112 GENERAL BEARINGS OF MAGNETIC OBSERVATIONS. 



Before dismissing - tlie subject of the above application of magnetic 

 observations, it may be remarked that we have now heavily armed 

 protected steel cruisers steaming over all parts of the world with less 

 change of deviation of the compass than the wood-built Erebus and 

 Terror of Boss's Antarctic expedition, and this remarkable result could 

 not have been achieved if the terrestrial magnetic observer had not 

 done his work. 



Moreover, if magnetic observations are not continued, the secular 

 change of the magnetic elements will soon commence to mar the pre- 

 cision with which our rapidly-moving ships traverse the globe. 



The voyage of the Challenger in 1872-1876 contributed the most 

 valuable series of observations of the magnetic elements in modern 

 times, when the large areas of the principal oceans traversed by that 

 vessel during three and a half years are taken into consideration. 

 These observations, combined with those taken from every available 

 source, both British and foreign, between the years 1865-1887, formed 

 the materials from which the magnetic charts of 1880 were compiled 

 (see Vol. ii, Bhysics and Chemistry, part 6, voyage of H. M. S. Challenger). 



The Challenger only crossed the Antarctic circle at one point in lon- 

 gitude 78° east, and, therefore, although we know large secular changes 

 to be going on south of 40° south, we have no measure of the amount, 

 nor anything like an accurate knowledge of distribution of the earth's 

 magnetism in those regions. This points to the necessity for a new 

 Antarctic expedition. 



In the year 1888 the late Prof. J. C. Adams was provided with a 

 complete set of magnetic charts for the two epochs 1842-1845 and 1880 

 previously mentioned, and as it was known he had directed his pro- 

 found mathematical ability to the analysis of the results contained in 

 them, it was hoped that some new and important light might be thrown 

 upon the bare facts presented. His lamented death occurred without 

 his publishing any results. 



If, however, reference be made to the report on the magnetical 

 results of the Challenger, a discussion of the secular change is con- 

 tributed, founded in a great measure on a comparison of those charts. 

 The outcome of this discussion is to throw considerable doubt upon 

 the theory that the motion of the magnetic poles round the terrestrial 

 is the cause of secular change; in fact, that the magnetic poles remain 

 fast, and we must look elsewhere for the cause, whatever it may be. 



Magnetic observations have so far been considered in their all- 

 important bearing as necessary to safe navigation in wood-built ships, 

 and in a far higher sense as indispensable to that of the iron or steel- 

 built ships which now cover the ocean, the magnetic charts hitherto 

 generally required for these purposes being those on which normal 

 lines of equal values have been given; but something more is now 

 needed. 



Observation in comparatively recent years has shown that not oidy 



