528 THE APPLICATIONS OF PHYSWAL FORCES. [BOOK v 



needle comes to rest after certain oscillations inclined to the horizon 

 at an angle which may be read off on the graduated circle. It is 

 this angle that measures the magnetic dip at the time and place of 

 observation. 



It remains to say a few words about an important application of 

 the declination and inclination compasses we mean the scientific 

 determination of these two elements and their diurnal, annual, and 

 secular variations at different points of the earth's surface. It is a 

 most interesting line of research, and at the same time of the greatest 

 use in navigation and geography. 



The study of the magnetism of the surface of the globe has shown 



that the declination, the inclina- 

 tion, and the intensity change 

 from one place to another in a 

 pretty continuous, but very ir- 

 regular, manner in relation to 

 geographical positions. To repre- 

 sent the state at a given time 

 Humboldt conceived the happy 

 idea of drawing on terrestrial 

 globes or charts three series of 

 lines. The isogonic lines are 



curves joining all the points 

 which have the same easterly 

 declination or the same westerly 

 declination. The isoclinic lines 

 similarly indicate the places on 

 the earth where the dip, either to north or south, is the same ; and 

 lastly, a third series is composed of isodynamic lines, that is, the 

 chains of points on the globe where the intensity of the force 

 of terrestrial magnetism has the same value. It appears from 

 an examination of these lines that there are in the neighbourhood 

 of the two geographical poles two points to which the isogonic lines 

 converge, and which are the common centres of the isoclinic but not 

 of the isodynamic curves. These are the magnetic poles of the 

 globe. At these two points the declination compass is indif- 

 ferent, while the needle of the inclination compass there maintains 

 a constantly vertical direction. As to the isogonic lines, not 



FIG. 341. Dip circle. 



