226 



REroRT— 1898. 



■we are in possession of records more strictly comparable it may be sub- 

 ject to considerable alteration : — 



All that this table tells us is that both preliminary tremors and large 

 waves exhibit a marked increase in period as they travel, and, whatever 

 the period of a given wave may be in the vicinity of its origin when it has 

 travelled a distance represented by a quarter of the circumference of the 

 earth, its period has increased twentyfold. 



VI. On Certain Disturbances in the Records of Magnetometers and the 

 Occurrence of Earthquakes. By John Milne. 



Although we are aware that the records from certain magnetic obser- 

 vatories rarely, and then only slightly, show that the magnetographs have 

 been disturbed at or about the time of large earthquakes, it is certain that 

 at other observatoi-ies these movements of the ground are accompanied 

 and possibly preceded by perturbations as shown upon magnetograms of 

 a very marked character. In some instances these disturbances have 

 evidently resulted from the mechanical shaking to which the magnetic 

 needles have been subjected, but there are other cases where such an 

 explanation is not so clear. 



To determine how far these movements may be attributed to mechani- 

 cal action, whether there is any reason to suppose that certain of them 

 may be the result of magnetic influences, to explain the observation that 

 what are apparently similar earthquakes with like origins are accompanied 

 by diffei'cnt results at the same observatory, and generally with the object 

 of throwing additional light upon a class of phenomena which at present 

 are not well understood, I have collected the materials contained in the 

 following notes. 



In addition to sending the list of ' Earthquakes recorded at Shide, 

 1897-98' (see p. 191), to vai-ious earthquake observatories, the same was 

 forwarded to magnetic observatories at the following places : Kew, Stony- 

 hurst, Greenwich, Falmouth, Potsdam, and Bombay. Accompanying 

 the list there was a request that the same might be returned with 

 notes respecting any magnetometer perturbations which might have 

 been noted at about the times of the earthquakes which were more pro- 

 nounced. 



Some time later I drew up a second list of earthquakes which had been 

 recorded in Italy, Germany, and England, the greater number of whicli 

 had orif^inated at great distances from these countries, and appended to 

 the same a request similar to that attached to the Shide list. On April .3 

 this was forwarded to magnetic observatories at the following places : — 



Pawlowsk (Odessa), Kasan and Tiflis^ (Russia), Irkutsk (Siberia), 

 Pra^'ue, Vienna, and Pola (Austria), O-Gyalla (Hungary), Utrecht 

 (Holland), Nice and Perpignan (France), Copenhagen (Denmark), Madrid 



