INSTRUMENT FOR MEASURING MAGNETIC ATTRACTIONS, &C. 251 



of 7° 30' thus reduced, calling the dip 71° 30', gives the true cor- 

 rection 7° 6'. This subtracted from 26°, the observed co-dip, leaves 

 18° 54' for the inclination of the magnetic equator, which, I be- 

 lieve, is not greatly wide of the truth. 



3. Before a magnet can attract iron, that is totally free from 

 both permanent magnetism and that of position, it infuses in- 

 to the iron a magnetism of contrary polarity to that of the at- 

 tracting pole. 



Exp. 1. — Bar No. 1., with the compass half an inch from it, had no 

 attraction or repulsion when the bar was at a mean angle of 35^-° ; 

 the same bar, with the compass at -^-ths of an inch distance, gave 

 the mean angle of no attraction 27|-° ; at 1-L inches distance, 24° ; 

 at 2-^g- inches, 20^° ; and at 2- 6 - inches distance, the mean angle 

 of no attraction was 19^°. This change, in the angle of no attrac- 

 tion, with the distance, evidently shews that some magnetism of 

 position was necessary to counteract the magnetism communicated 

 by the needle, which was considerable when very near, but became 

 scarcely sensible at the distance of two or three inches. 



Exp. 2. — To prove that the magnetic needle, at short distances, 

 comunicates magnetism to the bar, until it is balanced by the 

 magnetism of position, I placed the compass 1\ inches from the 

 bar No. 1. and found the mean angle of no attraction to be 24°; 

 then placing another small needle at the same distance from the 

 bar, with its north pole abreast of the north extremity of the bar, 

 I found the angle of no attraction changed to 20i°. In this case, 

 magnetism was communicated by the needle to the north extremi- 

 ty of the bar, and the same polarity by the other needle to the 

 south end of the bar, so as to counteract the influence of each other ; • 

 hence, had the magnetic power of both needles been the same, the 

 bar would have been freed from all communicated magnetism but 

 that of position, and would have pointed out the true line of no 

 attraction. 



vol. ix. p. i. i i 4. A 



