15 
empirical law beyond the limits of the observations from which 
it is deduced, even when it is much more scientific than a bare 
average. Lieut. -Col. Drayson’s circle is an approximation of the 
second order, and will satisfy the observations of four centuries 
much better than a simple average, which is of the first order 
only. But it will wholly mislead when carried beyond those 
limits ; for the true curve of the earth’s pole projected on the 
celestial sphere is not an excentric circle, but a kind of cycloid, 
or a circle of which the centre is ever moving, though within 
narrow limits. The pole of the equator does not move towards 
that of the ecliptic, but at right angles to the joining line, while 
the latter does approach to and recede from the pole of the 
equator. If the hypothesis were true, there is no reasonable 
doubt that it would involve the consequence of fierce extremes 
of summer heat and winter cold, over a large part of each 
hemisphere of the earth. 
VI. — The Theory op Increased Eccentricity. 
26. The most popular theory, at present, which offers a 
kind of geological chronology, is that of Mr. Croll, in his work 
entitled, “ Climate and Time in their Geological Relations.” 
It has been adopted by Mr. Geikie in his “ Great Ice Age,” 
by Sir C. Lyell, and apparently by many others, and has been 
developed, in a volume of live hundred pages, with great 
labour, research, and ingenuity. It professes to account for a 
recurrence of extremely cold or glacial periods by the coinci- 
dence of two astronomical elements, — an increased excentricity 
of the earth’s orbit at certain past dates, and the position of the 
northern winter solstice near the aphelion. It is held, further, 
that when the southern winter solstice was in the aphelion, there 
would be a similar period of glaciation of the southern hemi- 
sphere. Mr. Croll has calculated the excentricity, by Lever- 
rier’s formulas, at intervals of 50,000 years, for three millions 
of years of past, and one million of future time, and every 
10,000 years for the last million only. He discovers two 
maxima, 850 and 210 thousand years ago, and identifies them 
with a Miocene and a Post-Pliocene Ice Period, assumed to be 
proved by modern geology. The first signs of man’s presence 
on the earth are usually held to be either soon after, or else 
just before, the Boulder Drift, the second of these periods. 
The effect, then, of Mr. Croll’s theory would be to place the 
entrance of man on our planet above two hundred thousand 
years ago. During this vast interval, thirty times greater 
