Peirce Pendulum at the National Museum of American History, Smithsonian Institution, Washington DC. It is believed to be identical to the Peirce pendulum brought by Adolphus Greely into the Arctic. Photo courtesy of Dr. Geoffrey Clark.

Washington D.C., July 1881

Sargent Edward Israel arrived in Washington and made his way up Capitol Hill to the Coast Survey Building. He had an appointment with C.S. Peirce, Assistant at the Coast Survey, who would instruct Israel in the use of scientific instruments needed for the Greely Expedition, scheduled to depart for the Arctic in a few weeks. At twenty-two, Israel was the youngest member of the expedition. He had just finished his degree in astronomy at the University of Michigan where he had impressed faculty with his command of theoretical astronomy. He was comfortable with scientific equipment and well-prepared to do complicated calculations and reductions. Yet the biggest challenge that would face him on this expedition was in the gathering of data. His meeting with Peirce was not merely a lesson in how to use instruments, but how to use them in extreme conditions.^[1]

Lt. Edward Israel, from Greely Expedition photo, 1881.

When the Greely Expedition built its station on Ellesmere Island at 81°N latitude, it would be the northernmost outpost in the world, and one of the most difficult places on earth to do science. One of twelve stations to be established during the International Polar Year (IPY) of 1881-1882, the American outpost would record – along with all of the other stations – a variety of terrestrial phenomena including tides, weather, temperature, wind speed, and barometric pressure. The IPY was the brainchild of Austrian explorer, Karl Weyprecht, an attempt to redirect the energies of polar explorers away from flag planting and records of “Farthest North” towards something more substantial: a sustained and systematic  program of Arctic research.^[2]

 When Israel arrived at the Coast Survey Building, he was met by Peirce. The two men descended into the basement and entered Room 6.  There, anchored by concrete piers, suspended from a large trapezoidal frame hung a long brass bar, the Peirce Pendulum No. 1. Few people would have identified the object as a pendulum. It did not have a round weight or a thin arm. It was not an object one would find oscillating in the case of a grandfather clock. The Peirce No. 1 was unremarkable except for small projections, “knife edges,” that jutted out of the sides of bar near its top and bottom and allowed the pendulum to hang freely in its wooden frame.^[3]

Peirce set the pendulum in motion, swinging it a few centimeters off center.  The heavy bar, rocking back and forth on the slender pivot of its knife edges, swept out small, regular arcs. Israel did not record his impressions that day. He would not survive the expedition to write about it later. Perhaps his college experience with the instruments of astronomy, the telescopes that offered him spectacular views of planets and nebulae, made him jaded to the operations of the Peirce No. 1. Yet even someone less experienced with scientific instruments than Israel, some imaginary passer-by who found himself in Room 6 that day, would have struggled to find either drama or meaning in the Peirce No. 1’s slow monotonous motions. It appeared almost too simple to be useful.

Yet it was the monotony of this pendulum that gave it its power. Allowed to swing freely, a pendulum will repeat its journey back and forth in the same period of time, even as the height of its swing diminishes. It did not escape the attention of Galileo or other Renaissance scholars that the regularity of this motion offered a valuable way of measuring time itself. By the 1650s, the Dutch mathematician Christiaan Huygens understood the movement of a pendulum well enough to describe it mathematically:

T = π√(l/g)

 where the time (T) of the pendulum’s swing  varies directly with its length (l)  and indirectly with the force of gravity (g). Assuming that gravity remains constant, the most important variable determining the pendulum’s swing is the length of its arm. By lengthening or shortening this arm, the pendulum can be made to sweep out an arc of desired duration. After Huygens patented his first pendulum clock in 1657, clockmakers developed a “seconds pendulum” that offered a spectacular improvement in accuracy from earlier clocks, reducing error from fifteen minutes to fifteen seconds a day.^[4]

Christiaan Huygens’s design for a pendulum clock, 1673.

By the 1700s, the pendulum had also found more esoteric uses. Since clockmakers had succeeded in showing that a swinging bob could be used as a measure of time, it stood to reason that that swinging bob, marked by increments of time, could be used as a measure of gravity. Gravity appeared to be remarkably stable over time, but was it also stable over distance? On a perfectly spherical earth, this should be the case since the distance between the surface and the earth’s center of mass would never vary. A distortion of the planetary sphere, however, would produce variations in gravity from place to place, ones that might be detectible by the swinging of a pendulum.^[5] For this reason, in the 1730s, the French geodetic expeditions of Pierre-Louis Moreau de Maupertuis and Charles Marie de La Condamine carried gravity pendulums with them to the polar and equatorial regions respectively, attempting to resolve a dispute between French geographer Jean-Dominique Cassini, who believed the earth was slightly egg-shaped, and Isaac Newton who was convinced it was squashed like a jelly-donut. The expeditions proved Newton right, but did not give enough data to describe to the shape of the geoid with precision.^[6]

This was the objective of Israel and the Peirce No. 1: to determine the precise shape of the earth from the swinging of the pendulum. In so doing, it fit comfortably within the pendulum’s expanding role as an instrument of research, marking a procession of important instruments from Condamine’s pendulum in the 1700s to Foucault’s pendulum in the 1800s. Gradually the pendulum had evolved from a symbol of timekeeping to a symbol of science. As such, it conformed nicely to the broader objectives of the IPY:  to reinvent Arctic exploration as something serious, scientific, and collaborative.

As the simple brass bar swung on its knife edges in Room 6, Peirce recorded the duration of its swings. The meeting had provided Israel with a tutorial in operating the pendulum, but it had offered Peirce something equally important: a series of measurements that he could compare with those made by Israel in the Arctic. As they concluded their meeting, Israel departed. In Room 6, the Peirce No. 1 was carefully packed in a long wooden case and sealed with tin. In the days that followed, it was shipped north with other expedition equipment to St. John’s Harbor, Newfoundland where it was stowed below deck on Greely’s expedition ship, Proteus. On 7 July, the ship set sail with the expedition party for Lady Franklin Bay, an inlet on the northeastern shore of Ellesmere Island, the northernmost island in the Arctic Archipelago.^[7]  

[This essay was published by the journal Endeavour in December 2012: 36(4):187-90]