REPORT ON THE MAGNETICAL RESULTS. 9 



amount of 11', but quite confirm the general result that a remarkable movement of the 

 needle in a vertical direction is going on there. To estimate, however, the full value 

 of what has just been said, it is necessary to follow further the voyage of the 

 Challenger as far as the island of Ascension. With the marked local mag-netic 

 disturbance found on this island it has not been considered a trustworthy method to 

 compare land observations of different epochs not made exactly in the same position. 



Sabine's lines for 1842-45, however, are well supported by observation in and near 

 the island, and may be considered a near approximation to exact values. Comparing 

 the Challenger's results by swinging near the island with Sabine's, the following 

 values of the secular change are obtained. The declination is increasing 8' annually, 

 the south inclination increasing 14', and the horizontal force decreasing 0'013 (B.U.). 



There has been therefore not only considerable annual change going on at the two 

 positions, but the notable fact is made evident that the north-seeking end of the 

 needle is found to be moving in opposite directions, downwards at Sandy Point, and 

 more strongly upwards at Ascension. It was hardly to be expected that such large 

 and opposite movements of the needle should be confined to the spots where they were 

 discovered, and investigations in the surrounding countries and seas prove such to 

 be the case. If therefore the Challenger's observations in the North and South 

 Atlantic Oceans and seaboard were to be utilised satisfactorily for any given epoch, in 

 conjunction with those from various sources, and observed at different times, some means 

 must be adopted of gaining a fairly approximate knowledge of the secular change. 

 Although these remarks apply with special force to the Atlantic, there are sufficient 

 grounds for applying them to all parts of the world. 



For this purpose the author of this Report made a collection of a large number of 

 observations of the magnetic elements for all parts of the world — in many cases extend- 

 ing over a long number of years — and these have been discussed, and approximate 

 values of their secular change obtained. 



The several values were entered on maps against the positions where the observations 

 were made, and their relative accuracy noted. Thus results from fixed observatories, if 

 taken for a period of fifteen or twenty years, would be accepted as of full weight ; whilst 

 others at minor stations, where two or three observations only had been made, and the 

 exact positions of the observers were imperfectly known, one half or one quarter weight 

 would be allotted. This premised, lines of equal value of the secular change were then 

 drawn, and the following general results, as regards the annual angular movement of 

 the north-seeking end of the freely suspended needle during the epoch 1840-80, were 

 found clearly marked out. Commencing with the map showing equal lines of annual 

 change of the declination, it was found that there are two principal lines of little or no 

 change. The first took the following course — Starting from St. John's, Newfoundland, 

 it crossed the Atlantic in a south-easterly direction, striking the west coast of Africa 



(PHYS. CHEM. CHALL. EXP. PART VI. — 1888.) 2 



