364 FRAGMENTS OF SCIENCE. 



power over your implements. As you proceed, moreover, 

 you will be tempted to do more than I can possibly suggest. 

 Thoughts will occur to you which you will endeavor to fol- 

 low out ; questions will arise which you will try to answer. 

 The same experiment may be twenty things to twenty 

 people. Having witnessed the action of pole on pole 

 through the air, you will perhaps try whether the magnetic 

 power is not to be screened off. You use plates of glass, 

 \vood, slate, pasteboard, or gutta-percha, but find them all 

 pervious to this wondrous force. One magnetic pole acts 

 upon another through these bodies as if they were not 

 present. And should you become a patentee for the regu- 

 lation of ships' compasses, you will not fall, as some pro- 

 jectors have done, into the error of screening off the mag- 

 netism of the ship by the interposition of such substances. 



If you wish to teach a class you must contrive that the 

 effects which you have thus far witnessed for yourself shall 

 be witnessed by twenty or thirty pupils. And here your 

 private ingenuity must come into play. You will attach 

 bits of paper to your needles, so as to render their move- 

 ments visible at a distance, denoting the north and south 

 poles by different colors, say green and red. You may also 

 improve upon your darning-needle. Take a strip of sheet- 

 steel, the rib of a lady's stays will answer, heat it to vivid 

 redness and plunge it into cold water. It is thereby hard- 

 ened, rendered, in fact, almost as brittle as glass. Six 

 inches of this, magnetized in the manner of the darning- 

 needle, will be better able to carry your paper indexes. 

 Having secured such a strip, you proceed thus : 



Magnetize a small sewing-needle and determine its 

 poles ; or, break half an inch or an inch off your magnetized 

 darning-needle, and suspend it by a fine silk fibre. The 

 sewing-needle or the fragment of the darning-needle is now 

 to be used as a test-needle to examine the distribution of 

 the magnetism in your strip of steel. Hold the strip up- 



