292 THE POPULAR SCIENCE MONTHLY. 



angle from the geographical meridian, and this angle may vary 

 from 1° to nearly 90° in different parts of the globe. 



Thus, then, the variation may be defined as the angle between 

 the geographical meridian and the direction of the compass-needle. 



Another set of interesting magnetic lines are those of eqnal 

 DIP. They gird the earth in circles concentric with the magnetic 

 poles, just as the parallels of latitude do the geographical poles. 

 There is a magnetic equator along every point of which the com- 

 pass-needle is horizontal. As we travel from the magnetic equator 

 toward the north magnetic pole, the needle begins to incline, the 

 north end tending downward until, when we reach the vicinity of 

 the pole, the needle becomes vertical. 



If we travel toward the south magnetic pole, the same occurs 

 with the south end of the needle, now tending downward. An en- 

 tirely similar experience will result from carrying a small needle 

 through the magnetic field of the steel bar. At the neutral ground 

 it will be parallel to the bar, while, as we approach either end, the 

 dip toward the pole becomes more and more until it stands verti- 

 cal at the pole. And as it was stated, regarding the steel bar, that 

 the intensity of its magnetic field varies from point to point, so 

 with the earth, it also has a magnetic field which is powerful near 

 its poles and steadily moderates in strength as we aj^proach the 

 magnetic equator. 



A third set of lines are those of equal intensity. They are not 

 drawn in Fig. 2. In general contour they follow those of equal 

 dip, though, in point of fact, they are not identical with them. 



All these different systems of magnetic lines — variation, dip, 

 and intensity — have not on the earth that symmetry and regu- 

 larity which they would present around a steel bar ; on the con- 

 trary, they are often bent, looped, and turned into many a devious 

 path — wherefore, none can tell. The fact alone is well established, 

 while theories fail to account satisfactorily for the earth being an 

 irregular magnet. 



The observations that have determined the various magnetic 

 features of the earth have been made with delicate instruments in 

 stationary observatories in every country, and also on ships-of-war 

 in every sea. 



The magnetism of the earth is not fixed either in locality or 

 amount ; but the different systems of lines just described, and by 

 which it has been found convenient to represent this magnetism, 

 are ever varying — ever migratory. The hourly, daily, and other 

 periodic changes are all small, it is true ; but, however minute, 

 they are the object of inquiry at every magnetic observatory, with 

 the hope that in time, by the accumulation of data, a satisfactory 

 theory of the earth's magnetism may be deduced. 



As the part of the earth's magnetism which affects the com- 



