rigid body on the whole, and does not permit the axis about which it is turning to change its position.
It will now be easily understood how it comes to pass that the position of the North Pole upon the earth has not appreciably changed in the course of thousands of years. The axis around which the earth rotates has retained a permanent position relative to the earth itself; but with regard to its direction in space it has continuously changed, it is at this moment changing, and will continue to change.
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Fig. 10.—Rotation of the Pole.
So far our knowledge extended up to within the last few years, but quite recently a closer inquiry has been made into this, as into so many other physical subjects, and the result has been to disclose the important fact that, though the phenomena as just described are very nearly true, they must receive a certain minute
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