EL EMKNTA RY MA GNET1SM. 279 



change is very slow: it is therefore called the secular 

 change, and the observation of it has not yet extended over 

 a sufficient period to enable us to guess, even approximately, 

 at its laws. 



Having thus discovered, to some extent, the secret of 

 the earth's magnetic power, we can turn it to account. In 

 the line of " dip" I hold a poker formed of good soft iron. 

 The earth, acting as a magnet, is at this moment con- 

 straining the two fluids of the poker to separate, making 

 the lower end of the poker a north pole, and the upper end 

 a south pole. Mark the experiment: When the knob is 

 uppermost, it attracts the north end of a magnetic needle; 

 when undermost it attracts the south end of a magnetic 

 needle. With such a poker repeat this experiment and 

 satisfy yourselves that the fluids shift their position accord- 

 ing to the manner in which the poker is presented to the 

 earth. It has already been stated that the softest iron 

 possesses a certain amount of coercive force. The earth, 

 at this moment, finds in this force an antagonist which 

 opposes the decomposition of the neutral fluid. The com- 

 ponent fluids may be figured as meeting an amount of 

 friction, or possessing .an amount of adhesion, which 

 prevents them from gliding over the molecules of the poker. 

 Can we assist the earth in this case? If we wish to remove 

 the residue of a powder from the interior surface of a glass 

 to which the powder clings, we invert the glass, tap it, 

 loosen the hold of the powder, and thus enable the force of 

 gravity to pull it down. So also by tapping the end of the 

 poker we loosen the adhesion of the magnetic fluids to the 

 molecules and enable the earth to pull them apart. But, 

 what is the consequence? The portion of fluid which has 

 been thus forcibly dragged over the molecules refuses to 

 return when the poker has been removed from the line of 

 dip; the iron, as you see, has become a permanent magnet. 

 By reversing its position and tapping it again we reverse 

 its magnetism. A thoughtful and competent teacher will 

 know how to place these remarkable facts before his pupils 

 in a manner which will excite their interest. By the uso 

 of sensible images, more or less gross, he will first givo 

 those whom he teaches definite conceptions, purifying 

 these conceptions afterward, as the minds of his pupils 

 become more capable of abstraction. By thus giving 

 them a distinct substratum for their reasonings, he will 



