462 LIEUT.-GENERAL SABIXE ON TEEEESTEIAL MAGNETISM. 



precautions which were adopted to supply, as far as circumstances would permit, the 

 means of checking every part of the various processes. 



The difficulty which presented itself on the first aspect, and whilst the survey was as 

 yet only in the preliminary stage of contemplation, was to procure a proper haitis for the 

 determinations of the magnetic force. As the survey was designed to furnish not merely 

 a map of the Isodynamic lines corresponding to the present epoch, but also such deter- 

 minations as, repeated after the lapse of a century or centuries, should enable physicists 

 of future times to derive and place on a satisfactory foundation a general theory of the 

 secular changes to which the phenomena of each of the elements of terrestrial magnetism 

 are known to be subject, it was necessary that the values of the magnetic force should 

 be determined in absolute measure, at certain points which should serve as a base for the 

 whole operations of the survey, and should be so situated as to embrace them all. 



The difficulty which has been thus stated was surmounted by combining in one and 

 the same recommendation to Her Majesty's Government, the prosecution of the Southern 

 Magnetic Survey, and the establishment of fixed magnetic observatories at certain 

 localities in the British Colonies, two of which, the Cape of Good Hope and Hobarton, 

 were convenient of access and would comprehend between them nearly the whole of the 

 isodynamic lines which should be included in the survey. 



The groundwork of the survey, as regards the variations of the magnetic force, is 

 thus to be found in the determinations made at the magnetic observatories of Hobarton 

 and the Cape of Good Hope, of the absolute values of the magnetic force at those 

 stations in and about the years in which the survey was in progress. A summary of the 

 investigations on which these values are founded forms, therefore, a subject of primary 

 consideration in this section of the present Contribution: pages 463 and 464. 



The instruments and methods by which the variations of the magnetic force at other 

 land stations than Hobarton and the Cape of Good Hope, and in the almost daily 

 observations on board each of the ships, were investigated, were devised by Mr. Robert 

 Were Fox, F.R.S., and were described in publications at the date of their invention, 

 and more recently in the 'Admiralty Manual of Scientific Enquiry,' Ed. 1859, Appendix 

 No. 2. The mode of procedure for obtaining the ratios to an absolute value of the force 

 determined at a base station has been explained in No. III. of these Contributions, Phi- 

 losophical Transactions, 1842, Art. XI. page 9 et seq. In observations made at sea the 

 ratios, whether obtained by constant weights or by deflectors, are liable to be affected by 

 three sources of error, viz., (1) by the influence of the ship's iron, (2) by variations of 

 temperature producing corresponding variations in the magnetism of the needle, and (3) 

 by an alteration — progressive or sudden — which may possibly take place in the magnetism 

 of the needle in the course of the survey, and which when it does occur is usually a loss 

 of magnetism. Of these three sources of error the first is the most certain and impoilant, 

 and requires to be met by coiTections investigated and applied in modes analogous to 

 those already treated of in the cases of the Declination and of the Dip. The influence of 

 variations of temperature on Mr. Fox's needles has always been found on investigation 



