F. H. Bigelow — Solar and Terrestrial Magnetism. 459 



but this stratum sails eastward serenely while magnetic 

 theory demands that it should move westward ; and also no 

 physical cause has been proposed to show why electric currents 

 should especially originate at that level. There would be very 

 much more probability of the desired variations occurring in 

 the strato-cumulus or the alto-cumulus levels, if these had been 

 chosen for the purpose. 



But there are two main lines of argument against the theory, 

 which are really serious. One is that the magnetic tie Id 

 demands a cosmical cause, and the other is that this field 

 requires shorter vibrations, and also longer sustained states 

 than the meteorological conditions of the atmosphere can 

 furnish. Regarding the first, as already mentioned, it is evi- 

 dent that the magnetic field of the earth is changed as a whole 

 in every part of the globe instantly and synchronously ; also 

 these perpetual fluctuations can readily be grouped in a solar 

 period. Now meteorological conditions are very local affairs, 

 and we have no evidence of rapid short variations occurring 

 all over the world at the same time. If the deflecting forces 

 of the magnetic field are spread out in long tabulations such 

 as shown on pages 46, 47, 51-56, extending through several 

 years, it is to be seen by mere inspection that the changes are 

 periodic, and I made out the period to be 26*68 days, which 

 corresponds closely with the rotation of the sun at its equa- 

 torial belt, as measured by the motion of the sun spots. 

 Furthermore the direction of the deflecting vectors themselves 

 is such, that a field of magnetic force is directed upon the 

 earth, positive north and negative south of the ecliptic ; this is 

 also the direction from which the great disturbances come as 

 illustrated on pages 72-78 ; this period and its direction is per- 

 sistent through half a century as indicated on pages 100-102 ; 

 it is subject to two peculiarities, namely that the field is of 

 such a configuration at the earth's surface as to correspond to a 

 permeable shell placed within a uniform field, pages 61-72 ; 

 and it is as a whole subject to a peculiar inversion in the four 

 parts of the orbit of the earth about the sun, indicated on 

 pages 106-107. It appears that such phenomena are precisely 

 adapted to the explanation offered by the Bulletin, namely that 

 the earth is in an external field sustained by the sun, and on 

 the other hand one is driven to conclude that the electric cur- 

 rents of the cirrus region are powerless to produce such effects. 

 Of course we do not deny the existence of electric currents 

 entirely, since these must exist in all variable magnetic fields. 



The other difficulty consists in the proposition that the earth 

 is subject to such long sustained general states, extending over 

 years, as to preclude the electric current theory, though even 

 these too may well be included as a part of the effects of a solar 



