[11] Brown is not the right word for the tint of red where the visible
spectrum begins. I know, however, of no word properly expressing the
colour.
[12] Suppose there are two planets A and B of equal density, of which A
has a diameter twice as great as that of B. Then the volume of A is
eight times greater than B's volume. So that if the volume of its
atmosphere exceed the volume of B's air in the same degree, the planet A
has eight times as much air as the planet B. But the surface of A is
only four times as great as the surface of B; so that if A had only four
times as much air as B, there would be the same quantity of air above
each square mile of A's surface as above each of B's surface. Since then
A has eight times--not merely four times--as much air as B, it follows
that A has twice as much air over each square mile of surface as B has.
And similarly in all such cases, the general law being that the larger
planet has more air over each square mile of surface in the same degree
that its diameter exceeds that of the other.
[13] By age here I do not mean absolute age, but relative age. I speak
of Mars and the Moon as older than the earth in the same sense that I
should speak of a fly in autumn as older than a five-year-old raven.
[Illustration]