204 OBSERVATIONS OP THE TERRESTRIAL 



TABLE VI. {Continued.} 

 Intensity. Needle S (2). 



HL.~Itodytutmic and Isoclinal Lines. 



On a review of the preceding results of observation, it will be 

 seen that they exhibit much irregularity. The errors of observa- 

 tion in which we are to include the effects of the unsteadiness of 

 the magnetic state of the needles employed, as well as the various 

 other uncertainties arising from the imperfections of our methods 

 of observing have, of course, their share in these discrepancies ; 

 but they are by no means sufficient to explain the whole. The 

 action of the earth on the magnetic needle is itself subject to irre- 

 gularities, temporary as well as local ; and it is to these that the 

 observed anomalies must, in great part, be ascribed. 



To the variations of the former kind we have already referred. 

 The direction and intensity of the terrestrial magnetic force, at 

 a given place, are subject to fluctuations, or irregular oscillations 

 round their mean state, the cause of which is as yet little under- 

 stood ; and it is only by means of simultaneous observations, made 

 at some fixed station within the limits of the district through 

 which these effects take place, that we can hope to ascertain their 

 amount, and to correct for them. 



Of the local disturbing causes some are sufficiently obvious. 

 Thus the needle is in general affected by the vicinity of basaltic 

 rocks, owing to the quantity of iron they contain ; and instances 

 have been observed in which these rocks were even found to possess 

 magnetic polarity.* But there seem to be grounds for believing 



* A remarkable case of this kind has been noticed at Fair Head, on the north coast 

 of Ireland. The magnetic polarity of one of the columns which compose this wonder- 

 ful facade is said to be so strong as to invert the position of the compass ncedje, when 

 the poles of the same name are made to approach. 



