240 POPULAR LECTURES AND ADDRESSES. 



again, the magnetism once given by the little 

 magnet may be reversed by drawing the same end 

 of the same bar-magnet in the contrary direction a 

 sufficient number of times along the needle. If, 

 however, the needle has been magnetized by a 

 more powerful magnet, it may be found difficult or 

 impossible to reverse its magnetism by the simple 

 operation described above. A convenient way of 

 testing the direction shown by the needle, is to 

 draw a black line on a piece of white paper, and 

 place it below the tumbler or finger-glass. Turn 

 the paper round until the needle, resting in the 

 centre of the glass, is seen to be exactly over the 

 line. Deflect the needle from this position again 

 and again, and you will find it always coming with 

 great accuracy to the same line. 



Dr. Gilbert of Colchester, Physician in ordinary 

 to Queen Elizabeth, discovered the true explana- 

 tion of this wonderful phenomena to be that the 

 earth acts as a great magnet upon the movable 

 needle, and thus founded the science of terrestrial 

 magnetism. But an explanation of this discovery 

 must be reserved for a continuation of the present 



