Schiehallion looks like a mountain drawn by a child. From the east it rises out of the moor as a clean cone, symmetrical enough that it seems designed rather than weathered. It is not volcanic, whatever the shape suggests. It is quartzite — sand, turned to rock by heat and pressure, then shaped by ice.
That regularity is the reason a party of astronomers dragged brass instruments onto its flanks in the summer of 1774 and stayed for four months.
The problem they had come to solve was this. Newton had shown that mass attracts mass, and that a mountain should therefore pull a hanging plumb line very slightly out of true. He thought the effect too small to measure. Seventy years later, the Royal Society decided to try.
If you could measure how far a mountain deflected a pendulum, and you knew the mountain’s own mass, you could compare the two and arrive at the mean density of the Earth itself. From that: its mass. And from that, a value for the gravitational constant. Weighing the world, from a hillside in Perthshire, with a plumb line and a good deal of patience.
It needed the right mountain. Isolated, so that no neighbouring hill confused the measurement. Steep-sided, so the pull was concentrated. Regular enough in shape that its volume could actually be calculated. Charles Mason — of the Mason–Dixon line — was sent to find one, and settled on Schiehallion. He then declined the commission to run the experiment, and Nevil Maskelyne, the Astronomer Royal, took it on himself.
Maskelyne spent that summer in a bothy on the mountain, observing stars from north and south of the summit and measuring how far the local vertical had shifted. He got his answer. It was the first credible estimate of how heavy the Earth is, and it came out of a hillside a short drive from here.
The accident that mattered more
The more useful thing to come off Schiehallion was not the number.
Maskelyne’s surveyors had produced an enormous quantity of height readings across the mountain, and Charles Hutton — the mathematician tasked with turning them into a volume — could not hold the shape of the thing in his head. So he drew lines joining the points of equal height, and read the mountain’s form from the pattern they made.
Those are contour lines. Every Ordnance Survey sheet folded into every rucksack in Scotland descends from a mathematician trying to work out the volume of one Perthshire hill.
It is a fair example of how this landscape tends to work. The interesting thing is rarely the thing announced on the sign.
Going up
Schiehallion is a Munro, at 1,083 metres, and the standard route runs roughly four and a half kilometres from the car park at Braes of Foss. It is a straightforward walk in good conditions and an unforgiving one in poor visibility, because the summit ridge is a long boulder field where the path becomes a matter of opinion.
The original track eroded badly under its own popularity. By the late 1990s the John Muir Trust had built a new path on a different line, which is the one you will walk today. Worth knowing, if only because the mountain has now been repaired twice by people who cared about it — once for science, once for its own sake.
Take a compass. Quartzite is poor company in cloud.
Schiehallion is visible from a good deal of this part of Perthshire, and it is the sort of hill that becomes a fixed point in a week’s stay — the thing you check the weather against each morning. There is more on the walking in the area on our walking trails page, and on the rest of what is within reach on attractions.