50 



SCIENCE 



[N. S. Vol. XXXIII. No. 837 



activity. If, however, we make use of a 

 more direct physical quantity, viz., the 

 earth's magnetic moment, or let us say in- 

 tensity of magnetization per unit volume, 

 then we find that, in general, during a sun- 

 spot cycle the earth's magnetization de- 

 creases with increased solar activity. In 

 brief, on the average, the effect of magnetic 

 perturbations is to superpose on the 

 earth's magnetic field a magnetization op- 

 posite to that of its own, and hence the 

 effect is one of demagnetization. The 

 quantity involved is on the order of that 

 found some years ago when I raised the 

 question as to whether the earth is gaining 

 or losing magnetism. 



This question was attacked in two dif- 

 ferent ways; first, use was made of the 

 existing magnetic charts between 1840 and 

 1885; secondly, freshly accumulated data 

 between the years 1890 and 1900 were 

 utilized. Both investigations led to the 

 same result, viz., that the earth's magnetic 

 moment is at present being diminished by 

 about one twenty-four hundredth part an- 

 nually. Now, if the terrestrial magneti- 

 cian were permitted to make the same 

 apparently violent extrapolation as is 

 indulged in by the radio-physicist, he 

 would find that, at the present rate of de- 

 crease, the earth's magnetic moment wiU 

 have dwindled to one half of its present 

 value in about 1,660 years from now. 

 Note that this period is practically the 

 same as that of radium decay — probably 

 a mere coincidence! 



We may make use of magnetic pertur- 

 bations in another way, mainly, to get some 

 idea of the earth's magnetic permeability. 

 1 have already pointed out above that 

 when analyzing the effects of the magnetic 

 disturbance of May 8, 1902, it was found 

 that there were two systems involved, one 

 an external one composed of overhead 

 electric currents, and the other, an internal 



one having the characteristics of directly 

 induced magnetism. If we suppose that 

 the second system is the result of the first, 

 then the ratio of the potentials of the two 

 systems will give us the differential change 

 in the earth's magnetic permeability. 

 Various calculations of this kind are under 

 way. 



One of the most important bearings of 

 the facts above set forth regarding mag- 

 netic disturbances pertains to the slow, 

 progressive changes to which the earth's 

 magnetization is subject — secular changes. 

 It has already been hinted above that these 

 secular changes can not be explained 

 simply by a change in the direction of the 

 axis of magnetization, but likewise imply 

 changes in the intensity of magnetization. 

 Respecting the latter, our result was that 

 apparently the residual effect of a mag- 

 netic disturbance is a diminution of the 

 intensity of magnetization, which may last 

 for some period after the cessation of the 

 disturbance, two months, for example, as 

 occurred with respect to the notable mag- 

 netic storm of September 25, 1909. "Whether 

 the earth ever recovers completely from a 

 magnetic disturbance is questionable. 



Now, as to the effect of magnetic disturb- 

 ances on the axis of magnetization, let me 

 merely point out that if magnetic disturb- 

 ances do actually in general progress over 

 the earth more often in one direction than 

 in the other, the mechanical effect is to be 

 reckoned with. If the progression is gen- 

 erally eastward, as appears to be the case, 

 then the mechanical effect of the overhead 

 currents will be to increase the velocity of 

 the earth's rotation or, failing to do that, 

 which is more probable, the effect will be 

 to cause a displacement of the earth's mag- 

 netic axis eastward. We thus have dis- 

 closed to us one of the several systems 

 causing the secular variation of the earth's 

 magnetism which was pointed out in 1904 



