I tend to refer to Schiehallion in Perthshire as the Holy Mountain of Physics because it was this mountain that the Astronomer Royal Neville Maskelyne used in the late 1700s to measure the mass of the Earth. My pilgrimage to the top in 2009 was one of the early topics on this blog. I have returned to the northern shores at Kinloch Rannoch. What Maskelyne did was hang a pendulum near the mountain. You'd expect it to be pulled vertically downwards towards the centre of the Earth but he had figured that the mass of the mountain would attract it horizontally and pull it away from the vertical. If he could deduce the mass of the mountain and measure the deviation from vertical, he could work out the mass of the Earth from Newton's Law of Gravitation. He considered using Skiddaw - you can see that from Wigton! Schiehallion has a similar shape, like a Toblerone bar, but is more remote from lother mountains that Skiddaw. They mapped it accurately by inventing contours right here to get the correct volume, They knew the rock density so they could get the mass. But how to measure deviation from vertical? This had always puzzled me and my classes. I know what is vertical by hanging a pendulum - but clearly not if the pendulum itself is out of line. I read recently that Maskelyne used the stars. He could calculate which would be vertical and then observed the position of a pendulum relative to these stars,
