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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...
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