REPORT ON THE MAGNETIC AL OBSERVATIONS. 5 



Vincent and Kerguelen isJand. Tristan d'Acunha and the Fiji islands are open to 

 suspicion on this head, indeed, this remark may be said to apply to all islands, more 

 especially when situated far from a continent. 



Furnished as the " Challenger " was with instruments for the relative determination 

 of the magnetic elements at sea, the difficulties and doubts arising from local disturbing 

 influence on land were avoided in a great degree by swinging the ship (as before 

 described) ; thus, as it were, transferring the place of observation to another at which the 

 local disturbance was accurately known, namely, that caused by the iron of the ship. 

 The relative determinations so obtained were then referred to land base stations known to 

 be free from local disturbing causes. 



The results at the numerous stations where the ship was swung during the voyage bear 

 ample testimony to the value of the process. 



When the ship returned to England and was paid off, the magnetic instruments em- 

 ployed during the voyage were taken to the primary base station (Kew Observatory), for a 

 re-determination of the several constants. The satisfactory condition of the absolute instru- 

 ments after the long voyage as was then ascertained enhances the value of the results 

 obtained with the relative instruments, as these in the later part of the voyage, from 

 prolonged use, became dependent upon frequent comparisons with the results obtained 

 with the absolute instruments. 



In the abstracts of the magnetical results of the voyage will be found : (1.) A general 

 abstract containing the values finally adopted. (2.) The several results at land stations 

 from which the finally adopted values have been deduced. (3.) The original sea observa- 

 tions with their several corrections are given in detail. 



The sea and land observations of Inclination and Force, with the Declination results 

 by the unifilar, have, since the return of the "Challenger" to England, been re-examined, 

 and the necessary reductions, as also the corrections for the ship's iron, made by Com- 

 mander (now Captain) Maclear and Lieutenant Bromley. Those of the Declination made 

 at sea, and by the several compasses on shore, were made by Staff- Commander Creak. 



For the methods of reduction employed in the discussion of the sea observations, re- 

 ference must again be made to the " Magnetical Instructions." The formulae in the addenda 

 were used for computing the corrections of the Inclination and those due to the correction 

 of ship's head in the Intensity observations. The final correction of the latter element 

 was obtained from a comparison of the mean Total Force on board at swinging stations 

 with the Total Force at the adjoining land stations (when free from local disturbance) 

 the values at the intermediate positions at sea being derived by interpolation. This 

 method avoids the troublesome question of temperature correction always, it is true, 

 small, but thus reduced to inappreciable values. 



For the corrections of the Declination observations made at sea, tables were computed 



