1383.] 



On Irregularities in Magnetic Inclination. 



5 



Becquerel's comparative experiments (see "Electricity," first edition, 

 vol. ii, p. 235). 



This led me to revise tlie calculations given in the " Phil. Trans.," 

 1377, and I see that in the final formula in that paper 2R has been 

 used instead of R. This makes the value of the constant there given 

 double its true value, which is — 



w =l-52381xl0- 5 . 



This correction removes the discrepancy between my result and 

 Becquerel's. 



V. " Note on the Irregularities in Magnetic Inclination on the 

 West Coast of Scotland." By T. E. Thorpe, F.R.S., and 

 A. W. Rucker, M.A. Received October 20, 1883. 



In the Report of the results of the Magnetic Survey of Scotland, 

 undertaken at the request of', the British Association by the late 

 Mr. Welsh during the years 1857 and 1858, it is stated by Professor 

 Balfour Stewart (by whom the observations were reduced and the 

 report drawn up) that the values of all the elements as determined 

 in and adjacent to the Island of Mull were apparently largely affected 

 by local attraction, and from a comparison of the various observations 

 Professor Stewart was led to place the centre of the disturbance a 

 little to the south of the Mull stations, and at a considerable depth 

 below the surface. The effect of this local attraction was most 

 apparent in the determination of the dip, which at Tobermory was 

 upwards of 57', and at Glenmorven,.on the other side of the Sound, 

 was 14' in excess of the probable normal value, that is, the value 

 unaffected by local disturbance and dependent merely on geogra- 

 phical position, as deduced by combining together all the other 

 observations for Scotland, in the manner adopted by Sir Edward 

 Sabine in discussing the observations of the previous Survey of 1836. 



Dr. Stewart's localisation of the centre of disturbance was based 

 partly on a consideration of the abnormal values exhibited by the 

 observations made at tbe two stations on the Sound of Mull, and 

 partly on certain irregularities manifested by the determinations 

 taken on Islay and in Skye. So far as the Mull observations them- 

 selves were concerned, the clue as to tbe exact locality of the area of 

 disturbance was of the very slenderest. Mr. Welsh appears to have 

 made only a single observation of the dip at Tobermory, and 

 although observations were made with two needles at Glenmorven, 

 the divergence between the resultant values happens to be greater 

 than is exhibited by any other pair of dip observations throughout 

 the survey. Nevertheless, as we shall show, we are able to confirm 



