With the exception of attending the meetings of the Royal Society, Maskelyne hardly ever quitted his observatory. His life is therefore difficult to describe, except by its results. But in 1772 he went to Scotland, to pursue his celebrated experiment for the discovery of the earth’s density. The Newtonian doctrine of attraction, in the general form, that all matter attracts all other matter, could hardly be said to be finally established, except as a point of strong probability. That a planet, considered as a whole, attracts a planet, might be thought to be demonstrated, but there was no proof of matter being the agent of attraction upon matter, _on the earth_, except in the case of magnetised or electrified bodies. The notion that the attraction of a mountain, if it existed, would cause a slight deviation of the plumb-line, which should be perceptible in its effect on the observed position of the stars, had been entertained, and the effect even suspected, but without being reduced to absolute proof. To give an idea of the minuteness of the angle of deviation which was to be looked for, we may state that a pendulum ten thousand inches long, vibrating through an angle of ten seconds, would only move through half an inch at the end farthest from the point of suspension, and that ten seconds was, as it turned out, nearly double of the angle in question. Maskelyne chose the mountain Schehallien, in Perthshire, as the scene of his operations. By observing well-known stars with an instrument depending on a plumb-line, both north and south of the mountain, he determined the difference of latitude of two stations, subject of course to an error if the plumb-line were affected in its position by the attraction of the mountain. He then measured the difference of latitude of his stations by a trigonometrical survey, which gave their relative position by a method independent of the plumb-line and its errors. He thus found that his north and south plumb-lines were inclined to each other at an angle of about eleven seconds and a half more than they should have been from their difference of position on the earth, and that the direction of their inclination was towards the mountain. He deduced his results from those among his observations which he considered as the best, being about one out of ten of the whole; but it is much to his credit as an observer, that Baron Zach afterwards found that all his observations, good and bad, gave the same average result as those he had selected. Zach also established the same fact by his observations in the neighbourhood of Marseilles, namely, that the vicinity of a mountain affects the level, which was the instrument he used, and not the plumb-line.