ii8 ELEMENTARY LESSONS ON [CHAP. n. 



of latitude, but are irregular in form. Thus the line 

 joining places where the north-seeking pole of the 

 needle dips down 70 runs across England and Wales, 

 passes the south of Ireland, then crosses the Atlantic in 

 a south-westerly direction, traverses the United States, 

 swerving northwards, and just crosses the southern tip 

 of Alaska. It drops somewhat southward again as it 

 crosses China, but again curves northwards as it enters 

 Russian territory. Finally it crosses the southern part 

 of the Baltic, and reaches England across the German 

 Ocean. The chart of the world, given in Fig. 67, shows 

 the isoclinic lines of the Northern Hemisphere, and also 

 a system of " terrestrial magnetic meridians " meeting 

 one another in the North Magnetic pole at A. It was 

 prepared by the Astronomer-Royal, Sir George Airy, for 

 his Treatise on Magnetism. 



140. Variations of Earth's Magnetism. We 

 have already mentioned that both the declination and 

 the inclination are subject to changes ; some of these 

 changes take place very slowly, others occur every year, 

 and others again every day. 



141. Secular Changes. Those changes which re- 

 quire many years to run their course are called secular 

 changes. 



The variations of the declination previous to 1580 

 are not recorded ; the compass at London then pointed 1 1 

 east of true north. This easterly declination gradually de- 

 creased, until in 1657 the compass pointed true north. 

 It then moved westward, attaining a maximum of 24 

 27' about the year 1816, from which time it has slowly 

 diminished to its present value of 18 33' ; it diminishes 

 (in England) at about the rate of 7' per year. At 

 about the year 1976 it will again point truly north, 

 making a complete cycle of changes in about 320 years. 



The Inclination in 1576 was 71 50', and it slowly 

 increased till 1720, when the angle of dip reached 

 the maximum value of 74 42'. It has since steadily 



