18 THE VOYAGE OF H.M.S. CHALLENGER. 



[In the " Contributions to Terrestrial Magnetism," Nos. VII. and VIII., the values of q for seven 

 different needles furnished to Fox circles are recorded; namely -000072, -000116, -000117, -000069, 

 •000079, -000164, -000175, or a mean value of -000112. The influences of variations of temperatures 

 on such needles are therefore extremely small ; and, unless in cases when a more than ordinary 

 accuracy is desired, the corrections on this account may be regarded as insignificant.] 



Deflectors. — "When deflectors are used, there is also a correction required for differences of tempera- 

 ture at the different stations ; but as in this case the correction is for the changes of magnetism of the 

 deflector due to the variations of temperature, it must be applied in the opposite sense to the correction 

 when the constant weights are used ; that is, when weights are used an increase of temperature gives 

 an additive correction, and the contrary when the deflectors are used. 



[The N. and S. deflectors of two Fox circles furnished for the Antarctic voyage of the " Pagoda," 

 " Contributions to Terrestrial Magnetism," No. VIIL, were respectively -000123, -000154, and -000081, 

 ■000133.] 



General Remarks. 



Even at the risk of repetition, a brief recapitulation of the chief points to be observed in using 

 Fox's circle on board ship will not be without value. The testimony of the late Admiral Sir James 

 Ross on the merits of this instrument should not, however, be passed without notice, as it is probable 

 that the instrument is scarcely, if at all, practically known, to naval officers of the present day. 



In his " Antarctic Voyage," vol. i. chap, v., it is stated : " By means of the admirable contrivance 

 of Mr R. W. Fox, we were able, in tolerably moderate weather, to determine the magnetic elements 

 with even more precision on board our ships than they are susceptible of on shore, on account of the 

 unknown and indeterminable amount of local attraction ; and even in the heaviest gales, after a little 

 practice with his instrument, they may be observed with sufficient exactness to afford very useful and 

 important information." 



" Throughout the whole distance of between three and four thousand miles, from Kerguelen Island 

 to Van Dieman's Land, we could not have derived a single satisfactory result with the instruments in 

 common use ; and this portion of the ocean at least must for the present have remained a blank upon 

 our charts. But with Mr Fox's apparatus, the Dip and Intensity observations were accomplished in an 

 almost uninterrupted series of daily experiment." 



An able writer on Terrestrial Magnetism (author of the Adams Prize Essay for 1865)* describes 

 it " as one of the most important additions to the existing stock of magnetic instruments ever made, 

 not even excepting those introduced by Gauss." Alluding to the testimony of Sir James Ross, he adds: 

 " The principal cause of this superiority of Fox's instrument in taking observations at sea is its 

 stability, arising from the mode in which the needle is hung. As we have seen, the axis terminates in 

 very short cylindrical pivots which work in jewelled holes. By this construction any displacement of 

 the needle in consequence of the rolling or pitching of the vessel is prevented ; whilst the loss of sen- 

 sibility, which undoubtedly results from tliis mode of suspension, is a positive gain under such cir- 

 cumstances." f 



* Edward Walker, Esq., M.A., F.B.S. " Terrestrial and Cosniical Magnetism." The Adams Prize Essay for 1805. 

 Cambridge and London, 1866. 



t As there is no case recorded of Fox's circle having been used in a screw steam-ship, every precaution should lie 

 adopted to make observations at sea under favourable conditions. It is possible that the needle would require removal 



