1877.] Magnetic Observations at Kerguelen. 



101 



The declination magnet used throughout these observations was inca- 

 pable of rapid reversion ; and therefore the zero of the scale was tested 

 carefully by several reversions previous to our departure from the island, 

 and it was found to be 53*9. A very favourable series of reversions at 

 Cape-Town Observatory had previously given 54-2 as the zero division. 



The above readings of the declination show an absolute maximum of 

 35° 57' 12" at 10 a.m. on February 11th, and a minimum of 35° 39' 33" 

 at 2.45 p.m. on February 6th ; the range, therefore, for the month, as far 

 as observed, is 17' 39'', and the mean declination =35° 48' 22"-5. The 

 daily means give a slightly larger W. declination, with a range of only 

 10' 12" for the whole series of observations. 



Nothing more of course than the very roughest idea can be formed of 

 the diurnal range from the few observations taken ; but there is some 

 evidence at least of an easterly movement of the needle between 10 a.m. 

 and 3 p.m., followed by a westerly motion that continued till 7 p.m. The 

 greatest mean velocity of the magnet was about 3' 40" per hour at 2 p.m. ; 

 but the velocity once reached 8' an hour, viz. at 10 a.m., during the dis- 

 turbance on February 11th. As there are many disturbing causes that 

 probably affect the earth's magnetism as a whole, it may not be irrelevant 

 to remark that an examination of the Stonyhurst magnetograms on 

 all the days occurring in the above Table of Declinations shows that 

 February 11th was the only disturbed day in England, and that, with the 

 exception of a slight tremulous motion of the needle on November 13th 

 and 14th, on December 10th, and on February 8th, the observing days 

 were remarkably quiet. 



Besides the series of observations taken at Observatory Bay, other 

 determinations of the declination were made at Swain's Haulover, at 

 Thumb Peak, and at Molloy Point ; but as some doubt still remains to 

 be cleared up respecting the errors and rates of the chronometers em- 

 ployed in the sun observations, I will defer the publication of the results 

 until this essential point is satisfactorily settled. 



Previous to these observations, taken in connexion with the Govern- 

 ment Transit-of- Venus Expedition, the only magnetic observations at 

 Kerguelen on record, if we except any possible results obtained by Captain 

 Cook in the last century, are those of Sir J. B-oss in 1840 and of H.M.S. 

 * Challenger ' in 1874. The values contained in Sir Edward Sabine's 

 " Contributions to Terrestrial Magnestism, No. XI." (Phil. Trans. 1868), 

 furnish the data necessary for a comparison with our present work. 



Adopting 71° 56' 28" for the dip of the south end of the needle at 

 Observatory Bay on January 1st, 1875, and comparing this value with 

 the observation of Sir J. Boss in 1840, after correcting the dip of — 70 o, 

 at Christmas Harbour by — o, 4 for change of station to Royal Sound, 

 we find a secular variation of about — 2'*7. But if we consult the Table 

 of numerical coefficients deduced from all the collected observations, and 

 take the value of the dip in 1840 for the station whose longitude is 70° E. 



