174 TERRESTRIAL PHENOMENA. 



of the Carolinas. la investigating the laws of terrestrial 

 magnetism, it is no slight advantage that four-fifths of the 

 magnetic equator are oceanic, and are thus easily accessible, 

 and that we now possess the means of determining the de- 

 clination and inclination on board ship with great exactness. 

 The changes which alter the places of the nodes, and modify 

 the form of the magnetic equator, are felt in the remotest 

 regions of the earth, where also they produce changes of the 

 inclination and of the magnetic latitude. ( 157 ) 



We have spoken of the distribution of magnetism on the 

 surface of our planet according to the two forms of declina- 

 tion and inclination: it still remains to notice the third 

 form, that of the intensity of the force, which is expressed 

 graphically by isodynamic curves, or curves of equal inten- 

 sity. The investigation and measurement of this force by 

 means of the oscillations of a vertical or horizontal needle, 

 has excited a general and lively interest, which, in its appli- 

 cation to the distribution of the magnetic force on the 

 surface of the globe, commenced with the present century. 

 By the application of refined optical and chronometrical 

 means, the measurement of the horizontal force, in par- 

 ticular, has become susceptible of a degree of accuracy 

 exceeding that of all other magnetic determinations. 

 Doubtless the isogonic lines are of the greatest practical 

 importance, from their use in navigation ; but in respect to 

 the theory of terrestrial magnetism, the isodynamic lines are 

 those from which the most fruitful results are expected. ( 158 ) 

 The first fact in reference to these lines which direct observa- 

 tion made known was the increase of the intensity of the total 

 force in proceeding from the equator towards the pole.( 159 ) 

 It is to the unwearied activity of Edward Sabine, from the 



