4 
MR. A W. RUCKER AXD DR. T. E. THORPE OX A MAGNETIC 
Page.* 
Xoii-Basic Igneous Rocks. 653 
The Cheviots and Berwickshire . G54 
Galway . 655 
Faults. 655 
Regions of Upheaval . 656 
General Sumniary. 653 
Future Applications of the Results of the Survey . 659 
Errata in the “ 1890 Memoir ” . 660 
LIST OF MAPS. 
Xo. 1. Isogonals for the Epoch Jan. 1, 1891. 
,, 2. Lines of Equal Horizontal Force for the Epoch Jan. 1, 1891. 
3. Isoclinals for the Epoch Jan. I, 1891. 
,, 4. Comparison of the Principal Regions of Magnetic Attraction in the United Kingdom as indicated 
by the Surveys of 1886 and 1891. 
,, 5. Declinations and true Isogonals for the Epoch. 
,, 6. Horizontal Forces and true Lines of Equal Horizontal Force for the Epoch. 
„ 7. Dips and true Isoclinals for the Epoch. 
„ 8. Vertical forces and true Lines of Equal Vertical Force for the Epoch. 
„ 9. Differences between the Observed and Calmlated Values of the Declinations, 
,. 10. Differences between the Observed and Calculated Values of the Horizontal Forces. 
,, 11. Differences between the Observed and Calculated Values of the Dips. 
, 12. Differences between the Observed and Calculated Values of the Vertical Forces with Magnetic 
Ridge and Valley Lines. 
,, 13. Directions of Horizontal Disturbing Forces with corrected Ridge Lines. 
, 14. Relations between Magnetic Disturbances and Geological Phenomena. 
Inteoductiox. 
Five years ago we communicated to the Royal Society an account of a IMagnetic 
Survey of the British Isles, for the epoch Jan. 1, 1886 (‘ Phil. Trans.,’ vol. 131 (1 890), 
A, pp. 53-328).^" 
The necessary observations had been made by ourselves at 205 stations during the 
years 1884-88. The results were worked up in a manner which was, we believe, in 
some respects novel. The forms of the normal terrestrial isoraa.gnetlcs— i.e., the lines of 
equal Declination, Force, and Dip, as they would be if unaffected by local disturbance 
—were determined, and equations were found connecting the normal magnetic 
elements at any place with its geographical position. From these the undisturbed 
values at our stations were deduced, and by comparing them with the results of the 
observations, it was possible to calculate foi' each place the magnitude and direction 
of the horizontal and vertical components of the disturbing magnetic forces, regarded 
as acting on the north pole of a magnet. 
* Note. —As we shall frequently have to refer to this paper, we shall cite it by the short title of “ the 
1890 Memoir,” or “Mem., 1890.” 
