The Schiehallion Experiment

Having been up in Scotland, I looked out across at Schiehallion, an almost picture perfect mountain cone. It reminded me of the classic experiment conducted to achieve two aims. One prove Sir Issac Newtons Theory of Gravitation, and at the same time calculate the mass of the earth.

I find this whole concept rather interesting, and ever since I have got back I have been keen to google the experiment and find out just how the experiment worked, and why it resulted in another innovation for maps the humble contour line.

The reason that Scheihallion was chosen in the first place was because of not only its shape as an almost prefect cone, but its isolation from surround hills. These were both critical to the success of the experiment, as according to Issac Newton’s theory’s Gravity was based on a relationship between the mass of two objects and the inverse square of there distance. In this experiment these were the centre of the earth versus the centre of the mountain.

The perfect cone shape made it possible to calculate the mass of the mountain, and have the measuring device close to its centre of gravity. The mountains isolation would reduce the effects of gravity of the surrounding mountains.

Issac Newton and suggested that the effects would be measurable only by a couple of minutes of  one degree. So had decided not to do the experiment, instead in fell to a team that most famous included Nevil Maskelyne and Astronomer Royal and Charles Hutton who helped us work out longitude by the Lunar Distances, and Charles Hutton a renowned mathematician.

How they measured the effect was amazing, as I had imagines a pendulum with a degree mark on, however this would be to true. In order to measure the minute distance, it would be better to have a much longer pendulum to measure the relative distance between one pendulum on one side of the mountain, compared to one place on the other side. To achieve this they look to the stars, and leasure the relative distance between sighting staken precisely up the plumbline holding the pendulum.

But for the experiment to work, the team had to find the estimated mass  of the mountain. To do this it was expected that the team would use survey data to find equal slices of vertical prisms. After an extensive survey with thousands of points measured, Charle Hutton, though that his job would be easier if he interpolated lines of equal height to make these vertical prism slices. In doing so he helped bring brought the contour line to landmaps.

The experiment was extremely successful, and managed to calculate the density of the earth to within 20%, prove Sir Issac Newton right on his theory but wrong on think the effects of a mountain would be too small to measure. In 2007 the experiment was repeated with more accurate surveys going out to a 120 kilometre radius and of course the humble computer to crunch an awful lot of data. This showed that the basis of the experiment was extremely accurate, and this new repeat of the experiment got to with a hairs breadth of the Modern accepted figure.

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