20 PRESIDENTIAL ADDRESS. — SECTION A. 



present time for the dip and the horizontal intensity are shown 

 on Charts I. and II. respectively, and that for the declination 

 up to 1906 on Chart III. In connection with Charts I. and II. 

 it is to be noted that they really contain the secular changes 

 for two distinct periods, one from about 1900 to 1904, the other 

 from 1903 to 1908. The stations common to the iwo periods 

 have in each case two numbers attached to them; the upper 

 number in each case refers to the later period. The results at 

 Pretoria, Potgietersrust, Newcastle, Illovo River, and Gin- 

 ginhlovu refer to the later period. The other stations with a 

 single number refer to the earlier period. The most striking 

 result in the case of the secular changes of dip is the decrease in 

 value as one goes north and east; the yearly change in the 

 Cape Peninsula and along the south coast of Cape Colony is 

 an increase of southerly dip amounting to between 8' and 9' per 

 year. In the Transvaal the value is about 6', and in Natal 

 between 3' and 4'. Another point to be noticed is the fact that the 

 rate of change at the present time is decreasing. In connec- 

 tion with Chart II., in which the secular change of the hori- 

 zontal intensity is given for the same periods, we see that the 

 rate of decrease is less in Bechuanaland and Rhodesia than it 

 is in Natal, and the latter change again less than over the 

 Cape Colony. In the case of this element the change shows 

 signs of increasing at the present time. In Chart III. the most 

 striking fact is the extremely high values of the declination 

 secular change along the eastern side of the continent ; the 

 figures in fhis chart all refer to the earlier period when in Cape 

 Town the yearly decrease of declination was about 4'. while 

 the change along the east coast from Port Elizabeth up to 

 Beira was about 10'. It will be noticed that the secular change 

 of declination shown in the earlier charts in their main outlines 

 •can only be a rough approximation to the truth; the isogonics 

 do not keep the same relative positions throughout the cen- 

 turies. 



The charts giving the isomagnetics for the earlier epochs, 

 prepared by Van Bemmelen and others, have been published 

 comparatively recently. The first to make use of this method 

 of showing magnetic results by means of isomagnetics was 

 Halley. His " Tabula Nautica," published probably in 1701, 

 f^ives the isogonics for the Atlantic and the Indian Oceans; he 

 does not attempt to draw the lines over the continents. Halley 

 himself says :• — 



" What is here properly New, is the Curve Lines drawn over the several 

 Seas, to show the degrees of the variation of the Magnetical Needle, or Sea 

 Compass : Which are designed according to what I myself found, in the 

 Western and Southern Oceans, in a Voyage I purposely made at the Publick 

 Charge, in the year of our Lord 1700, or have Collected from the Comparison 

 of several Journals of Voyages lately made in the Indian Seas, adapted to 

 the same year." 



He goes on further to say that the line of no declination — he 

 uses the sailor's term, variation — passes near " the Bermudas, 

 the Cape de Verde Isles, and St. Helena," and that 



" This Chart, as is said, was made by observations of the year 1700, but 

 it must be Noted, that there is a perpetual, tho' slow Change in the Variation, 



