REPORT OX THE MAGXETICAL OBSERVATIONS. 3 



voyage, proved to be of great value, especially at the commencement, as then results 

 unlocked for in a wood-built ship like the "Challenger" necessitated an "Addenda 

 to the Magnetical Instructions " to be drawn up and forwarded to the observers. Sug- 

 gestions as to fresh observations in certain directions, arising from an examination 

 of the abstracts occasionally transmitted to the Admiralty, were frequently sent to the 

 observers. 



When the " Challenger " was ready for sea, with all the iron fittings in their places, 

 the requisite swingings to determine the magnetical coefficients of the ship preparatory 

 to the voyage were undertaken. As these swingings may be taken as an example of 

 those which followed, the details are here given somewhat fully. 



The ship still lay in Sheerness harbour, where she had been fitted out, and it being 

 the first week in December, with short days, strong tides, and unsettled weather, the 

 operation occupied four days. The first observation was to ascertain the deviation of the 

 Standard and Fox compasses by well known methods. Then followed a series of observa- 

 tions for determining the mean Horizontal and Vertical Forces at the two positions on 

 four azimuths, distributed round the compass. These forces were determined from the 

 vibrations of a small horizontal and a vertical needle, made on four widely separated 

 azimuths of the ship's head. 



The Fox circle was next mounted at its position, and observations made for Dip, both 

 direct and with deflectors, then for Intensity with the N. and S. deflectors taken separately, 

 the ship's magnetic azimuth being noted by the Standard compass. It was intended to 

 have observed on the sixteen principal points, but those on the E.X.E. and W.S.W. 

 points were unavoidably not completed. These several observations were carried out 

 under the superintendence of Staff- Commander E. W. Creak, of the Admiralty compass 

 department. 



On the investigation of these observations a strong Vertical Force acting upwards 

 at the Fox position, and of an intensity unusual in a wood-built vessel was found to 

 exist. As this was unexpected, an examination of the structural drawings of the ship 

 was made, which proved that this force proceeded from a number of diagonal iron 

 stringers, placed to strengthen the after section of the ship's hull, and extending from the 

 upper deck to near the keelson. As the value of the constituent parts of this vertical 

 force whether proceeding from the effects of hard or soft iron could not be ascertained 

 in one latitude, it was necessary to await the results of the Inclination observed on board 

 near the magnetic equator before framing additional instructions to meet the case. 

 Eevised formulae were compiled and forwarded as an " Addenda to the Magnetical Instruc- 

 tions " to the " Challenger " immediately the nature of the disturbing force was ascertained. 



The " Challenger," in a magnetical sense, was now ready for the prosecution of her 

 voyage. On the 20th of December 1872, shortly after sailing from Portsmouth, observa- 



