The Experiment that led to an invention of Contour Lines

The title of Maskelyne’s 1775 paper is splendid: An Account of Observations made on the Mountain Schehallien for finding its Attraction. He spent 4 long summer months high on a mountainside, gazing nightly up into the heavens, how could he not find it attractive?

He was not set on writing poetry though. His aim was an: “astronomical experiment of the attraction of a mountain, as what could hardly fail of throwing light on the principle of universal gravitation, and was likely to lead to new discoveries concerning the constitution of the earth which we inhabit, particularly with respect to the density of its internal parts.”

There was mystery surrounding the shape and makeup of the earth. By the time he presented his paper to the Royal Society he had come to some rudimentary conclusions (the computations were to be left to mathematician Charles Hutton and subsequent papers): “…the mean density of the earth is at least double of that at the surface … the great density of the internal parts of the earth, is totally contrary to the hypothesis of some naturalists, who suppose the earth to be only a great hollow shell of matter; supporting itself from the property of an arch, with an immense vacuity in the midst of it.”

Why then would he need to look at stars to know what was under his feet? There were two parts to his experiment both of which involved finding the exact latitude of his two temporary observatories on Schiehallion; one through a trigonomical survey of the mountain and its surrounds, and the other through plotting the paths of various stars much in the way a mariner would. The results of the two calculations would – if earth and mountain were both dense masses, and gravity function horizontally (as well as vertically), by the ‘attraction’ of a plumb line to the mighty mass of the mountain – result in: “…the apparent difference of latitude of the two stations of the observatory, and, when compared with the difference of latitude which should result from the trigonometrical measures, will give the sum of the contrary attractions of the hill.” Thus proving solidity all round.

I can’t pretend to understand what these calculations entailed, when I think of looking at stars it is purely for the loveliness of them. When I look at maps, it is partially to find my way, but again mostly for the loveliness of them. But if it weren’t for this experiment and its need for a precise knowledge of the shape and volume of Schiehallion, then Newcastle born Hutton would not have invented contour lines. This is not to say they would remain ‘un-invented’, their submarine counterparts already had been.

This blog will be word heavy and image light. What I wanted to show here was a segment of a notebook Maskelyne kept on Schiehallion I saw when visiting The Royal Society archives. But they did not hold copyright and put me onto Greenwich Royal Observatory who steered me to Cambridge University (where it is no longer with Maskelyne’s papers as the 7^th Astronomer Royal removed it and kept it with his own). I am pursuing permission to use this image now simply for the love of a good back story, I’ll let you know how I get on.