ELEMENTARY MAGNETISM. 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 use 
of sensible images, more or less gross, he will first give 
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 
