Mathematical and Physical Paper's. 201 



four hours. The axis of the earth about which it rotates points 

 to the center of that little circle, and any change can therefore be 

 determined by observing the motions of the pole star. It is found, 

 too, that the size of the circle increases or diminishes, thus indi- 

 cating that the earth's axis is pointing farther away from the pole 

 star, or nearer to it. This movement is due to precession, as has 

 been known ever since the days of Hipparchus. The circle de- 

 scribed by the present pole star is now getting smaller, but in the 

 course of years it will increase in size, until after thousands of 

 years it will cease to be the north star. Other stars, one succeed- 

 ing the other, each at about twenty-three and one-half degrees from 

 the pole, will become in turn the pole star. 



Again, the center of the circle may change its position, and no 

 longer be at the same height above the horizon of any given place. 

 If this is true, then the earth's axis must be shifting in the earth 

 itself; in other words, the north pole is not remaining in the same 

 place. If the change in position should ever become a large one 

 terrestrial climates would be subject to very great changes ; but the 

 truth is that the changes are small and the effect on climate is 

 scarcely appreciable. Fifteen or twenty years ago no one believed 

 that the north pole of the earth moved. Even the finest astro- 

 nomical observations of that time failed to detect any such move- 

 ment. Yet we have reached a point where the facts clearly show 

 that the north pole does move — not very much, but still to an ex- 

 tent that is easily measured by astronomical observations. 



Mr. Chandler found that there is a revolution of the earth's pole 

 in 427 days, from west to east, with a radius of thirty feet, meas- 

 ured at the earth's surface. He assumed, for the purpose of state- 

 ment, that this is "a motion of the north pole of the principal axis 

 of inertia about that of the axis of rotation ; the direction of the 

 former from the latter lay towards the meridian of Greenwich about 

 the beginning of the year 1890. This, with the period of 427 days, 

 will serve to fix approximately the relative positions of these axes 

 at any other time for any given meridian." This was in flat con- 

 tradiction to all accepted views. The only period in which the 

 earth's pole could revolve was believed at that time to be ten 

 months. The radius of its path had been declared to be insensible, 

 and now comes Mr. Chandler affirming that the period was a variable 

 one, and that the path had a radius of thirty feet. 



In confirmation of his theory, an examination of the observa- 

 tions of a German astronomer, of Berlin, furnished Mr. Chandler 

 another fact, namely, when the Cambridge, United States, latitudes 



