290 Variation in the Obliquity of the Ecliptic. |July, 
revolution of the equinoxes, or, as it may be termed, one 
second rotation of the earth; and the analogy between the 
“‘ orbit of a planet’ and ‘‘ the course traced by the pole” is 
as appropriate as would be the course of a line of rails and 
the movement of the piston of a locomotive. 
The problem, however, connected with the possible varia- 
tion to a great extent in the obliquity of the ecliptic, is one 
which is considered entirely of a mathematical nature, and 
the assertion that no variation greater than about 1° 12’ could 
occur in the obliquity is the result of the investigations of 
theorists. 
When we examine the manner in which this problem has 
been treated, we find that certain important omissions have 
occurred, on which the real question at issue is dependent ; 
and instead of the question being one that can off-hand be 
resolved into a mathematical problem, it is in reality one 
that can and must be decided long before it assumes%he 
form of a mathematical investigation. 
The problem of a possible change in the value of the 
obliquity to any great amount has been confined almost 
entirely, hitherto, to an enquiry as to the extent to which 
the plane of the ecliptic could vary from a mean position; and 
when M. La Place concluded that the plane of the ecliptic 
could vary only 1° 21', it was assumed that the obliquity of 
the ecliptic could vary only that amount. Such a conclu- 
sion is incorrect. The obliqiity of the ecliptic being the 
complement of the angle which the earth’s axis makes with 
the plane of the ecliptic, it follows that the direction in 
which the earth’s axis moves is the problem for enquiry,— 
far more important as regards the solution than is that of 
the variation in the plane of the ecliptic. 
That the earth’s axis was formerly supposed to trace a 
circle round the pole of the ecliptic, as a centre, is evident 
from the writings and calculations of all the olden astrono- 
mers, and any change in this movement was supposed 
impossible. It is to this portion of the problem that we 
will now earnestly call attention. 
The present accepted theory of the producing cause of 
the conical movement or change in dire¢tion of the earth’s 
axis is as follows :—The earth is supposed to be a homo- 
geneous spheroid, with a protuberance of about 1-300 at 
the equatorial diameter. This protuberance,* being acted 
on by the mass of the moon and sun, causes a gyratory 
movement of the axis of diurnal rotation, and produces a 
* See Airey’s Lectures. 
