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



from the coefficients B, C, D, the coefficients A and E being neglected, and a value for 

 D deduced from the several swingings made under the most favourable conditions. 



The magnetic effects arising from the ship's heeling have been closely investigated. 

 At the Standard compass position these were so small as to be safely neglected, they 

 were, in short, masked by errors of observation. 



At the Fox position, the errors arising from the heel of the ship were more appre- 

 ciable ; their maximum value affected the Inclination and Force with the ship's head on 

 the east and west points of the compass, vanishing on the north and south points. [As 

 the motion of the ship at sea was generally that of rolling or lurching, rather than that of 

 a steady heel or incline from an upright position, an oscillation of the suspended needle 

 instead of a determinable deflection resulted; it was unnecessary to apply any correction 

 on this account to the results recorded in the tables.] 



The term H, as chiefly employed in the " Addenda to the Magnetical Instructions " for 

 the correction of the Inclination and Force observations at sea, has been obtained from the 

 observations of Horizontal Force (in absolute measure) at the land stations, expressed in 

 terms of the Horizontal Force at Kew as unit, the intermediate values of H (at sea) being 

 interpolated with the aid of the chart of Horizontal Force given in the "Admiralty 

 Manual" for the deviations of the compass, 4th edition, 1874. 



A few remarks upon the magnetic character of the ship as affecting the determina- 

 tions made at the standard and Fox positions are here offered, in order to show how fully 

 this character has been realised for all periods of a voyage extending from the 51st 

 parallel of north latitude to the 67th parallel of south latitude, and more than this, crossing 

 and recrossing the magnetic equator six times ; also as a measure of assurance to those 

 who may be surprised at the magnitude of some corrections recorded in the abstract of 

 sea observations. 



The swinging of the ship for the determination of her coefficients in so many places 

 where the Inclination and Horizontal Force had been ascertained through the medium of 

 absolute instruments, furnished a ready means to this end. 



Accompanying the tables of coefficients will be found values as computed from them 

 of the constant parameters P, Q, R, c and d at the two instrumental positions. From 

 these we learn that by far the greater parts of the ship's force proceeded from the effects 

 of permanent magnetism common to hard iron, that from induction common to soft iron 

 being comparatively small. 



It is interesting to follow the changes : the magnetic character impressed upon the 

 ship previously to her leaving England continues nearly unchanged until the magnetic 

 equator is reached ; it then gradually alters in the south magnetic hemisphere ; shortly 

 after the highest southern latitude is attained, the extreme limit of change is accom- 

 plished. This new magnetic condition remains constant until again passing the 



