January 13, 1911] 



SCIENCE 



49 



comparatively slowly over the earth? Or, 

 are we to suppose that at each recurrence 

 the disturbance was formed anew? No 

 matter what view we adopt, it is evident 

 that we are about to find out another im- 

 portant fact. 



Determining the local mean time of the 

 extreme stations at which the disturbance 

 was recorded whenever it occurred, it is 

 immediately seen that only the observa- 

 tories in the daylight zone were affected. 

 At those observatories where the local time 

 was somewhere between 4 p.m. and 6 a.m., 

 no effect was obtained. Hence, the conclu- 

 sion is inevitable that solar radiations of 

 some kind must have played an important 

 part in the production of these disturb- 

 ances. There were at the time on the sun 's 

 visible disk some peculiarly eruptive spots 

 which may have to be held responsible for 

 the peculiar magnetic disturbances. 



A mathematical analysis of this type of 

 magnetic perturbation is at present under 

 way, but it has already become sufficiently 

 evident that we are dealing here with a 

 much more complex system than in our 

 first type. 



GENERAL DEDUCTIONS RESPECTING MAGNETIC 

 DISTURBANCES 



From the two types of disturbances con- 

 sidered it has been found that not only 

 may our most sudden magnetic disturb- 

 ances begin at measurably different times 

 for various points on the earth's surface, 

 but also that magnetic perturbations may 

 even be confined to but a very limited por- 

 tion of the globe. 



The possibility of a regional magnetic 

 disturbance was foretold with the aid of a 

 law which I found to hold regarding mag- 

 netic changes in general: 



"Alterations in the earth's magnetic 

 condition, whatever their nature or origin 

 may be, appear to be distributed over the 



globe according to a law profoundly de- 

 pendent upon that governing the distribu- 

 tion of the earth's oivn primary magnetic 

 forces." 



The prediction made on the basis of this 

 law 'last spring, before the facts had be- 

 come known respecting the disturbances 

 mentioned, was as follows : 



I confidently expect, as soon as a complete 

 analysis has been made of magnetic disturbances 

 covering the greater portion of the earth, it will 

 be found, that the disturbance field, in general, pre- 

 sents all the characteristics of the terrestrial, 

 primary one, the disturbances will themselves re- 

 veal effects from terrestrial, continental, regional 

 and even local causes (earth currents, for ex- 

 ample, whose path and intensity depend upon local 

 character of soil, etc. ) . 



Were this the place, I should like the 

 privilege of setting before you the full im- 

 port of this law. How, for example, we 

 find characteristics in the magnetic fields 

 composing the so-called permanent mag- 

 netization of the earth analogous to those 

 which represent the systems producing the 

 time variations. Suffice it to say that, if 

 we were to establish a mathematical expres- 

 sion for the respective systems involved, 

 the same terms would appear in the space 

 variations as in those of the time. "We 

 might tersely put it thus: "In terrestrial 

 magnetism space and time are often rela- 

 tively interchangeable." 



Before leaving the subject of magnetic 

 disturbances let me point out to you two 

 or three additional interesting facts which 

 may serve to guide us in our study of 

 causes. 



It is the usual custom to exhibit the de- 

 pendency of the fluctuations in the earth's 

 magnetic condition during a sunspot cycle 

 by means of changes in certain particular 

 magnetic elements, as for example, the 

 change in the amplitude of the diurnal 

 variation during the sunspot cycle, this 

 amplitude increasing with increased solar 



