402 TERRESTEIAL MAGNETISM. 



TERRKSrUIAL MACiNETISM AND (lEOTAKiY. 



1 hiivc iilrciuly refenvtl to tlic (juestioii of local magnetic disturbance 

 as one of great importance in magnetic surveys. The causes of these 

 disturl)ances were at one time a matter of opinion, l)ut the evidence of 

 the ela})orate magnetic surveys 1 have alluded to, when compared with 

 the geological maps of the same countries, ])<)ints clearly to magnetic 

 rocks as their chief origin. 



Magnetic rocks may ))e present, but from their peculiar position fail 

 to distui-b the needle; on the other hand, as lliicker writes in his 

 sunnnary of the results of the great magnetic survey' of the British 

 Isles conducted by Thorpe and himself, "'the magnet would be capable 

 of detecting large masses of magnetic rock at a depth of several miles," 

 a distance not yet attained by the science of the geologist. 



Again, Doctor Rijckevorsel, in his survey of Holland for the epoch 

 1891, was convinced that "''in some cases, in many, perhaps, there 

 must be a direct relation between geology and terrestrial magnetism, 

 and that many of the magnetic features nuist he in some way deter- 

 mined by the geological structure of the underground.'' 



During the years 189T-1.S!)9 a nuignetic survey was made of the 

 Kaiserstuhl, a mountainous district in the neigh])orhood of Freil)urg, 

 in Baden, by Dr. G. Meyer. Exact topographical and geological sur- 

 veys had l)een previousl}' made, and the ol)ject of the magnetic survey 

 was to show how far the magnetic disturl)ances of the needle were con- 

 nected with geological contirmations. Here, again, it was found that 

 the magnetic and geological features of the district showed consider- 

 able agreement, basaltic rocks being the origin of the disturbance. 

 This was not all, for in the level country adjacent to the Rhine and 

 near Breisach unsuspected masses of basalt were found by the agency 

 of the magnetic needle. 



More recentl}^ we find our naval officers in H. M. S. Pengiiln^ with 

 a complete outfit of magnetic instruments, making a magnetic surve}'^ 

 of Funafuti atoll and assisting the geologist l)y pointing out, by means 

 of the observed distui'bance of the needle, the probabh^ positions in 

 the lagoon in which rock would ))(» most accessible to their boring 

 apparatus. 



Lea\'ing the geologist and the magnetician to work in harmony for 

 th(Mr common weal, let us turn to some other aspects of the good work 

 already accomplished and to be accomplished by magnetic observers. 



MAGNETIC CHARTS. 



Of the valuable work of the several fixed magnetic observatories of 

 the world, 1 may remark that the}' are constantly recording the never- 

 ceasing movements of the needle, the kej^ to man}^ mysteries to science 

 existing in th(^ world and external to it, but of which we have not 3'et 

 learned the use. Unfortunately, many of these once fixed observatories 



