1875.] Variation in the Obliquity of the Ecliptic. 289 
avoid admitting a fact which is not popular with a certain 
arty. 
z Sie writers who have opposed the idea of any change 
in the obliquity greater than 1° 12’ have asserted that it is 
easy to find a curve that will correspond with the recorded 
obliquity during three or four hundred years. Such a state- 
ment plainly shows that the writers have never attempted 
to test their assertion; for not only will no other curve 
agree with the arc thus defined, but if we assume a centre 
of even 5° 50’, instead of 6°, from the pole of the ecliptic, 
the curve then traced out will in twenty or thirty years 
differ several seconds from recorded observations, instead of 
agreeing with them to within one-tenth of a second for two 
hundred years. It is unfortunate that writers should make 
such incorrect statements, and also that persons, in their 
endeavour to oppose an original investigation, should publish 
assertions relative to this subject which—had they really 
read the books* which treat of it—they never could with 
truth have ventured to write. 
As an example of the very small amount of enquiry or 
thought which has been devoted to this problem, we will 
call attention to the supposed unanswerable objection which 
certain writers have brought against the proof of the course 
traced by the earth’s axis being a circle round the point 
6° from the pole of the ecliptic. ‘These objectors assert that 
such a course is ‘‘ impossible,” dnd is against the laws of 
gravitation, and is opposed to all the analogy of the solar 
system. They add that the orbits of all planets are ellipses, 
and that therefore no such thing as a circular course can be 
possible for the pole of the heavens. The repetition, by 
copying of this statement, shows how powerful it is sup- 
posed to be. 
In the first place, these objectors forget that the present 
orthodox belief, which they are endeavouring to defend, is 
that the pole of the heavens traces a circle round the pole 
of the ecliptic asa centre. When they assert that because 
the orbit of a planet is an ellipse, therefore the pole of the 
heavens cannot trace a circle, as such a course is opposed to 
certain laws, they forget that every zenith traces an ex.1ct 
circle round the pole of the heavens every twenty-four 
hours,—also that the conical movement of a planet’s axis 
is geometrically the same as a second rotation of that 
planet. Consequently it would be opposed to experience if 
the earth’s axis traced anything but a circle during one 
* The Last Glacial Epoch. The Motion of the Fixed Stars. 
