NOTES ON THE POSSIBILITY OF USING AVAILABLE VESSELS FOR DETERMINING 



MAGNETIC SECULAR-VARIATION 



In view of the improbability of building a nonmag- 

 netic vessel, at least in the near future, it becomes im- 

 portant to consider the possibilities of magnetic work on 

 vessels not especially constructed for the purpose. 



The principal object in building the Carnegie was to 

 eliminate ship-deviations so that observations could be 

 made frequently without the necessity of computing cor- 

 rections that require an Immense amount of office work, 

 that occasion delays in publication, and that are not very 

 satisfactory after all is done. 



The mathematical theory of ship-deviations as- 

 sumes that the ship may be magnetic from two sources, 

 that is, from transient magnetism induced in the soft 

 iron of the ship by the earth's field, and from permanent 

 magnetism in the hard iron. It assumes that the tran- 

 sient magnetism at any instant corresponds with changes 

 in orientation of the iron or changes in the earth's field 

 and that the permanent magnetism is constant. Experi- 

 ence indicates, however, that neither assumption is 

 strictly true. Changes due to time, buffeting of the seas, 

 and holding one course for many days in succession are 

 factors in the problem. Their effects cannot always be 

 controlled to the required degree of accuracy. 



Since the observations already made by the Galilee 

 and the Carnegie are fairly well distributed over large 

 ocean areas, the principal desiderata now in magnetic 

 surveys at sea are secular variation observations, which 

 are essential for keeping the magnetic charts up to date 



and for theoretical studies. It might be noted here that 

 the errors found in the magnetic data on charts by the 

 Galilee and Carnegie appear to be largely the result of 

 imperfect knowledge of secular change. Since secular 

 stations may be rather widely spaced over the oceans, 

 sufficient data could be obtained, say, once a week, de- 

 pending on the progress of the cruise. 



In accordance with these ideas, it is now proposed 

 to utilize the opportunities offered by the ship of any 

 scientific expedition provided she is not too magnetic, 

 and to eliminate the harmonic part of the deviations due 

 to her magnetic character by swinging her at every sta- 

 tion and to reduce the nonharmonic part by installing the 

 instruments at the most favorable location on board. 



The practical limits admissible in future work at 

 sea for the magnitude of the nonharmonic part of the de- 

 viations may be inferred from the parameters (see table 

 1) of the Erebus, Challenger . Gazelle. Gauss , Terra 

 Nova , and Discovery . These were all sailing vessels 

 with auxiliary steam or steamers with aiixiliary sail, 

 having wooden hulls iron-fastened. On the Challenger 

 and Gauss there was no iron within about thirty feet of 

 the magnetic instruments, other than the hull fastenings. 



In table 1 A, D, E represent so-called deviation- 

 coefficients for the magnetic elements, the parameters 

 c, f, g, h, k depend on the amount, arrangement and in- 

 ductive capacity of the soft iron of the ship, and P, Q, R 

 are parameters depending on the amount, arrangement. 



Table 1. Parameters of some wooden vessels 



Vessel and date 



D 



g 



f 



Q 



R 



cgs cgs cgs 



Erebus (1) 



1839-1842 0.991 0.0 +0.4 small +0.027 small +0.026 small +0.003 small small small 



Challenger (1) 



1873-1876 0.999 +0.1 +0.3 0.0 0.000 0.000 +0.008 0.000 -0.033 +0.013 0.000 -0.040 



Gazelle (1) 



1874-1876 0.980 +0.3 +0.6 -0.1 +0.013 +0.009 +0.021 -0.007 -0.021 +0.008 -0.003 -0.002 



Gauss (1) 



1901-1903 1.003 +0.3 +1.2 0.0 -0.005 0.000 -0.012 +0.001 -0.013 +0.002 0.000 -0.002 



Terra Nova (2) 



1913 1.000 +0.037 



Discovery (3) 



1904 0.973 0.0 +1.1 0.0 



Galilee (4) 



1905-1908 1.000 0.0 0.0 0.0 -0.001 -0.006 0.000 +0.001 -0.008 0.000 -0.001 -0.001 



+0.009 +0.007 



+ 0.003 0.000 -0.022 +0.003 0.000 +0.004 



1. Bidlingmaier, F. 1905. Anleitung zu wissenschaftlichen Beobachtungen auf Reisen--Magnetische Beobachtungen 



an Bord. p. 486. 



2. British Antarctic Expedition, 1910-1913. Terrestrial Magnetism, p. 447. 1921. 



3. National Antarctic Expedition, 1901-1904. Physical Observations, pp. 148-149. 1908. 



4. Researches of the Department of Terrestrial Magnetism. Carnegie Inst. Wash. Pub. No. 175, vol. 3, pp. 78-92. 



1917. 



91 



