ADAMS AND EVANSJ. — TERRESTRIAL MAGNETISM. 11 



be brushed off the sheet by means of a feather as soon as all 

 the snow has melted. They must then be carefully preserved in 

 a tube or stoppered bottle and brought home for analysis. 



The same observer noticed that the black magnetic particles 

 were frequently seen in the " firn " or granular old snow above 

 which several layers of recent snow had accumulated ; it would 

 therefore be well to look out for the black grains below the snow. 



II.— TERRESTRIAL MAGNETISM. 



1. Memorandum on Determination of Elements and use of 

 Magnetical Instruments, bv Professor J. C. Adams, 

 M.A., F.R.S., and Captain Evans, C.B., F.R.S. 



The determination anywhere in the Arctic regions of the 

 elements, by means of which the earth's magnetic force is usually 

 expressed {^Declination, Inclination and Intensity) will be valu- 

 able : — if made within the limits of former voyages, by affording 

 the means of determining the approximate amount of the secular 

 changes by comparison with earlier observations ; — if made beyond 

 the limits of former explorations, by materially adding to our 

 knowledge of the distribution of the magnetic force over the earth's 

 surface, and thereby contributing towards the perfection of the 

 theory of Terrestrial magnetism. 



The multiplication of the observations to be made in the Arctic 

 Expedition being so much dependent on circumstances and 

 climate, no definite suggestions can be offered on this head ; it 

 may, however, be borne in mind that the several elements above 

 mentioned must be considered as possessing an equal importance, 

 and that the value of each new station is proportional to its 

 distance from those where observations have already been made. 



The Article on Terrestrial Magnetism in the Admiralty Manual 

 of Scientific Enquiry, by Sir Edward Sabine, K.C.B., with its 

 appendices on the use of the principal instruments furnished to 

 the Expedition, will be found an excellent guide for observers, 

 and should be carefully consulted. In lieu of the maps therein 

 referred to, provisional maps of the Magnetic Elements suitable 

 to the requirements of the Expedition are appended to this memo- 

 randum.* 



The instruments furnished comprise— 



1. Portable unifilar magnetometers (in duplicate) for determin- 

 ing the absolute horizontal intensity at a fixed station. 



2. Barrow's Circles (in duplicate) for determining the inclination. 

 These circles are further provided with additional needles for the 



* Magnetical instructions (in duplicate) for the use of portable instruments 

 adapted for magnetical surveys and portable observatories, &c. by Lieut. 

 C. J. B. Hidden, R.A., F.R.S., 1844, are also furnished. 



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