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184 in the direction opposite to the direction of rotation.

Now, in Figure 49, let CP be the axis of a spinning

top, CQ the vertical line; the immediate tendency of the force of gravity is to bring the axis CP further from CQ (or to make the top fall) and if the top were not spinning, it would make CP recede further from CQ; but it will be found that, in consequence of its spinning, the inclination of CP to CQ does not sensibly alter (till the spinning motion is retarded by friction), but CP revolves slowly round CQ in the same direction as the direction of rotation. So that if Figure 50 represent a view of the top from above, if the top be spun in the direction marked by the arrow, its axis will reel in the direction PRS. This, as I have said, is strictly analagous to the precessional motion of the earth, although, from the immediate tendency of the forces being of an opposite kind, the ultimate effect is of an opposite kind.

The following astronomical effects of this precession deserve attention.

First, the celestial equator, (which, as I said in a former lecture, is the great circle in the heavens, which at every point is 90 degrees from the Pole,) is in fact