X CONTENTS. 



AP.T. PA6B 



23. More accurate methods of determining the Local Magnetic 



Meridian ; Reversed Telescope carried by Magnet ; elimination 

 of corrections for position of Magnetic Axis, and for torsion 

 power of suspension-thread .46 



24. Terrestrial Magnetic Meridians; Historical Physical Changes 



in the system of Magnetic Meridians ..... 50 



25. Imperfect Method of measuring the horizontal directive force 



of terrestrial magnetism at any locality, by vibrations of a 

 magnetic needle. Correction for the torsion-power of the 

 suspension-thread ........ 53 



23. Experimental determination of the proportion of the magnet- 

 power of a magnet to the Earth's horizontal magnet-power 

 by the method of deflexions. First process, the disturbing 

 needle applied broadside-on . .... 56 



27. Experimental determination continued. Second process, the 



disturbing magnet applied end-on . . . . . 61 



28. Experimental determination continued. Each of the preceding 



operations performed with the needles at different distances. 

 Reference to numerical observations. The discussion of the 

 first process alone, the discussion of the second process alone, 

 and the comparison of the two processes, shew independently 

 that the attraction or repulsion of magnetic masses is in- 

 versely as the square of the distance between them : Final 

 Law of Magnetism . . . .. .. . . .62 



^9. Inference as to the numerical value of the proposfcion of A to 



E. Remark on the unit of measure for A and E . .Co 



30. Investigation of the most advantageous proportion of the 



two distances of the disturbing magnet from the disturbed 

 needle 67 



31. Incidental inference as to the effect of temperature upon the 



magnet-power of the disturbing magnet, and necessity for 

 correction for temperature in various experiments . .68 



32. Accurate determination of the Earth's local horizontal magnet- 



power, founded on the method of deflexions, used in com- 

 bination with the measure of the Earth's horizontal action 

 upon the disturbing magnet 70 



33. Investigation of the proportion in which the numerical value 



for E will be altered, when, instead of using the foot and 

 the grain as units, we use other units, as the millimetre 

 and milligramme 73 



