The Earth; Its Life and Death (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III THE FORM, MAGNITUDE, AND MASS OF THE EARTH WE have now reviewed the successive states through which the Earth has passed before reaching that with which we are familiar to-day. The periods with which we have dealt in the course of the preceding two chapters correspond to the birth, the infancy, the adolescence, and the youth of the Earth. To-day it is mature; let us see in what manner it exists and how it lives. We must first of all gain some idea of its external aspect, and we will commence by investigating its form and dimensions. The proofs that the Earth is a spheroidal body, isolated in space, have been summarised in our earlier pages. It is possible to give a preliminary notion of its dimensions by remarking that a telescope, whose axis is truly horizontal, mounted on the summit of a mountain overlooking the sea, would not show the sea horizon. In order to have this horizon in view in the centre of the field ofthe instrument, the latter must be rotated downwards through an angle which astronomers call the angle of depression. If this angle be carefully measured, and if the height of the mountain be also known, we can deduce the Earth's radius by means of elementary geometry, on the assumption that it is a true sphere. As a first approximation, the result so obtained is 6,366,000 metres 4000 miles]. It is noteworthy that if this experiment be repeated in different parts of the Earth nearly the same result is always obtained. We may therefore assert that the Earth is sensibly spherical and that its radius is 6,366,000 metres, to a first approximation. If we now make a more precise and accurate determination of the angle of depression by employing a more powerful telescope, capable of rotation about a more exactly divided circle, and if, furthe...