er question arises out of this discussion. I have spoken of the
excentricity of the earth's orbit as decreasing. Was it always
decreasing? and if so, how far back was it so excentric that at
perihelion the earth passed quite near the sun? If it ever did thus pass
near the sun, the inference is manifest--the earth must at one time have
been thrown off, or been separated off, from the sun.

If a projectile could be fired so fast that it described an orbit round
the earth--and the speed of fire to attain this lies between five and
seven miles a second (not less than the one, nor more than the
other)--it would ever afterwards pass through its point of projection
as one point of its elliptic orbit; and its periodic return through that
point would be the sign of its origin. Similarly, if a satellite does
_not_ come near its central orb, and can be shown never to have been
near it, the natural inference is that it has _not_ been born from it,
but has originated in some other way.

The question which presented itself in connexion with the variable
ellipticity of the earth's orbit was the following:--Had it always been
decreasing, so that once it was excentric enough just to graze the sun
at perihelion as a projected body would do?

Into the problem thus presented Lagrange threw himself, and he succeeded
in showing that no such explanation of the origin of the earth is
possible. The excentricity of the orbit, though now decreasing, was not
always decreasing; ages ago it was increasing: it passes through
periodic changes. Eighteen thousand years ago its excentricity was a
maximum; since then it has been diminishing, and will continue to
diminish for 25,000 years more, when it will be an almost perfect
circle; it will then begin to increase again, and so on. The obliquity
of the ecliptic is also changing periodically, but not greatly: the
change is less than three degrees.

This research has, or ought to have, the most transcendent interest for
geologists and geographers. You know that geologists find traces of
extraordinary variations of temperature on the surface of the earth.
England was at one time tropical, at another time glacial. Far away
north, in Spitzbergen, evidence of the luxuriant vegetation of past ages
has been found; and the explanation of these great climatic changes has
long been a puzzle. Does not the secular variation in excentricity of
the earth's orbit, combined with the precession of the equinoxes, afford