CONTENTS. XI 



ART. PAOI: 



3i. Special values of E\ historical physical change in the value ; 



lines upon the Earth's surface, passing through points of 

 equal horizontal force . 74 



SECTION V. 



ON TERRESTRIAL MAGNETISM, AS ACTING IN THE VERTICAL AT EACH 

 PLACE OF OBSERVATION: AND ON THE COMBINATION OF THE 

 HORIZONTAL AND VERTICAL FORCES, AND THE TOTAL TERRESTRIAL 

 MAGNETIC TORCE, AT EACH PLACE OF OBSERVATION. 

 Articles 3542. Pages 7799. 



35. First evidence of the existence of a vertical magnetic force . 77 



36. Description of the Dipping Needle 78 



37- Manipulation of the Dipping Needle; reversion of its pivot- 

 bearing ; rotation round its vertical axis ; reversal of its 

 magnetic poles ......... 81 



38. Mathematical Theory of the Dipping Needle : first, on the 



supposition that the magnetic intensity after reversion is 

 equal io that before reversion; simplification when the 

 needle- is very nearly balanced 83 



39. Mathematical theory of the Dipping Needle continued; 



secondly, on the supposition that the intensity is not the 

 same after reversion, and that the needle is not nearly 

 balanced .......... 86 



40. Theory of the Dipping Needle, when the dips are observed 



in different vertical planes, inclined to the plane of the 

 magnetic meridian 88 



41. Determination of the Total Terrestrial Magnetic Force at 



any locality: lines upon the Earth's surface passing through 

 points of equal dip, and lines passing through points of 

 equal total force ; historical changes . . . . .90 



42. Reference to the points of principal interest in Figures 20, 21, 



28, 29, 35, 36; secular change in the place of North 

 Magnetic Pole 94 



SECTION VI. 



THEORIES 'ON THE PHYSICAL CAUSE OR REPRESENTATION 



OF TERRESTRIAL MAGNETISM. 

 Articles 4349. Pages 100121. 



43". Reasons for believing that Terrestrial Magnetism is not pro- 

 duced, in any important degree, by magnetic forces external 

 to the Earth . 100 



