1883.] 



On Irregularities in Magnetic Inclination. 



7 



Our plan of investigation was briefly as follows : — In the first place 

 we sought for a station sufficiently remote from the supposed area of 

 disturbance, and itself free from any suspicion of local attraction, 

 where we could hope to obtain a perfectly normal value for the inclina- 

 tion. We then proceeded to confirm this value by observations made 

 at distances of a few miles and in different directions from this 

 station on spots as free as possible from local attraction. Our normal 

 station we found on the Island of Shuna in Loch Linnhe ; this island 

 is composed of alternations of micaceous schist and limestone, and a 

 careful examination of the locality by our colleague Professor Green, 

 to whom we are indebted for the geological notes which accompany 

 our observations, revealed nothing which could exercise any disturbing 

 influence on the dip. The subsidiary stations were near Carron Point 

 in Loch Aber and on the shore of Loch Corrie. We then approached 

 Mull along lines converging to the supposed place of maximum 

 disturbance, making observations at tolerably regular intervals as we 

 came towards the volcanic district as indicated on Professor Judd's 

 map of Mull and Morvern. In this manner we imagined we should 

 be able to determine the influence of the attractive mass upon the 

 dipping needle. We next made observations near but not actually 

 upon the extreme edge of the basalt in Morvern and on the opposite 

 shore of the Sound ; and lastly we made a number of determinations 

 on Mull itself with a view of ascertaining, if possible, the position of 

 the maximum disturbance. 



The results are contained in the following table (see next page) : — 



These results serve, in the first place, to establish most unequivo- 

 cally the existence of the area of local attraction which Dr. Stewart 

 found to be indicated by the observations of Mr. Welsh. In the 

 table showing the results of Mr. Welsh's observations, contained in 

 the Report to the British Association, no particulars are given as to 

 the exact spot in Tobermory on which the tripod was placed, nor is 

 anything to be obtained on this point from Mr. Welsh's MS. notes of 

 his survey work which are kept at the Kew Observatory. Neverthe- 

 less, when regard is had to the secular change in the inclination, our 

 results are as concordant as could be expected. Nothing is, of 

 course, known as to the precise amount of this change at Tobermory, 

 but assuming that it is identical with that calculated by Dr. Stewart 

 from the whole of the results of the surveys of 1836-7 and 1857-8, 

 viz., an average yearly decrease of 2'23, our value would become 

 72° 43', which agrees closely with that observed by Mr. Welsh, 

 viz., 72° 46 /, 8. This agreement would in all probability be actually 

 closer if we assume that we have in Scotland as in England a 

 progressive augmentation in the amount of secular change with an 

 increasing westerly position. 



Our observations further tend to show that the centre or focus of 



