754 COSMICAL PHYSICS 



there are other cases of similar connection, it is not likely that there 

 will be found an instance in which the data are as complete and as 

 accurate as in the present case. In the mightier eruption of Kraka- 

 toa, no magnetic disturbance affecting the entire earth simultaneously 

 was noted. A discontinuous disturbance occurred at the near-by 

 Batavia magnetic observatory, which lasted merely during the rain 

 of volcanic ashes upon Batavia, and the observer attributed the mag- 

 netic effect to the magnetic character of the ashes. If there was any 

 general magnetic effect referable to the eruption, it was of a totally 

 different character from that of Mt. Pele", for Whipple deduced a 

 velocity for the rate of propagation of a magnetic disturbance which 

 occurred on the day of the Krakatoa eruption of about 1000 miles an 

 hour. At this rate, it would have taken the Mt. Pel6 magnetic dis- 

 turbance several hours to travel around the whole earth. 



The coincidence of the magnetic disturbance with the Mt. Pel6 

 eruption was such a striking one as to suggest, as already stated, some 

 physical connection. And the first thought might naturally be that 

 the displacement of masses in the earth's interior produced a redistri- 

 bution of the electric currents inside the earth, which in turn gave 

 rise to the magnetic disturbance observed on the earth's surface. 

 We have had, namely, repeated instances in which seismic disturb- 

 ances, known to have occurred, were recorded not on seismographs, 

 but on magneloyraphs. This might occur if, for example, the me- 

 chanical displacement of masses below the surface resulted in either 

 the formation, destruction, or redistribution of the electric currents, 

 which in turn produced the magnetic effect. This magnetic effect 

 would then propagate itself more rapidly to the surface of the earth 

 than the mechanical vibration, and hence might be recorded first or 

 even give a record when the mechanical vibrations by the time they 

 reached the earth's surface would be too feeble to leave their trace 

 on seismographs. 



However, in the case of the magnetic disturbance before us no such 

 simple explanation is possible. While the mathematical analysis has 

 not yet been completed, it has progressed sufficiently far to show that 

 the cause of the magnetic disturbance cannot be referred to any dis- 

 tribution of electric currents below the earth's surface, but that, on the 

 other hand, the observed phenomena are better satisfied by assuming 

 a distribution of electric currents in the regions above us. As is 

 known, it is with the aid of the changes in the vertical component of 

 the earth's magnetism that we can decide whether the forces produc- 

 ing the observed disturbance have their seat in the earth's interior or 

 in the regions outside. The question now is, was the coincidence be- 

 tween the magnetic disturbance and the Mt. Pel6 eruption a mere 

 chance connection? If not, then the further analysis of the magnetic 

 disturbance is going to be of the greatest interest. 



