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



The observations for Declination or Variation of the compass made both on land and 

 at sea, with the various "compasses furnished for that purpose, were under the charge of 

 Staff-Commander Tizard. The compasses were previously examined at the Admiralty 

 Compass observatory at Deptford, and rendered practically free from error. 



As the fitting out of the " Challenger " at Sheerness drew towards completion, the 

 most favourable positions for the Standard compass, and also for the gimbal table to 

 receive the Dip and Intensity apparatus designed by Mr E. W. Fox, were selected. 



The Standard compass was placed on a pedestal on the steering bridge above the 

 quarter-deck, its card being 12 feet 9 inches above the deck, 43 feet 3 inches from the 

 tafrail, and 97 feet from the steam funnel. This will hereafter be referred to as the 

 Standard position. 



The " Fox " gimbal table was placed on the after side of the mizenmast, supported 

 by four metal pillars secured to the upper deck ; the centre of the " Fox " circle when 

 mounted for observation was 3 feet 9 inches above the deck, 28 feet 1 inch from the tafrail. 

 This will hereafter be referred to as the Fox position, and the compass used there to 

 determine the magnetical coefficients of the position as the Fox compass. 



The system of observation to be pursued throughout the voyage, given in detail in 

 the "Magnetical Instructions for H.M.S. Challenger," may here be described briefly 

 as follows : — 



(1.) At each land station the absolute Declination, Inclination, and Horizontal Intensity 

 were to be determined, and especially at places where fixed Observatories were established. 

 When time did not admit of the more elaborate and delicate instruments being used, 

 Fox circles were to be employed in obtaining observations for Inclination and Total Force. 

 With this view, and also to secure a base for the relative observations made at sea, 

 complete sets of observations were directed to be made with one or more Fox circles at 

 all land stations. 



(2.) As magnetic elements determined at sea were known to be affected by th e 

 influence of a ship's iron, it was directed that the " Challenger" should be swung for ascer- 

 taining the errors of the Standard compass, and of the Fox circles and compass in given 

 localities. Beginning with the port of departure, the ship was afterwards to be swung 

 on a change of magnetic Inclination of 30° to 40°, also on any considerable alteration in 

 the distribution of the iron in the ship's equipment, should such take place, and in regions 

 where the Inclination was very small or very large, and finally at the termination of the 

 voyage. 



(3.) At sea, observations of the three elements were to be made daily, subject of 

 course to vicissitudes of weather. 



(4.) Copies of all observations were directed to be transmitted at an early date to the 

 Admiralty for examination. This rule, which was faithfully observed throughout the 



