of steel, the
instantaneous axis would revolve in 441 days; in the actual earth, the
process is accomplished in 428 days. By this new path, accordingly,
astronomers have been led to an identical estimate of the consistence of
our globe with that derived from tidal investigations.

Variations of latitude are intrinsically complex. To produce them, an
incalculable interplay of causes must be at work, each with its proper
period and law of action.[894] All the elements of the phenomenon are
then in a perpetual state of flux,[895] and absorb for their continual
redetermination, the arduous and combined labours of many astronomers.
Nor is this trouble superfluous. Minute in extent though they be, the
shiftings of the pole menace the very foundations of exact celestial
science; their neglect would leave the entire fabric insecure. Just at
the beginning of the present century they reached a predicted minimum,
but are expected again to augment their range after the year 1902. The
interesting suggestion has been made by Mr. J. Halm that such
fluctuations are, in some obscure way, affected by changes in solar
activity, and conform like them to an eleven-year cycle.[896]

In a paper read before the Geological Society, December 15, 1830,[897]
Sir John Herschel threw out the idea that the perplexing changes of
climate revealed by the geological record might be explained through
certain slow fluctuations in the eccentricity of the earth's orbit,
produced by the disturbing action of the other planets. Shortly
afterwards, however, he abandoned the position as untenable;[898] and it
was left to the late Dr. James Croll, in 1864[899] and subsequent years,
to reoccupy and fortify it. Within restricted limits (as Lagrange and,
more certainly and definitely, Leverrier proved), the path pursued by
our planet round the sun alternately contracts, in the course of ages,
into a moderate ellipse, and expands almost to a circle, the major axis,
and consequently the mean distance, remaining invariable. Even at
present, when the eccentricity approaches a minimum, the sun is nearer
to us in January than in July by above three million miles, and some
850,000 years ago this difference was more than four times as great. Dr.
Croll brought together[900] a mass of evidence to support the view,
that, at epochs of considerable eccentricity, the hemisphere of which
the winter, occurring at aphelion, was both intensified and prolonged,
must have undergone ex