TERRESTRIAL MAGNETISM, ETC. 237 



round stem of a fine kind of wooden lucifer match, 

 sometimes met with, answers very well, the head 

 with the inflammable substance being of course cut 

 off; but the stem of an ordinary match may be 

 taken, one end of it slightly flattened to allow the 

 needle to be pressed through it easily, and the 

 whole thinned away so much that it will just 

 barely float the needle. The needle must be ad- 



FlG. 31. 



justed so that it will rest horizontally with the 

 wooden bar vertical over it. The bar ought to be 

 longer than half the length of the needle, otherwise 

 there is a difficulty in preventing one end or other 

 of the needle from rising to the surface of the 

 water. If the bar is seen to project even so much 

 as one-tenth of an inch above the surface of the 

 water, it should be cut shorter ; and the part of it 



