Ascent of Man – The Mountain Scientists: Weighing the Earth and Air Pressure

The mountain Schiehallion tells a story, a fairly stunning story that started with the birth of science in the pre-Greek era and went through Galileo in 17th Century and onto Robert Hooke who suggested that gravitational forces are dependant on the an inverse square of the distant. As a result Sir Isaac Newton managed to derive mathematical proofs of Kepler’s law’s of planetary motion.

Newton wrote this down in his Principia Mathematica, one of the most important books for science.

I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.”

That simple theory was used to predict that there was yet another planet in our solar system Neptune, as the behaviour observed by Uranus could not be accounted for by the other planets. There was one other thing that his theory suggested that it might be possible to weight the earth with a suitable mountain, although Newton dismisses it fairly abruptly towards the end of his magnum opus.

“Nay, whole mountains will not be sufficient to produce any sensible effect. A mountain of an hemispherical figure, three miles high and six broad, will not, by its attraction, draw the pendulum two minutes out of the true perpendicular; and it is only in the great bodies of the planets that these forces are to be perceived…”

To scientist who pawed over Principia looking for ways to out smart the smart guy, this was a red rag to a bull. In 1738 having set out from France to try to measure the meridian arc length of one degree of latitude near the equator two French astronomers Pierre Bouger and Charles Marie de La Condamine took advantage of the opportunity to carry out what has been called the ‘deflection experiment’ on a Chimborazo that was over 6km tall and more than 20km across.

Whilst the deflection experiment ultimately failed to produce decent result, they did conclude that the earth was not an empty sphere. Perhaps the first real proof that this was the case.  The first successful deflection experiment was to take much closer to home in Scotland on the edge of Rannoch Mor in the Highlands on a mountain called Schiehallion.

The experiment was carried by some of the leading scientist at the Royal Society from 1773, it included the astronomer and surveyor, Charles mason, Neville Maskelyne, Charles Hutton and Reuben Burrow. The experiment meticulously measure the deflection of a pendulum from vertical as a result of the gravitation pull of the mountain. They measured this minuscule angle by sighting onto stars in the night sky. A phenomenal achievement for the time on the side of a mountain in Scotland.

A plague describing the Schiehallion Experiment.

Given Newton’s equations they could in theory work out how much the earth weigh, if they could discern the weigh of the mountain that was causing the deflection. So after the pull was measured a surveyor was sent to find an accurate weight of the mountain. The current surveying process at the time revolved around taking many sighting from many different spots and linking them by distance, elevation difference and bearing. This created a grid of thousands of triangle that represented the mountain as a mesh.

The problem was that there was no way to work out the volume of the mountain this way. However there was one thing they could do and so the surveyor interpolated points of equal height between each surveyed point. In linking them together because of the conical shape of the mountain the survey could  find the volume of a series of what were essentially truncated cones.

In doing they not only introduced contour lines to land based maps for the first time. Which if you are a hillwalker or mountaineer will have since allowed you to navigate through even the most complex terrain. More important to the scientist was they got one of the most accurate scientifically derived approximations of the mass and density of the earth.

This wasn’t the only experiment that required a mountain or at least a hill. The other perhaps more famous experiment involved Blaise Pascal some 100 years early in 1648, who after hearing about Torricelli’s experiments with a barometer and his idea that we were at the bottom of an ocean of air. Pascal enrolled his brother in law Florin Peter to carry one of his mercury barometers to the top of Puy De Dome and compared the reading to that of one left in the town of Clermont. Florin Peter recalls the experiment,

“Taking the other tube and a portion of the quick silver…I walked to the top of Puy-De-Dome, about 500 fathoms higher than the monastery, where upon experiment…found that the quicksilver reached a height of only 23″ and 2 lines…I repeated the experiment five times with care…each at different points on the summit…found the same height of quicksilver…in each case…”

The mercury read over three inches lower, meaning that the pressure had dropped as the altitude had increased. It was this experiment that helped immortalise Pascal’s name as the unit of measure for air pressure. It was also a first sign as to what might happen years later when climbers were to go to much greater elevations.

These are just two of the reasons that scientist were starting to take to the hills and mountains of the world. Whilst these people didn’t go to these places at first for pleasure. They were one of the necessary steps mountaineering had to make. As before scientists started to turn to the mountains in many places there were myths and legends that kept most people at bay. Stories of dragons and monsters in the Alp, probably from the billowing clouds from Avalanches that looked like smoke.

After the scientists turned up killing some of these myths they turned to study of natural science in the mountains like glacial movement, the effects of altitude, geology and other phenomenon. Indeed the earliest alpine mountaineers would never be seen on the hill without a barometer in hand, which invariably would break at some point. What these scientist had in common was they were regularly heading to the mountains to carry out there research and eventually one or two of them would quite literally leave the barometers behind and start climbing for its own sake.


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