170 TERRESTRIAL PHENOMENA. 



for example, the true direction of the mountain Chains of 

 the Apennines and Pyrenees always remained unknown. 



The magnetic force of our planet is manifested at its 

 surface by three classes of phenomena ; one of these is the 

 varying intensity of the force, and the other two its varying 

 direction, shewn in the inclination of the magnetic needle 

 in the vertical plane, and in its declination from the geo- 

 graphical meridian. The aggregate effect may therefore be 

 represented graphically by three systems of lines, called 

 isodynamic, isoclinal, and isogonic ; or, of equal force, equal 

 dip or inclination, and equal variation or declination. The 

 distances apart, and the relative as well as absolute positions 

 of these lines, are undergoing continual change. At parti- 

 cular points on the Earth's surface, ( 15 ) for example in the 

 western part of the Antilles, and in Spitzbergen, the mean 

 declination of the magnetic needle has scarcely undergone 

 any sensible change in the course of the last hundred years. 

 Elsewhere, when the isogonic curves, in their secular move- 

 ment, pass from the surface of the sea to that of a continent 

 or island of considerable extent, they appear to be retained 

 for a time, and the curves become thereby inflected. The 

 gradual change in the forms of the lines which accompanies 

 their translation, and modifies the extent of the spaces which 

 are occupied by east or west declination, makes it difficult 

 to recognise the nature of the changes and the analogies of 

 form in graphic representations belonging to different cen- 

 turies. Each branch of a curve has its history ; but in no 

 case does that history reach farther back, among the nations 

 of the West, than to that memorable epoch (13th Sept., 

 1492), when the re-discoverer of the new world found the 

 line of no variation three degrees to the westward of the 

 meridian of the Island of Mores, one of the group of the 



