282 Variation in the Obliquity of the Ecliptic. (July, 
ago admitted that the obliquity of the ecliptic had been 
35 instead of 233°, geologists would at once have accepted 
this as a full and complete explanation of the great 
climatic change shown to have occurred on earth, and they 
would never have searched for those feeble or incomplete 
causes which are so inefficient when we endeavour to explain 
facts by their aid. 
Thus it may be fairly claimed that geology, the science 
above all others of facts, is most completely in favour of a 
great variation in the obliquity, and almost demands itas a 
cause to explain the known effects. 
We will next take the analogy of the planetary system, 
and we there find that there is no apparently arbitrary law as 
regards the inclination of the axis of a planet to the plane of 
its orbit. We find that the axis of Uranus lies almost in the 
plane of its orbit, so that the arctic circle extends from the 
poles almost to the Equator. In Venus, nearly the same 
condition exists, the arctic circle extending to within about 
15° of the Equator. In Jupiter, however, the axis of diurnal 
rotation is nearly at right angles to the plane of the planet’s 
orbit. The inclination of the axis of diurnal rotation of 
Saturn and Mars does not differ much from 663°,—the 
earth’s condition. 
Analogy in Nature is an argument that must be used with 
great caution. It does not at all follow that because some 
planet has an axis of diurnal rotation, so inclined to the 
plane of its orbit that the ar¢tic circle reaches nearly to the 
Equator, therefore our own planet was once in the same 
condition. Whilst, however, we use this caution, we must 
not forget that, if the analogy of the solar system had been 
allowed to weigh in the arguments long ago fought out, 
relative to the form and movement of the earth, its spherical 
or spheroidal form, and its rotation on its axis, might have 
been received without much opposition, when it became known 
that other planets were spheroidal in form, and that they as 
well as the sun rotated round an axis. The link wanting to 
bring that which seems probable into the region of proof, is 
to show that there is now actually going on some movement 
or some change in the earth’s axis which will, may, or 
might formerly, have caused the angle of inclination of this 
axis to the plane of the earth’s orbit to be a very different 
angle to that which it now is. This link, therefore, is the 
problem on which rests nearly the whole question connected 
with the probable or possible variation in the obliquity. 
It is a fact known to observers that, during the past 
2000 years at least, there has been a change in direction of 
