The other important sighting from the top of Ben Nevis was a cone shaped mountain to the south east called Schiehallion. In some ways, it is the holy mountain of physics.In 1774, the Astronomer Royal Nevil Maskelyne travelled to Scotland and used Schiehallion in an experiment to find the mass of the Earth.
What he did was to hang a pendulum. The Earth attracts the pendulum downwards but the mountain attracts the pendulum sideways.
By measuring the angle of dangle, he was able to use Newton's Law of Gravitation to calculate the mass of the Earth, providing he knew the mass of the mountain.
That's why he chose Schiehallion. As you can see, it has a very regular shape meaning its volume is easily calculated. It is isolated so other mountains have little effect. Using the density of the rocks and the volume, he could calculate the mass.
My two unanswered questions are these:
- The angle of dangle was very small, much less than a degree. How did he measure it?
- How did he know which direction was vertical? You usually hang a pendulum (plumb line) to work that out.
