THE VOYAGE OF H.M.S. CHALLENGER. 



II— MAGNETICAL INSTRUCTIONS for the Voyage of 

 H.M.S. Challenger, Captain G. S. Nares, 1872. 



As in the forthcoming voyage of H.M.S. Challenger a large extent of the surface of the globe 

 will be passed over, it is desirable in the interests of nautical, as well as of natural science, that 

 " Terrestrial Magnetism," among other subjects of inquiry, should engage attention ; and when the 

 ship is at sea be considered a matter of daily observation. For this purpose, instruction has been 

 afforded at Kew Observatory, and in this Department, to certain of the officers in the use of 

 instruments necessary for the determination of the magnetic elements both on land and at sea, and a 

 supply of these instruments furnished. 



The instruments comprise — 



(1.) A portable Unifilar Magnetometer for determining the absolute horizontal intensity and the 



declination on land ; 

 (2.) A Barrow's circle with two reversible needles for deternrining the inclination on land; 

 (3.) A Kater's azimuth compass, with levelling table, for determining the declination on land, — 



to be considered as an auxiliary to the declination apparatus of the unifilar magnetometer ; 

 (4.) Fox's apparatus or circle (in duplicate) for observing the magnetic inclination and force at 



sea primarily ; 

 (5.) A gimballed table complete, adapted for Fox's circle as to be used at sea ; 

 (6.) A compass bowl and standard card, adapted to occupy the place of the Fox circle, when 



necessary to obtain certain of the ship's magnetic coefficients at the assigned position of 



the Fox circle, as also the compass deviation there ; 

 (7.) A small vibrating needle to be employed for determining independently the ship's mean 



Horizontal Force to the north, necessary for the reduction of the Inclination and Intensity 



observations made at sea with the Fox circle. 



The article on Terrestrial Magnetism given in the " Admiralty Manual of Scientific Enquiry," by 

 Sir Edward Sabine, late President of the Eoyal Society, affords most valuable information on the 

 subject, not only in the aim of the several observations which are required and the ultimate object in 

 view, but also in the use of the several instruments. The careful consideration and study of this 

 article cannot be too strongly urged on those entrusted with the duties defined in its several sec- 

 tions ; indeed, it must be held as forming the basis of instructions to all officers conducting magnetic 

 surveys at sea, especially those paragraphs (7, 11, 12) concerning the relative measurements of the 

 Magnetic Force, the summary of the observations to be made, and the record and transmission of the 

 observations. 



In the Appendixes, No. 1, — No. 2 so far as it relates to Inclination observations, and in No. 3 

 (omitting paragraphs 5 and 6 of Section II.), will be found clear directions for using the instruments 

 furnished to the " Challenger." 



Considering, however, the delicate nature of magnetical observations, and the exactitude required 

 by modern science in their residts, the experience of those who have been engaged in the performance 



