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 90° 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 2° 45′, and the monthly equation, at a maximum, is about 15′, being a plus correction in the northern hemisphere, where the moon is between her descending and ascending node, reckoned on the plane of the vortex, and a minus correction, when between her ascending and descending node. And the mean longitude of the node will be the same as the true longitude of the moon's orbit node,--the maximum correction for the true longitude being only about 5° ±. [Illustration: Fig. 7]