1 2 o NUTA T1ON OF THE EAR TH. 



hence, this slow deviation will have become very percep- 

 tible; astronomers will then see the pole of the world in 

 another constellation, and, as this displacement is con- 

 tinuous, the prolonged axis of the earth will have traced 

 on the firmament, in 25,000 to 26,000 years, a circle 

 parallel to the plane of the Ecliptic, and having for its 

 centre the pole of that plane. This circle is the base of 

 a cone whose summit rests upon the earth. (Fig. 27, a.) 



But this imaginary defined circle (which appears elliptic 

 on account of the perspective) is but the mean of a series 

 of oscillations around the pole of the world, which changes 



b its position, as we 



have just shown. (Fig. 

 27, b.) These oscil- 

 lations originate in 

 the circumstance that 

 the axis of the earth 

 inclines alternately for- 

 ward and backward, 

 FIG. c?. in such wise, that a 



star, after having approached the Pole, immediately after- 

 wards recedes from it; they cause the terrestrial globe to 

 resemble the head of a man who, by an alternation of 

 gesture, says alternately yes and no. Only, while man (the 

 puppet !) occupies but a second or two in affirming and 

 denying the same thing, the earth employs about eighteen 

 years and a half in inclining once forward to say yes (in 

 Latin, adnuere\ and once backward to say no (in Latin, 



