, are diametrically in opposition. And yet, for one as for
the other, the bottom is the soil they are held to, and the top is
space above their heads.

The Earth turns on itself in twenty-four hours. Whatever is above us,
_e.g._, at midday, we call high; twelve hours later, at midnight, we
give the same qualification to the part of space that was under our feet
at noon. What is in the sky, and over our heads, at a given hour, is
under our feet, and yet always in the sky, twelve hours later. Our
position, in relation to the space that surrounds us, changes from hour
to hour, and "top" and "bottom" vary also, relatively to our position.

Our planet is thus a ball, slightly flattened at the poles (by about
1/292). Its diameter, at the equator, is 12,742 kilometers (7,926
miles); from one pole to the other is a little less, owing to the
flattening of the polar caps. The difference is some 43 kilometers
(about 27 miles).

Its circumference is 40,000 kilometers (24,900 miles). This ball is
surrounded by an aerial envelope, the atmosphere, the height of which
can not be less than 300 kilometers (186 miles), according to the
observations made on certain shooting stars.

We all know that this layer of air, at the bottom of which we live, is a
beautiful azure blue that seems to separate us from the sidereal abyss,
spreading over our heads in a kind of vault that is often filled with
clouds, and giving the illusion of resting far off on the circle of the
horizon. But this is only an illusion. In reality, there is neither
vault nor horizon; space is open in all directions. If the atmosphere
did not exist, or if it were completely transparent, we should see the
stars by day as by night, for they are continually round us, at noon as
at midnight, and we can see them in the full daylight, with the help of
astronomical instruments. In fact, certain stars (the radiant Venus and
the dazzling Jupiter) pierce the veil of the atmosphere, and are visible
with the unaided eye in full daylight.

The terrestrial surface is 510,000,000 square kilometers (200,000,000
square miles). The waters of the ocean cover three-quarters of this
surface, _i.e._, 383,200,000 square kilometers (150,000,000 square
miles), and the continents only occupy 136,600,000 square kilometers
(55,000 square miles). France represents about the thousandth part of
the total superficies of the globe.

Despite the asperities of mountain ranges, and the abysses hollowed ou