Scientific Lectures. 171 



axis, pointed at its lower end, which rests in a cup of agate placed in 

 the top of this wooden column. This axis is a powerful magnet, and 

 I place it on the column, near this lantern, so that its pole and the 

 center of the needle of the vertical lantern are in the magnetic meri- 

 dian. You observe that the needle is at rest in that position while 

 the axis of the ring is vertical. 1 now set the ring in rapid rotation, 

 and, inclining its axis, I let it take care of itself. See how majesti- 

 cally it slowly tilts its ring around the horizon, always keeping 

 the same angle to it, while at the same time its magnetic axis 

 revolves and swings around a vertical line. Thus the magnetic 

 pole of this axis, revolving in a circle, is now to the east and 

 now to the west of the undisturbed position of the needle, and you 

 behold on the screen the lantern needle, slowly swaying back- 

 ward and forward, in subjection to the guidance of the revolving 

 pole. And observe how completely it represents, in a few minutes, 

 what takes place only through centuries in the grand cycle of the 

 needle's variation. The axis, in revolving around the vertical, has 

 now gained its extreme easterly position, and the motion is nearly in 

 a line with the length of the needle, and therefore the latter is at rest ; 

 but now the magnetic axis is moving westerly and the needle keeps 

 pointing towards its pole, and is moving faster and faster, until now 

 the axis is sweeping directly across the needle's natural position. The 

 needle is with a gradually diminishing velocity approaching the end 

 of its western swing, and now is again momentarily at rest, for the 

 revolving pole is running on the western bend of its orbit. Thus 

 every swing of the needle follows the same law of motion as rules 

 the vibration of a pendulum, which is also the motion of the mag- 

 netic needle as through 320 years it moves from one extreme side of 

 the meridian to the other. 



Various hypotheses have been formed to account for this wonder- 

 ful phenomenon. As far as observations extend, they seem to show 

 that the very magnetic pole of the northern hemisphere is revolving 

 in a direction opposite to that of the hands of a watch around a fixed 

 geographic point, just as you have in like manner seen the pole of 

 this magnetic axis rotating around a fixed vertical line ; and into this 

 polar revolution the magnetic equator is supposed to tilt its plane 

 around the terrestrial equator.* Thus the change of position of the 

 magnetic poles and equator f cause constant movements in the lines 



♦Observations on the Magnet Orbit, by the Rev. H. Grover, London, 1850. 



tSee Sabine's Chart for years 1825 and 1837, in his Contributions to Terrestrial Magnetism, 1840. 

 Humboldt, Cosmos, vol. i, p. 178. 



