Page:Popular Astronomy - Airy - 1881.djvu/270

256 There is another inference from these theoretical investigations of the figure of the earth, which it is proper to mention. Though we do not know what the law of the earth's internal structure is, yet we can assume some law of densities of successive strata gradually changing from the surface to the centre which shall give a value for the earth's ellipticity agreeing with the results which I have mentioned: and from this law we can find what the mean density of the earth is. The inference was thus made by Clairaut and his successors, that the mean or average density of the earth is about twice as great as at the surface, and that in some parts at and near to the centre, the density must be considerably greater than that mean density. Remarking that the mean density of the earths and rocks at the earth's surface, taking one with another, is about twice and $6⁄10$ that of water, it was inferred that the mean density of the earth is more than five times the density of water. After this, another experiment was made, applying to the determination of the earth's mean density I have mentioned the liberality of George the Third in supplying funds for the observation of the transit of Venus. The same monarch (as I believe) supplied the funds for another experiment of great importance; it was the Schehallien experiment. Probably some of my auditors who have travelled in the highlands of Scotland, have seen the Schehallien Mountain; it may be observed from the banks of Loch Tay. If you go from Killin to Taymouth, it is on the left hand. Now this mountain was selected for observations of a very remarkable kind. It was argued that if the theory of gravitation were true, (that is to say, if attraction were produced not by a tendency to the centre of the earth, or to any special point, but