ELEMENTARY LESSONS ON [CHAP. n. 



netic charts (Art. 1 39) must be prepared, and the declina- 

 tion at different places accurately measured. The upright 

 pieces P P', on the " azimuth compass " drawn in Fig. 

 65, are for the purpose of sighting a star whose position 

 may be known from astronomical tables, and thus 

 affording a comparison between the magnetic meridian 

 of the place and the geographical meridian, and of 

 measuring the angle between them. 



137. Inclination or Dip. Norman, an instrument- 

 maker, discovered in 1576 that a balanced needle, 

 when magnetised, tends to dip downwards toward the 



north. He there- 

 fore constructed a 

 Dipping -Needle, 

 capable of turning 

 in a vertical plane 

 about a horizontal 

 axis, with which he 

 found the " dip " 

 to be (at London) 

 an angle of 71 50'. 

 A simple form of 

 Dipping-needle is 

 shown in Fig. 66. 

 The dip - circles 

 used in the mag- 

 netic observatory 

 at Kew are much 

 more exact and 

 delicate instru- 

 ments. It was, 



Fig. 66. 



however, found that the dip, like the declination, differs 

 at different parts of the earth's surface, and that it 

 also undergoes changes from year to year. The " dip " 

 in London for the year 1881 is 67 39'. At the 

 north magnetic pole the needle dips straight down. 

 The following table gives particulars of the Declination, 



