ADDENDA TO MAGNETICAL INSTRUCTIONS. 21 



ADDENDA to the MAGNETICAL INSTRUCTIONS for the Voyage of 

 H.M.S. Challenger, 1872. 



The most favourable positions having been selected for placing the Fox circle, and the Standard 

 compass in H.M.S. Challenger, a complete series of observations for the Dip, Intensity, and Devia- 

 tion were made on board at Sheerness, just before the ship left in December 1872 for her voyage of 

 exploration. The Dip and Intensity observations revealed the fact of a force acting upwards, unusual 

 in amount for a wood-built ship. An inquiry into the nature of the construction of the ship, causing 

 this unusual and unlooked-for disturbance, proved that this force resulted from a series of diagonal iron 

 stringers, extending from the upper deck to near the keelson, employed to strengthen the frames of the 

 after section ; a condition quite unsuspected. 



The f ormuLse given in the " Magnetical Instructions " for correcting the observations of Inclination 

 and Intensity, were those which had so well represented the effect of the " soft " iron of ILM. Ships 

 "Erebus" and "Terror." 



A discussion of the observations made in the tt Challenger " between England and the Magnetic 

 Equator in the Atlantic Ocean shows, however, that this upward Vertical Force is principally produced 

 by " hard " iron (not strictly constant), and involves the necessity of adding to the above-mentioned 

 formulae, coefficients representing the unsuspected force. 



In the " Contributions to Terrestrial Magnetism," Xo. VI., by Sir Edward Sabine, published in the 

 Philosophical Transactions for 1844, a memorandum by the late Mr Archibald Smith, MJL, LLJX, 

 F.K.S., is inserted, in which formulae are given introducing the required coefficients. 



Let E represent the Vertical Force produced by the " hard " iron of the ship ; then the following 

 additions to the original Instructions are necessary : 



T> 



Page 10, line 21, after "the additional coefficients c, d, A'" introduce . , 



T> 



11, 27, after " we require to know the coefficients B, D, c, d, A' " introduce ., TT . and 



p 



if we consider d tan 6 + ., = N , then 



A H 



11, 29, after " the variable coefficients c and," omit d and substitute N. 



The two succeeding formulae at the bottom of page 11 become 



e cos f + N = (1 2 sin D) sin f cosec f tan ff . 

 c cos f + N = (cos f + sin B) sec f tan ff . 



12, 7, after " With the values of c and d" substitute N for d. 

 The two succeeding formulae become 



tan ff = c -. . cos 



cos f 4- sin IS 



