Ckas. Davison — Magnetic Disturbance by Earthquake. 211 



to the west of the South American continent. In his classical 

 memoir, "On the Connexion of certain Volcanic Phenomena in South 

 America, etc.," ^ Mr. Darwin quotes the observation of Mr. Douglas, 

 made at the moment of the shock in the island of Chiloe, as follows : 

 "Pocket compass placed level on the ground, N. point set to lubbers' 

 point; remarked that it vibrated during the violent shocks two 

 points to westward and only half a point to eastward ; stood at N. 

 when the motion was less violent. Four minutes afterwards, a shock 

 more violent than any of the preceding ones, affecting the compass 

 as before ; another violent shock, and then the movements became 

 gradually less distinct, and eight minutes after the first commence- 

 ment, they entirely ceased." In this case, though one not quite free 

 from doubt, the north end of the compass-needle seems to have 

 moved in the direction opposite to that of the earthquake-shock. 



Explanation of the Movement. — In the memoir above mentioned, 

 Mr. Darwin continues: "I have quoted Mr. Douglas's statement 

 with regard to the compass, although it is not clear how any move- 

 ment could have forced it to oscillate towards one side more than to 

 another. I presume, however, if the needle with its card had not 

 been acted on by the magnetic force, it would have been thrown in 

 the trough (if such an expression may be used) of the undulation, 

 that is, in a N.W. and S.E. line, and, therefore, that the recurrence 

 of this tendency acting against the polar attraction, caused the un- 

 equal oscillations described." 



Mr. Mallet has shown that the impulse communicated to a body 

 by the shock of an earthquake is, cceteris paribus, proportional to the 

 mass of the body, and acts in the direction opposite to that of the 

 earthquake-motion at the instant considered. If, therefore, the masses 

 of the two halves of the magnetic needle were equal, the impulse 

 communicated to both would be the same, and no motion would 

 ensue. But, to counteract the dipping tendency of the magnetic 

 needle, the end that would naturally be higher is weighted so that 

 the needle may swing horizontally. Hence, the masses of the two 

 halves being unequal, the needle will receive a differential impulse 

 proportional to the added weight, and the heavier end will swing 

 round in the direction opposite to that of the earthquake motion. 

 The resultants of the magnetic force and of the subsequent impulses 

 communicated during the earthquake will tend to bring the needle 

 back to its position of rest, or cause it to describe further oscillations 

 of greater or less magnitude. 



Now, in Europe, the dip-needle inclines towards the north, and 

 the south end of the compass-needle must therefore be weighted. 

 In Chili, the needle dips towards the south, and the north end must 

 be weighted. In the first two of the above cases, then, and probably 

 in the third also,'^ tlie first movement of the heavier end teas in the 

 direction opposite to that of the earthquake motion. 



' Trans. Geol. Soc, second series, vol. v. pt. iii. pp. 602-3. 



2 These are the only instances I have been able to find in which the direction of 

 the movement of the compass-needle is stated. Others only mentioning a movement 

 at the instant of the shock are of course much more commonly recorded. 



