1. If the earth’s surface be an ellipsoid of revolution, whose moments of inertia round the polar and equatorial axes are C and A, and if
μ
be the mass of a mountain placed on any meridian, with coordinates
z, x
, it is required to find the change of position in the earth’s axis caused by the addition of the mass
μ
(supposed in the first instance to be placed upon the earth
ab extra
). If λ be the latitude on which
μ
is placed, we have tan λ =
z/w
; and if
θ
be the angle made with the earth’s axis by any axis x in the meridian of
μ
, if I be the total moment of inertia round this axis, we have I=A sin
2
θ
+C cos
2
θ
+ μ (
x
2
cos
2
θ
+
z
2
sin
2
θ
–2
xz
sin
θ
cos
θ
) . . . (1) The new axis of rotation is that which makes I = maximum, or
d
l = 0, from which we find, after some reduction, —tan 2
θ
=2
μxz
/(C–A)+μ(
x
2
–
z
2
. . . . (2) If we make
θ
= maximum, or
dθ
=0, we ascertain the position in which the mass
μ
must be placed so as to produce the maximum shift in the position of the earth’s axis.