.

There are evidences of longer climatic cycles than eleven or 35 years.
Bruckner calls attention to the fact that sometimes two of his periods
seem to merge into one. E. Richter shows much the same thing for the
Alpine glaciers. Evidence of considerable climatic changes since the
last glacial period is not lacking. But as yet nothing sufficiently
definite to warrant general conclusions has been brought forward.

_Geological Changes in Climate._--Changes of climate in the geological
past are known with absolute certainty to have taken place: periods of
glacial invasion, as well as periods of more genial conditions. The
evidence, and the causes of these changes have been discussed and
re-discussed, by writers almost without number, and from all points of
view. Changes in the intensity of insolation; in the sun itself; in the
conditions of the earth's atmosphere; in the astronomical relations of
earth and sun; in the distribution of land and water; in the position of
the earth's axis; in the altitude of the land; in the presence of
volcanic dust;--now cosmic, now terrestrial conditions--have been
suggested, combated, put forward again. None of these hypotheses has
prevailed in preference to others. No actual proof of the correctness of
this or that theory has been brought forward. No general agreement has
been reached.

_Conclusion._--Without denying the possibility, or even the probability,
of the establishment of the fact of secular changes, there is as yet no
sufficient warrant for believing in considerable _permanent changes over
large areas_. Dufour, after a thorough study of all available evidence,
has concluded that a change of climate has not been proved. There are
periodic oscillations of slight amount. A 35-year period is fairly well
established, but is nevertheless of considerable irregularity, and
cannot as yet be practically applied in forecasting. Longer periods are
suggested, but not made out. As to causes, variations in solar activity
are naturally receiving attention, and the results thus far are
promising. But climate is a great complex, and complete and satisfactory
explanations of all the facts will be difficult, perhaps impossible, to
reach. At present, indeed, the facts which call for explanation are
still in most cases but poorly determined, and the processes at work are
insufficiently understood. Climate is not absolutely a constant. The
pendulum swings to the right and to the left. And its swin