et from the sun does not
exceed the least distance by one per cent. Its mean distance from the
sun is about 67,000,000 miles, and the movement in the orbit amounts to
a mean velocity of nearly 22 miles per second, the entire journey being
accomplished in 224.70 days.

CHAPTER IX.

THE EARTH.

The Earth is a great Globe--How the Size of the Earth is
Measured--The Base Line--The Latitude found by the Elevation of the
Pole--A Degree of the Meridian--The Earth not a Sphere--The
Pendulum Experiment--Is the Motion of the Earth slow or
fast?--Coincidence of the Axis of Rotation and the Axis of
Figure--The Existence of Heat in the Earth--The Earth once in a
Soft Condition--Effects of Centrifugal Force--Comparison with the
Sun and Jupiter--The Protuberance of the Equator--The Weighing of
the Earth--Comparison between the Weight of the Earth and an equal
Globe of Water--Comparison of the Earth with a Leaden Globe--The
Pendulum--Use of the Pendulum in Measuring the Intensity of
Gravitation--The Principle of Isochronism--Shape of the Earth
measured by the Pendulum.

That the earth must be a round body is a truth immediately suggested by
simple astronomical considerations. The sun is round, the moon is round,
and telescopes show that the planets are round. No doubt comets are not
round, but then a comet seems to be in no sense a solid body. We can see
right through one of these frail objects, and its weight is too small
for our methods of measurement to appreciate. If, then, all the solid
bodies we can see are round globes, is it not likely that the earth is a
globe also? But we have far more direct information than mere surmise.

There is no better way of actually seeing that the surface of the ocean
is curved than by watching a distant ship on the open sea. When the ship
is a long way off and is still receding, its hull will gradually
disappear, while the masts will remain visible. On a fine summer's day
we can often see the top of the funnel of a steamer appearing above the
sea, while the body of the steamer is below. To see this best the eye
should be brought as close as possible to the surface of the sea. If the
sea were perfectly flat, there would be nothing to obscure the body of
the vessel, and it would therefore be visible so long as the funnel
remains visible. If the sea be really curved, the protuberant part
intercepts the view of the hull, whi