Page:Popular Astronomy - Airy - 1881.djvu/274

260 The next thing was, to draw from this observation a determination of the mean density of the earth. The general form of the process was this: the mountain was surveyed, mapped, levelled, and measured, in every way, so completely, that a model of it might have been made; it was then (for the sake of calculation) conceived to be divided into prisms of various forms: the attraction of every one of these was computed, on the supposition that the mountain had the same density as the mean density of the earth; and by means of this, the attraction of the whole mountain was found on the same supposition.

Thus it was found, that if the density of the mountain had been the same as the mean density of the earth, the sum of the effects of the attractions of the mountain at N and S would have been about $1⁄9933$ part of gravity. But the observed sum of effects was 12 seconds, which corresponds to $1⁄17,804$ part of gravity. Hence the density of the mountain is only about $5⁄9$ of the earth's mean density; or the earth's mean density is nearly double of the mountain's density. The nature of the rocks composing the mountain was carefully examined, and their density as compared with that of water was ascertained; and thus the mean density of the earth was found to be something less than five times the density of water: a result agreeing nearly with that found from the assumption of the law of density of the earth's strata, connected with the observed variation of gravity, and observed ellipticity.

This was the nature of the celebrated Schehallien experiment, which was so extremely creditable to the parties by whom it was promoted and undertaken, and so important in its results.

After this, another set of experiments was made;