TERRESTRIAL MAGNETISM. 



293 



and intensity at various points on the earth is that which would 

 be produced by a large magnet inside of the earth with its axis 

 slightly inclined to the axis of rotation of the earth, as shown in 

 Fig. i. 



2. The compass. Definition of declination. The compass 

 needle is a horizontal magnet which is free to turn about a verti- 

 cal axis. The direction in which such a needle points at a given 

 place on the earth is called the magnetic meridian at that place, 

 and the angle between the magnetic meridian and the geographic 

 meridian is called the declination * of the earth's magnetic field 

 at a given place. 



3. The dip needle. Definition of inclination. The needle of 

 a compass is usually weighed at one end to make it lie in a hori- 

 zontal plane. A steel bar which is magnetized after being 

 accurately balanced on a horizontal pivot constitutes a dip needle. 

 When the horizontal pivot of the dip needle is placed at right 

 angles to the magnetic meridian, the 



needle points in the actual direction 

 of the earth's magnetic field, as 

 shown by the two suspended mag- 

 nets ns and ns in Fig. I, and the 

 angle of inclination of the needle is 

 called the inclination or dip of the 

 earth's magnetic field at the given 

 place. Figure 2 is a general view of 

 a dip needle, or dip circle, as it is 

 usually called. 



4. Magnetic elements. The di- 

 rection and intensity of the earth's 



magnetic field at a place is completely specified when the decli- 

 nation, the inclination, and the value of the horizontal component 



* Sometimes called the variation of the compass. This word variation, how- 

 ever, is here used to designate the changes which are continually taking place in the 

 earth's magnetic field. 



Fig. 2. 



