190 



REPORT — 1859. 



Such groups of stations will in fact represent districts, some of them of 

 considerable extent, which thus appear to be similarly affected by local 

 attraction. 



The observations are probably insufficient to enable us to determine the 

 disposition of this attractive matter ; but there is one very marked case to 

 which I may be permitted to refer. 



If we examine Tables VI. and XI. we shall find that the stations in the 

 Island of Isla have their dip diminished and their total force increased by 

 local attraction. On the other hand, the Mull stations have both dip and 

 total force increased, while those in Skye have their dip increased and their 

 total force diminished. 



Such a state of things might be brought about by a powerful source of 

 attraction for the north pole of the needle situated a little to the south of the 

 Mull stations at a considerable depth below the surface. This supposition 

 derives confirmation from the fact that the errors due to local attraction 

 are exceedingly large in Mull. 



Table XIX. 



