1877.] 



On Magnetic Observations at Kerguelen. 



95 



III. " Magnetic Observations at Kerguelen." By the Rev. S. J. 

 Perry, S.J., F.R.S. Received March 15, 1877. 



The Government expedition to Kerguelen Island for the observation 

 of the Transit of Venus on December 8th, 1874, presented a very favour- 

 able opportunity for the accurate determination of the magnetic elements 

 of an important station in the South-Indian Ocean, and one which will 

 at most be visited only at distant intervals for the purpose of scientific 

 investigations. When, therefore, I heard of my appointment to that 

 station, I at once brought the question of terrestrial magnetism under 

 the notice of the Astronomer Royal, and he readily agreed to my pro- 

 posal of taking a complete set of magnetic instruments to Kerguelen, and 

 of making any observations that would not interfere with the main object 

 of our expedition. 



Being in charge of the whole Kerguelen party of observers, I could not 

 expect to be able to devote much time personally to the magnetic work ; 

 but the Rev. W. Sidgreaves, whom long experience, both at the Stony- 

 hurst Observatory and during our magnetic survey of Trance in 1868 

 and 1869, had made perfectly conversant with all the details both of 

 instruments and observations connected with terrestrial magnetism, had 

 already been placed on the staff of astronomical observers for Kerguelen. 

 The assistance of a very efficient observer being thus secured, the next 

 step was to procure the necessary instruments. Fortunately I experi- 

 enced no difficulty in this matter, as the authorities at Kew immediately 

 placed at my disposal a Jones unifilar and a Barrow dip-circle. There 

 was no question of the want of a good chronometer, as the astronomers 

 were to be supplied with nine of these, besides the eight reserved exclu- 

 sively for longitude connexions, which remained always undisturbed in 

 their quiet berth on shipboard. 



It was at first proposed by Sir Edward Sabine that we should take a 

 series of magnetic observations at sea during our vo^yage from England 

 to the Cape of Good Hope and thence to Kerguelen, and a special 

 instrument was ready for the purpose ; but as it was finally arranged 

 that we should perform the first part of our journey in the mail-steamer, 

 it was thought advisable by the Hydrographer of the Admiralty to relin- 

 quish all idea of taking magnetic observations at sea. 



The land instruments were made use of on almost every available 

 occasion, both at Kerguelen and during our journey ; but I will confine 

 myself in this paper to the Kerguelen results, reserving the other obser- 

 vations for a separate communication. 



The constants for the temperature-correction, and for other data 

 regarding the magnets employed, were kindly determined for me by 

 Mr. "Whipple at Kew. 



No correction for error of graduation of deflection-bar was found 

 necessary. 



