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



into the outer jewelled hole, and the opposite pivot should be carefully guided into the hole at 

 the back as the bracket is screwed up tight. 



4. Unless there are particular reasons for a change, the same needle should always be used at sea, 

 and kept always mounted, being clamped before it is put away for the clay. 



5. As the instrument is furnished with two needles for the observations of Intensity, each needle 

 must be successively used in land observations, and the Inclination as well as the angles of deflection 

 with constant weights observed with it. 



6. Although every care is taken that all the weights which have the same nominal value should 

 be equiponderant, it is desirable, if possible, to preserve the same identical weights throughout the 

 whole observations of the same relative sines. 



7. The thermometer attached to the circle mast be observed at the commencement and close of 

 the observations of Intensity, whether with deflectors or weights. 



8. In reading the limb, be careful to bring the division nearest to the needle to coincide with the 

 corresponding division of the second graduated circle immediately behind it, by which means 

 parallax is avoided. 



9. In preparing for an observation at sea, the circle must be turned in azimuth till its plane 

 coincides with the magnetic meridian, when the ship is steady on her course, i.e., the course by the Fox 

 position compass. If the zero division of the horizontal circle coincides with the fore and aft line of 

 the ship, the plane of the magnetic meridian will be denoted by the vernier of the horizontal circle, 

 showing an angle corresponding to the difference between the ship's course and the magnetic 

 meridian. 



10. As a general rule, the observations for the magnetic elements should be multiplied when the 

 course of the ship lies at right angles to their several lines of equal amount, as delineated on magnetic 

 charts. This specially applies to the determination of the Inclination and Force when in the neigh- 

 bourhood of the magnetic equator (or line of no Dip), and also in the regions of least force. 



Magnetic charts of the several elements are appended to these instructions for the requisite 

 guidance. 



