12 ADAMS AND EVANS. — TERRESTRIAL MAGNETISM. 



purpose of determining the total force by Dr. Lloyd's method, 

 which is independent of any changes in the magnetic moments 

 of the needles employed. It should be observed that the poles of 

 the additional needles so furnished are never to be reversed or 

 disturbed* 



3. Azimuth compasses (in duplicate), fitted with Admiralty 

 Standard circle, special needles, and levelling foot screws, for 

 determining the absolute declination. The declination as determined 

 by this instrument at a fixed station is to be considered as the zero 

 to which the observations made with the difi^erential declinometer 

 (mentioned in the following paragraph) are to be referred. 



4. Portable declination magnetometer, for differential observa- 

 tions only at a fixed station. 



5. Mr. Fox's apparatus for observing the inclination and force 

 (in triplicate) for use in sledge or travelling parties. [This instru- 

 ment is generally known as Fox's Circle]. 



6. Three-inch prismatic compasses (nine in number) for observ- 

 ing the declination (variation of the compass), to be used by sledge 

 or travelling parties. 



At winter quarters, and in an observatory established at a 

 distance from the ship, so as to be free from the disturbance of her 

 iron, it is assumed that the declination magnetometer will be firmly 

 secured on its pedestal ; and pedestals or stands arranged for the 

 reception of the unifilar magnetometer and Barrow's circle, at 

 suitable distances apart, to avoid inter-disturbance among the 

 magnets of the several instruments. The necessary observations 

 should, if possible, then be made as follows : — 



It may be expedient frequently to determine the absolute declina- 

 tion with the azimuth compass specially furnished, for the purpose 

 before winter darkness sets in, so that a reliable zero may be 

 obtained for the differential observations of declination. From 

 6 to 9 a.m., and from 4 to 6 p.m., will probably be found the best 

 times for observing, on the assumption that the declination is then 

 at or near the mean daily value. 



The line of de torsion of the declination magnetometor should 

 be carefully adjusted at the outset. 



The inclination by Barrow's circle, and also the total (relative) 

 force by means of Lloyd's needles to be observed once a week. 

 Occasionally, it is desirable that the observations with Lloyd's 

 needles should be repeated several times in the day, in order to 

 find the approximate amount of the diurnal changes. 



The absolute horizontal intensity to be determined once a month, 

 avoiding days of unusual disturbance as denoted by the declination 

 magnetometer. At the same time, or nearly so, observations 

 should be made with Lloyd's needles employed as deflectors at 

 different distances, in order to obtain absolute results. | 



* See Admiralty Manual, Appendix 2 B., p. 105. 

 t See Lloyd's Treatise on Magnetism, Art. 97, p. 99. 



