Introduction

The Earth, as will appear, is not typical of the planets. It is the largest of the inner planets, it is the only one on which active tectonic development of the surface appears to be going on at present and, so far as we know, it has the most complex structure. Yet it is the only one which can be studied in detail; from it we may derive empirically equations of state of the materials of the inner planets; and the methods that have been used to study the structure of the Earth are those we should like to use, but are inhibited from using by the difficulties of observing, in the investigation of the other planets. For these reasons it is helpful to preface an account of the methods used to study the planets and of the results that have been obtained with a review of the way in which the Earth is examined and what has been discovered.

Our knowledge of the internal structure of the Earth comes by two routes. In the first place the mass, size and density of the Earth provide a rough idea of the overall composition and of the central pressure, while the value of the moment of inertia shows that the density increases strongly towards the centre. Naturally a wide range of models could be constructed to fit just three facts and so it is necessary to turn, in the second place, to seismology to provide more detailed information.