THE BEARINGS OF MAGNETIC OBSERVATIONS. 89 



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 contributed 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 indispens- 

 able to that of the iron- or steel -built ships which now 

 cover the ocean ; the magnetic charts hitherto generally re- 

 quired 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 only are there large " regional " magnetic dis- 

 turbances extending over large areas of land, but that in 

 moderate depths of water where the largest ship can navi- 

 gate freely, the land below is also found to have considerable 

 areas of local magnetic disturbance which, if not allowed for, 

 may in thick or foggy weather lead ships into danger by 

 seriously disturbing their compasses. 



The United States have done excellent work in pro- 

 ducing charts of iso-magnetic lines, or charts in which the 

 chief local magnetic disturbances are recognised, and the 

 full results of observation recorded. The maonetic sur- 

 veys of Riicker and Thorpe in the British Isles, of Moureau 

 in France, of Rijckevorsel in Holland and elsewhere, have 

 thrown considerable light on the magnetic conditions of 

 those countries, but there remain whole continents to be 

 covered by the observer. 



The direction of the iso-magnetics too from the deep 

 sea to the dry land of the coasts is an extension of the 

 subject, which the observer has hardly touched as yet, but 



