250 POPULAR LECTURES AND ADDRESSES. 



removed to thousands of miles from the earth. In 

 experiments with a terella the needle is attracted 

 obliquely or directly towards the globe with a very 

 perceptible force. This is because the length of the 

 needle is so considerable in proportion to the 

 diameter of the globe that the magnetic forces on its 

 two ends are not equal and parallel. But the 

 length of the largest of mariner's compass needles 

 is not more than about -4 oWo-<rs-o> an d the length of 

 the largest bar magnet that has ever been suspended 

 so as to show by its movements any motive ten- 

 dency it may experience from the force of terrestrial 

 magnetism is not more than 1 ^ -, of the earth's 

 diameter, and therefore magnetic needles or bar 

 magnets experimented on in any part of the world 

 experience as wholes no sensible attraction towards, 

 or repulsion from, the earth, and show only a direc- 

 tional tendency according to which a certain line 

 of the magnet called its magnetic axis takes the 

 direction indicated by the curved lines of force in 

 our diagram. The word pole has been much used, 

 but somewhat vaguely, to express a point in, or 

 near, the surface of a body where there seems some- 



