870 FRAGMENTS OF SCIENCE 



done, into the error of screening off the magnetism 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 

 movements 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, heat it to vivid redness and plunge ' 

 it into cold water. It is thereby hardened; rendered, ia 

 fact, almost as brittle as glass. Six inches of this, mag- 

 netized in the manner of the darning-needle, will be bet- 

 ter 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 mag- 

 netized 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 distribu- 

 tion of the magnetism in your strip of steel. Hold the 

 strip upright in your left hand, and cause the test- needle 

 to approach the lower end of your strip; one end of the 

 test- needle is attracted, the other is repelled. Eaise jrour 

 needle along the strip; its oscillations, which at first were 

 quick, become slower, opposite the middle of the strip 

 they cease entirely; neither end of the needle is attracted; 

 above the middle the test- needle turns suddenly round, its 

 other end being now attracted. Go through the experi- 

 ment thoroughly: you thus learn that the entire lower 

 half of the strip attracts one end of the needle, while the 



