cm) high.

The zenith sector was a big instrument. Transporting it by boat from Tornio to Pello required a lot of work.

The sector was intended to measure the difference in degree between the Tornio and Pello observation points. This could be found by measuring the angle of the same star in the zenith sector in both Tornio and Pello.

The star chosen by the expedition was δ Draconis or the delta star of the constellation of the Dragon. The star crossed the meridian near the zenith, the highest point in the sky. The star was also such that it could be seen well in both Pello and Tornio.

The measurements with the sector were made as a working group. One observed the pendulum, another turned the telescope and a third read the micrometer attached to the telescope.

To ensure the measurement results, the tasks were changed daily.

The measurements were repeated to ensure that the result was as accurate as possible. The variation between the different measurements was only 2 to 3 seconds.

The degree difference between the extreme ends of the triangulation chain, Pello and Tornio, was calculated by measuring the positions of the objects in relation to the selected fixed star. By comparing the angles between the zenith and the fixed star, the exact position on the meridian arc was found out.

Pendulum – measuring gravity

Since the 1660s, it had been known that the position on a latitude affected the movement of the pendulum.

The French astronomer Jean Richer (1630–1696) noticed while working in Cayenne, French Guiana, that his pendulum clock was not keeping time.

Later, Isaac Newton (1642–1727) proposed, based on his theory of gravitation, that Richer’s observation proves that the centre of the Earth is farther from the Equator in Cayenne than further north in Paris.

Drawing of Jean Richer at work in Cayenne. The pendulum can be seen on the wall in Richer’s background. Excerpt from engraving by Sébastien Leclerc.

Maupertuis’s expedition had with it several similar pendulums, with which Richer had observed the difference in gravity at the Equator.

In the autumn of 1736, a room in the Korteniemi house in Pello was harnessed for measurements with gravity pendulums.

A hole was made in the ceiling of the room and a stone pillar was built underneath it. Both the telescope and the pendulums were attached to the pillar. The telescope was used to determine the oscillation time of the pendulums using fixed stars.

Although Newton had used Richer’s pendulum observations to calculate the Earth’s lithification ratio, the Maupertuis expedition could only use the pendulum observations to complement their geodetic and astronomical observations.

Newton’s theory of gravitation was not generally accepted in France at the time, which limited its application in calculations.

Outhier’s drawing of the Korteniemi house in Pello. The room marked with the letter A in the picture is where astronomical research was done with various pendulums and a telescope.

A telescope – a help in astronomical observations

The telescope was invented in Holland at the beginning of the 17th century. Galileo Galilei (1564–1642) made the first astronomical observations with a telescope in 1609.

The expedition had several telescopes at its disposal. One of them was a telescope made by George Graham, which was used to determine the direction of the meridian.

Le Monnier left one of the telescopes to Anders Hellant, who acted as the expedition’s interpreter from Tornio. Its length was 7 feet or about 2.3 meters. Thanks to the telescope, Hellant was later able to establish his own observatory in Tornio.

Kittisvaara observatories – the northern base of the sector

The expedition built two observatories in Kittisvaara. They were the northernmost astronomical observatories of their time.

The big observatory was constructed in a building bought from the courtyard of the Saukkola house (called cotta in Outhier’s book), which Camus moved to Kittisvaara. It was a log building, taller than the house, which, according to Outhier, was used in winter, for example to melt snow to provide drinking water for livestock. It was moved to Kittisvaara in sections and rebuilt.

A zenith sector was placed in the large observatory. In his travelogue, Outhier mentions how the sector was closely monitored. Each of the expedition members took turns to stay overnight at the observatory and monitor the precious instrument.

In addition to this, another slightly smaller observatory was built on the hill. There, measurements were made with a Graham pendulum and a device to determine the orientation of the meridian in relation to the triangles. The device was precisely positioned at the Kittisvaara marker.

The observatories built on top of Kittisvaara can be seen in the background of Maupertuis’ portrait. Excerpt from painting.
Kittisvaara’s observatories are depicted in Outhier’s drawing of the Korteniemi house. Excerpt from the drawing.

Tornio observatories – the southern base of the sector

The observatory in Tornio was also built in a building similar to the one in Pello. The observatory was located in Helander’s house, where Le Monnier and Celsius stayed. A sturdy stone base was built for the sector and an opening was made in the roof of the building.

Unlike in Pello, the weather in Tornio favoured the researchers and they took measurements over several days in one go, starting on the last day of October.

In addition to the observatory set up in Helander’s outbuilding, the expedition needed another observatory from which they could see the horizon. They therefore had a small building built on the banks of the Torne. There they took measurements with a meridian measurement device, a pendulum and a quadrant.

The observatory intended for the sector was built in the courtyard building of Helander’s house. Helander’s house is marked in red on the map. The location of the smaller observatory built on the riverbank is marked on the map with a white circle.
Sources:

Encyclopedia.com, writer Daumas, Maurice : https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/langlois-claude

Karttunen Hannu, Ursa and Observatory of Tuorla, https://www.astro.utu.fi/zubi/astro.htm

Maupertuis, Pierre Louis Moreau de. “Maan muoto”. Maan muoto ynnä muita kirjoituksia Lapista. Ed. Osmo Pekonen. Väyläkirjat, 2019 (orig. 1738).

Outhier, Réginald. Matka Pohjan perille. Maupertuis Foundation and Väyläkirjat, 2011 (orig. 1744).

Pekonen, Osmo. “Esseitä. Viisi akateemikko Lapissa”. Maan muoto ynnä muita kirjoituksia Lapista. Ed. Osmo Pekonen. Väyläkirjat, 2019.

Pekonen, Osmo and Stén Johan. “Geodeettiset mittauslaitteet”. Maan muoto ynnä muita kirjoituksia Lapista. Ed. Osmo Pekonen. Väyläkirjat, 2019.

Terrall, Mary. Maupertuis. Maapallon muodon mittaaja. Trans. Osmo Pekonen. Väyläkirjat, Tornio, 2015 (orig. 2002).

Wikipedia: https://en.wikipedia.org/wiki/Jean_Richer

Wikipedia: https://en.wikipedia.org/wiki/Julien_Le_Roy


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