h, we must
remember there are two different kinds of matter,--one ponderable, the
other not ponderable; yet both subject to the same dynamical laws. If we
consider the axis of the terral vortex to coincide with the axis of the
lunar orbit, the moon and earth are placed in the equatorial plane of
the vortex, and consequently there can be no derangement of the
equilibrium of the vortex by its own rotation. But even in this case,
seeing that the moon's orbit is inclined to the ecliptic, the
gravitating power of the sun is exerted on the moon, and of necessity
she must quit the equatorial plane of the vortex; for the sun can exert
no influence on the _matter_ of the vortex by his attracting power. The
moment, however, the moon has left the equatorial plane of the vortex,
the principle of momentum comes into play, and a conical motion of the
axis of the vortex is produced, by its seeking to follow the moon in her
monthly revolution. This case is, however, very different to the
illustration we gave. The vortex is a fluid, through which the moon
freely wends her way, passing through the equatorial plane of the vortex
twice in each revolution. These points constitute the moon's nodes on
the plane of the vortex, and, from the principles laid down, the force
of the moon to disturb the equilibrium of the axis of the vortex,
vanishes at these points, and attains a maximum 90d from them. And the
effect produced, in passing from her ascending to her descending node,
is equal and contrary to the effect produced in passing from her
descending to her ascending node,--reckoning these points on the plane
of the vortex.

[Illustration: Fig. 6]

INCLINATION OF THE AXIS.

By whatever means the two planes first became permanently inclined, we
see that it is a necessary consequence of the admission of these
principles, not only that the axis of the vortex should be drawn aside
by the momentum of the earth and moon, ever striving, as it were, to
maintain a dynamical balance in the system, in accordance with the
simple laws of motion, and ever disturbed by the action of gravitation
exerted on the grosser matter of the system; but also, that this axis
should follow, the axis of the lunar orbit, at the same mean
inclination, during the complete revolution of the node. The mean
inclination of the two axes, determined by observation, is 2d 45', and
the monthly equation, at a maximum, is about 15', being a plus
correction in the northern hemisp