1878.] 



during Transit of Venus Expedition. 



3 



to the epoch 1840-45, by Sir Edward Sabine (Cf. Phil. Trans., 1868), 

 it may be well to compare the values for the two epochs, in order to 

 ascertain the secular variation. 



Table III. 



Station. 



1312-5. 



1875. 



Annual change. 



Cape Town .... 



- 53°-5 



- 56° 2' 22" 



- 4'-69 





18 -4 



19 14 40 



+ 1 56 



Aden 



5- 1 



5 28 9 



+ 63 



Malta 



53- 5 



51 36 42 



- 3 49 



Naples (1845) . . 



58° 43' 45" 



57 23 



- 3 45 



Rome (1847) . . 



59 49 4 



58 50 55 



- 2 09 



Florence (1844) . 



61 54 30 



60 13 



- 3 28 



The values of the dip at the Cape of Good Hope, from 1751 to 1851, 

 have been collected in vol. i of the Cape Magnetic Observations, and 

 these give, for the yearly increase in the south dip of the needle, 

 - 6'-94 from 1751 to 1840, - 5'-45 from 1841 to 1846, and - 5'*58 

 from 1843 to 1851. The quantity in Table III shows that this decrease 

 in the acceleration is still continuing. 



The inclination observed at the College in Bombay is very extra- 

 ordinary, and would at first sight appear to have been entered incor- 

 rectly. The result, however, agrees perfectly with the observed angles, 

 and as two complete observations were taken with Needle Xo. 3, and 

 these fairly agree, it may be supposed that the instrument was stand- 

 ing at the time over an unsuspected mass of iron capable of deflecting 

 the suspended needle, through an angle of nearly a degree. The paper 

 by Mr. C. Chambers, in the Phil. Trans, for last year, gives 19° 8'*5 as 

 the most probable value of the dip for January 1st, 1871, at the Colaba 

 Observatory, with a secular change of + l'"9. This magnetic element 

 would, therefore, appear to be altering more rapidly at present than its 

 mean rate of change during the last 30 years. The angle observed in 

 1875 is very slightly in excess of that computed for the same epoch 

 from the data of Mr. Chambers. 



The low value of the secular change at Rome is quite exceptional, 

 but if we compute this quantity for the epoch 1849 from the figures 

 given in Sir E. Sabine's report, we again obtain - 2' '09, showing 

 that, if there be an error, it is due to local causes. 



The Magnetic Intensity. 



At each station the horizontal component of the magnetic force was 

 determined absolutely by vibrations and deflections, and the total 

 intensity deduced from the horizontal force combined with the dip. 

 The magnets employed throughout were the same as those used at 

 Kerguelen, and no departure from the usual methods was ever found 

 necessary. 



B 2 



