TERRESTRIAL MAGNETISM. 399 



del Fuego focus, it will !)«' found moving to the westwai'd on the east 

 side of the whole line of no declination from that focus to near Van- 

 couver Island; to the eastward on the west side. 



A similar result may be seen in the line passing through a minor 

 focus of the dip near Hongkong. 



Judging from analogy, there should be another focus of change in 

 the dip in latitude T0° north, longitude 115'- east, or about the posi- 

 tion assigned to the Siberian focus of greatest force. " 



On Map No. 2 are shown lines of equal value of the declination — the 

 red lines for the year 1880, the black lines for the year 189.5. From 

 these, when shown on a large scale, w^e may deduce the mean annual 

 change which has taken place in the declination during the fifteen 

 years elapsed. 



In this map we are reminded of the ditferent results we obtain in 

 diti'ereiit localities; for if a line be drawn from Wellington, in New 

 Zealand, past Cape York, in Australia, to Hongkong, little or no 

 change will be found in the neigh])oring region since 1810. Again, 

 the line of no change in the declination shown on Map No. 1, to be 

 following nuich the same direction as the great mountain ranges on 

 the west side of the American Continent, has hardly moved for many 

 years, according to the observations available. 



On the other hand, let us now turn to an example of the remarkable 

 changes which may take place in the declination unexpectedly and 

 locally. The island of Zanzibar and the east coast of Africa were con- 

 stantly being visited b}^ our surveying ships and ships of war up to 

 the year 1880, observations of the declination being made every year 

 at Zanzibar during the epoch 1870-1880, The results showed that from 

 Cape Town nearly to Cape Guardafui the annual change of that element 

 hardly exceeded 1'. 



During the succeeding years of l81H»-!tl o1)servations were made I)}' 

 the Germans at Dar-es-Salaam and some other places on the neighbor- 

 ing coasts, with the result that the declination was found to he chang- 

 ing at first 3 minutes annually, and since that period it had leached 10 

 to 12 minutes at Dar-es-Salaam. Subsequent observations at the 

 latter place in 1896-1898 confirmed the fact of the great change, ;uid 

 in addition our surveying ship on the station, specially ordered to 

 "swing" at different places in deep water off the coast, generally con- 

 firmed the results. It is remarkable that while such great changes 

 should have taken place between (-ape Town and (^ape Guardafui, 

 Aden and the region a])out the St I'aifs of lia)) el Maiidel) seem to be 

 comparatively unafi'ected. 



''The results described in (he four precediiii,' paragraplis aiul given in Maj) No. 1 

 were also exhibited on a 12-inch glohe with tlie addition of magnetic nieridiann for 

 the epoch 1880, terminating in the MippoHcd positions of the magnetic poles. This 

 12-inch globe is now in the South Kensington Museum, London. 



