24 POPULAR LECTURES AND ADDRESSES. 



of the earth's surface, as the reader will readily 

 see and understand by looking at the annexed 

 magnetic charts, Figs. 5 and 6 (pp. 22 and 23). 

 Observation shows that nowadays the lines of 

 horizontal magnetic force are as represented on 

 the diagrams before you. But a comparison with 

 observations made within the last 300 years shows 

 us that the magnetic poles and lines of force are 

 changing. Three hundred years ago (1576), in 

 London, the compass pointed to the east of 

 north. Two hundred and seventeen years ago 

 O^SQ)) the compass pointed due north there. 

 After that, for 164 years, it showed an increasing 

 westerly direction, till in 1823 it pointed 24' 30' 

 to the west of north, and began to come back 

 towards the north. Now its deviation in London 

 is only 20 30' west, and it is decreasing about 

 6' annually. Here, at Glasgow, the deviation is 

 at present about 24 west. 



20, The dip at London is now about 67 40', 

 at Glasgow 71, and for the British Islands it is 

 at present decreasing at about 2*69' annually. It 

 is ascertained to have been decreasing during 



