SCHIEHALLION, with its distinctive conical summit visible from all over Perthshire, is one of Scotland's most celebrated mountains. Known as "the fairy hill of the Caledonians" in folklore, the mountain was also believed to hold magical powers and to stand in the centre of Scotland. Even a hardened sceptic, when presented with a map, would be forced to admit that it does sit almost bang smack in the middle of the country.
At 3,547ft, and just two hours' drive from the nation's central belt, it's also one of the most southerly Munros, one of the 284 mountains in Scotland over 3,000ft. This means it also plays host to around 20,000 walkers a year who scale the summit using the well-worn path on the east side of the mountain, an area owned and maintained by the John Muir Trust since 1999.
Long before mountaineering made it famous however, Schiehallion earned its place in history in 1774 when Nevil Maskelyne, the Astronomer Royal, spent 17 weeks living in a
bothy (wooden hut) on its northern slopes conducting measurements to calculate the weight of the earth.
The Maskelyne experiment has become one of the most famous in the history of science. Whilst many are interested enough to read the commemorative plaque during their visit, Dr Peter Clive of Glasgow-based science consultancy Counting Thoughts went one further and recreated the experiment.
"We carried out a similar experiment using the same sort of principles used by Maskelyne, but where we did it in a single day at a cost of about £100, Maskelyne took 17 weeks and bankrupted the Royal Society," he explains.
Simply put, Maskelyne's experiment fell into two parts. Firstly he would calculate what fraction of the world the mountain represented using the circumference of the earth (which was known at the time) and a practical measurement of Newton's Gravitational constant G (known as big G) using pendulums. He would then measure the mass of the mountain (its volume multiplied by density) before going on to estimate the mass of the earth.
After initially proposing the experiment to the Royal Society in order to determine the weight of the earth, he then needed a mountain whose volume could be easily measured, which meant one with a regular geometric shape. The Royal Society sent astronomer Charles Mason on a reconnaissance tour of the Highlands and he returned with news that he had found a suitable mountain in the wilds of Scotland.
With Schiehallion fitting the bill, Maskelyne's mountain experiment was under way, but even travelling there from London was a task in itself in the 1770s. We can perhaps best conceive of how poor the road network was by Maskelyne's decision to sail to the mountain by boat, only travelling overland for the last 20 miles of the journey from Perth.
Maskelyne arrived at Schiehallion on the 30 June 1774 and set about using local labour to build two observation stations on the north and south side of the peak from which to make his measurements. He observed the stars, more specifically, the transit of Venus, to record a measurement for true vertical before recording the sideways gravitational pull exerted by the mountain on the pendulums. Now all he needed to do was measure the volume of the mountain. For this he turned to mathematician Charles Hutton.
As Clive, the science consultant, explains: "Hutton's method for measuring the volume of the mountain was to survey the whole thing, notionally divide the mountain into horizontal slices at regular vertical intervals, the volume of which was easily calculable due to their regular shape, and then add the volume of these together."
Interestingly they discovered that if you drew lines between each of the marker points of these slices, then the brain could interpret these to give a three-dimensional shape of the mountain. Hutton had invented contour lines, a system still used in mapping today.
From Maskelyne's results of the Schiehallion experiment Hutton calculated the mass of the earth to be 4.5 x 1024kg, with actual value using current methods being 5.98 x 1024kg, a surprisingly accurate figure given the resources available to him and a result accurate enough see the experiment regarded as a success with hindsight.
"He really pushed the cutting-edge technology of his day to the limit," says Clive. "Although it might be difficult to comprehend now given what we now know about the earth it was only after Maskelyne that we could assert with some confidence that the earth was not hollow - it was just far too heavy."
Certainly, 17 weeks in a mountain
bothy taking measurements every day must have left its mark on Maskelyne, as 1774 was recorded worst year on record in terms of rain - which may in turn explain why he took so long. Shiehallion, Maskelyne was helpfully told later, meant "constant storm" in the ancient Erse language.
Despite the area being virtually uninhabited at the time, the people of nearby Pitlochry and surrounding villages had a great reputation for hospitality. As a mark of his gratitude he threw a party in the northern observatory on the last evening of his project. "Evidently the party was pretty wild" explains Clive, "as it climaxed not only in the fiddler burning his own fiddle, but in the whole observatory being burnt to the ground."
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