136 ON MAGNETISM. 



attracts the blue end of the compass-needle : if the white 

 or upper end is at the level of the compass, it attracts 

 the red end of the needle : if the middle of its length is 

 at the level of the compass, it produces no sensible 

 disturbance. 



Yet this magnetism does not imply any permanent 

 modification in the state of the iron bar. For, invert 

 the bar, so that the white end is downwards, and apply 

 it in the same way to the experimental compass. Now, 

 the white end of the bar attracts the blue end of the 

 needle (instead of attracting the red end as it did before) 

 and the black end of the bar attracts the red end of the 

 needle (instead of attracting the blue end as it did be- 

 fore). The iron bar is for the time a magnet, but its 

 poles are in the direction opposite, as regards the struc- 

 ture of the iron, to that in which they were before. 



But they are in the same direction as regards up 

 and down. The upper end (whether white or black) is 

 always a blue pole, and the lower end, (whether black 

 or white), is always a red pole. 



These experiments are described as they are seen in 

 the northern magnetic latitudes of the earth. In the 

 southern magnetic latitudes, the lower end of the bar 

 has blue magnetism. At the magnetic equator, the ex- 

 periment fails in this form ; but a slight variation in the 

 form of the experiments, applicable in every place, ex- 

 hibits the induced magnetism in the greatest possible 

 intensity ; the variation is merely the following : 



Instead of holding the bar in the vertical position, 

 hold it in the direction of the local dip. Then it will 



