210 Chas. Davison — Magnetic Disturbance by Earthquake. 



5. Granite. 6. Felsite. 7. The later basaltic rocks — diabase and 

 melaphyre. 8. Trachyte. 



The above is given as the results of field observations in the 

 summer of 1883, the specimens not having been unpacked since 

 collected. It is intended to make a microscopic examination of 

 the rocks at the earliest time my duties will permit, and to publish 

 the results with further details of the field relations. 



IV. — On a Possible Cause of the Disturbance of Magnetic 

 Compass-Needles during Earthquakes. 

 By Charles Datison, B.A., 

 Mathematical Master at King Edward's School, Birmingham. 



EEOM time to time, during earthquake- shocks, magnetic compass- 

 needles within the disturbed areas have been seen suddenly 

 deflected from their position of rest, returning to it shortly, after one 

 or more oscillations. This movement admits of two very different 

 interpretations. Either it may be attributed to an intimate connexion 

 between earthquakes and terrestrial magnetism, or it may be regarded 

 as merely a consequence of the shoch of the earthquake. The object 

 of this note is to point out a possible mechanical cause, and to guard, 

 therefore, against a too hasty assumption of a magnetic origin for 

 these disturbances. 



The following examples may be given to illustrate the nature of 

 these movements : 



(1.) On August 28, 1787, an earthquake was felt at Stuttgart, 

 Munich, Augsburg, Landshut, Innspruck, Pappenheim, Ansbach, 

 Empten, Dillingen, Eatisbon, Zurich, and Bale. At Innspruck, the 

 direction of the shock was from S.W. to N.E. A magnetic needle 

 at the same place deviated 0° 12' to the east.^ Here, then, the south 

 end of the needle moved in the direction opposite to that of the 

 earthquake-shock. 



(2.) On December 2, 1841, an earthquake was felt at the following 

 places: — At St. Eambert-en-Bugey at 7*47 p.m.; Lyons at 7-50; 

 Geneva and Chambery at 7'53 ; Chessy, Anse, and in the neighbour- 

 hood of Berne at 8 ; and at Lons-le-Saulnier at 8-15, and again at 

 8-30. At Billey the direction of the shock was said to be from N. to 

 S. ; at Geneva from S.W. to N.E. ; and at St. Eambert-en-Bugey, 

 apparently fi'om E. to W. From the recorded times and directions, 

 it follows that the focus of the earthquake must have lain to the 

 east of the meridian of Lyons. At this town a compass-needle 

 suddenly turned from N. to N.N.W.^ Here again, then, the south 

 end of the needle moved in the opposite direction to that of the 

 shock. 



(3.) On February 20, 1835, occurred the great earthquake of 

 Concepcion, described by Fitzroy, Caldcleugh, Darwin, and others. 

 There is some uncertainty as to the direction of the shock, but the 

 balance of the evidence is in favour of the seismic focus having been 



1 R. Mallet, Catalogue of recorded Earthquakes, Brit. Assoc. Rep. 1854, p. 18. 



2 Ibid. p. 314. 



