Early pendulum experiments helped calculate the force of gravity. By taking different readings at different locations across the planet, it is possible to calculate the contours of the earth, as well as the density of the interior. (A later Foucault pendulum was used to demonstrate the rotation of the Earth.)
Lt Charles Wilkes, as part of the US Exploring Expedition, came to Hawai‘i in 1840 with a 68-inch long pendulum and a pendulum clock. After suspending the pendulum from a tripod, he set up the pendulum clock nearby.
Both the clock and pendulum were swung – since they had different lengths, they swung at different rates. Every so often, they would coincide. Observing the pendulums through a telescope, he would record the time of the coincidence.
Over time, enough data was accumulated to determine the duration of a single swing of the pendulum. With the time and the length of the pendulum, he was able to calculate the force of gravity. (Philbrick)
Just after new year’s in 1841, Wilkes conducted pendulum observations on the top of Mauna Loa, at a site they named Pendulum Peak.
In the days before Christmas, with temperatures in the teens, and “water in the bags, under my pillow, froze,) “it blew a perfect hurricane for several hours, causing an incessant slamming, banging, and flapping of the tents, as though hundreds of persons were beating them with clubs”.
At other times, “at sunset, we had a beautiful appearance of the shadow of the mountain, dome-shaped, projected on the eastern sky: the colour of a light amethyst at the edges, increasing in intensity to a dark purple in the centre; it was as distinct as possible, and the vast dome seemed to rest on the distant horizon.”
“The night was clear, with moonlight, the effect of which on the scene was beautiful: the clouds floating below us, with the horizon above them, reminded us of the ice bergs and ice-fields of the Antarctic: the temperature lent its aid to the deception.”
“The 3d proved fine, and the pendulum-clock and apparatus being arranged and adjusted, the clock was put in motion, and a comparison made with the three chronometers every two hours.”
They made other observations, as well, “During our stay on the summit, we took much pleasure and interest in watching the various movements of the clouds; this day in particular they attracted our attention; the whole island beneath us was covered with a dense white mass, in the centre of which was the cloud of the volcano rising like an immense dome.”
“All was motionless, until the hour arrived when the sea-breeze set in from the different sides of the island: a motion was then seen in the clouds at the opposite extremities, both of which seemed apparently moving towards the same centre, in undulations …”
“… until they became quite compact, and so contracted in space as to enable us to see a well-defined horizon; at the same time, there was a wind from the mountain, at right angles, that was affecting the mass, and driving it asunder in the opposite direction.”
“The play of these masses was at times in circular orbits, as they became influenced alternately by the different forces, until the whole was passing to and from the centre in every direction, assuming every variety of form, shape, and motion.”
“(T)he outward variation (in temperature) still continued from 17° to 50° during the twenty-four hours.”
“On the 8th, we had a change to cold, raw, and disagreeable weather; snow began to fall, and a kona or southwest gale set in; the temperature fell soon to 20°.”
“At 10 pm, I was unable to proceed with the pendulum observations; for such was the fury of the storm, that the journeyman-clock, with a loud beat, although within three feet of my ear, could not be heard. I was indeed apprehensive that the whole tent, house, and apparatus would be blown over and destroyed.”
“This storm continued until sunrise of the 9th, when it moderated. I have seldom experienced so strong a wind; it blew over and broke one of the barometers, although its legs had been guarded carefully by large stones; and the wind was so violent at times, that it was with difficulty we could keep our footing.”
“On the 10th of January, we had snow again. The temperature rose to 32°: the snow melted fast, causing excessive dampness within and without, while other discomforts that may be imagined prevailed.”
“On the 11th, having the eprouvette mortar (a small cannon) with me, I tried some experiments on the velocity of sound, comparing it with our measured bases and the sides of the triangles: these gave results as satisfactory … The great difference was in the sound itself: the report of the gun producing a kind of hissing noise.”
“The eprouvette was of iron, and was fired with a plug driven into it very tightly after it was loaded. When fired near the level of the sea, it was necessary to close the ears when standing within twenty feet of it. The sound could be heard six miles, and the report was equal to that of a large gun.”
“But on the summit we stood close to it without any precaution whatever, and the noise it there made was more like that of a squib (a small firework that burns with a hissing sound before exploding.) … This night we finished the pendulum, and all the dip and intensity observations”.
“When day broke, on the 13th January, all was bustle on the summit of Mauna Loa. Every one was engaged in taking down and packing up the instruments and equipage, loaded with which the native labourers scampered off.”
“Previous to our departure, I had the words ‘Pendulum Peak, January 1841,’ cut in the lava within our village. JG Clarke, one of the seamen belonging to the Vincennes, who made these marks came to me and desired, on the part of the men, that I would allow them to add to it US Ex Ex, in order that there might be no mistake as to who had been there; to this I readily gave my consent.”