Page:Popular Astronomy - Airy - 1881.djvu/266

252 to gravity at the equator, is as 180 to 179. This shows that Newton was wrong.

The question then is, in what was he wrong? Now, it must be remarked, that Newton's calculation was founded entirely on the supposition that the earth is of equal density thoughout. When we consider the matter, it is very unlikely that, if the interior of the earth is fluid, its density is equal in every part. Accordingly in the last century, investigations were made, supposing the earth to be of different densities in different parts, and specially that the density increased as we approach nearer to the centre. The principal investigation (to which in fact, nothing important has been added in later times) was made by an eminent French mathematician, named Clairaut. The supposition on which he went (and which is really the only kind of supposition to be made at all in this investigation) was, that the earth consists of strata of different densities, but that each stratum is in some degree elliptical; the ellipticity of one stratum being different from that of another; and the investigations leave these ellipticities to be determined by considerations connected with the equilibrium of fluids.

For instance, in Figure 62, setting aside the water and floating matter on the top; suppose that F is the region of lava, if you please, that G is the region of melted iron, and that H is the region of melted platinum. Suppose you conceive one tube to be drawn from the Pole, and another from the equator, meeting at E; you make this condition, that when you have investigated properly the gravity, (arising from the attraction of every particle of the spheroid upon any one particle, and modified by the centrifugal tendency from the axis Aa ,) acting on the different substances at the different parts of these tubes, and when you have