U1RKELAND : THE NORWEGtAH AURORA POLARIS EXPEDITION, 1902 1903. 



field will produce a change in the orbits of the cathode-ray particles coming from the sun. It lias been 

 found by STORMKR that rays which are to arrive at the earth must start in directions that lie within very 

 narrow limits. Now the magnetic field of the moon might change the direction of the rays, and thus a 

 number of rays may reach the earth, which otherwise would escape from it. At present, mathematical 

 investigation has not been carried so far that the magnitude and variation of such an effect can be exactly 

 calculated. 



From a simple consideration, however, we are able to estimate the character and magnitude of tin- 

 indirect effect of the moon's field, compared with the direct effect of the radiation from the sun. 



The earth and the moon are put into an almost uniform field of electric radiation. Let us imagine 

 a sphere (S) drawn with the earth as centre and with a radius equal to the distance between the earth 

 and the moon. The radiation which must consist of very stiff" rays, will enter mostly on one hemisphere 

 and pass out of the sphere on the opposite side. On the surface of this sphere there will be a number of 

 spots a\ .a->... . a, t where those rays enter that reach the earth. 



Let us first consider the case of the moon being so far from the areas a\ a., . . . a n that ii> 

 magnetic field at those spots is very weak. This only requires the distance from the moon to the spots 

 to be of the order of the radius of the sphere, because we know that the direct magnetic effect of the 

 moon upon the earth is very small. 



On this assumption, the moon has no appreciable influence on the rays that come directly from 

 the sun to the earth; but we nevertheless have to consider the effect of those rays which pass near 

 to the moon and are so greatly deflected that their previous history, so to speak, is totally wiped out 

 so as to leave the moon in every variety of direction. The earth will be exposed to the action of t\y. 

 fields of radiation, one from the sun and the other from the moon. But the rays, of which the history 

 is effaced are scattered in all directions, so that the field of radiation from the moon must be extremely 

 weak as compared with that from the sun. As the plane of the moon's orbit forms a comparatively 

 small angle with the ecliptic, the directions from the moon are distributed very nearly in the same way 

 in relation to the earth's magnetic field, as the directions from the sun ; so that on an average the magnetic 

 effects produced by the two fields must be in proportion as the intensities of the radiation. 



A ''similar consideration will show that the effect of any secondary radiation caused by the impact 

 of electric radiation on the moon must be very small compared with the direct effect from the sun. 



If, however, the secondary electric rays are caused by radiations such as light or y rays which do 

 not produce- magnetic effects themselves, or if the moon is the source of primary electric radiation, 

 are a priori unable to say anything definite about the order of magnitude of the effect of the moon as 

 compared with that of the sun. We shall have to look at the observed magnetic effects for information 

 regarding this point, and, in fact, the diurnal distribution of disturbances will give us some information 

 in this respect. 



In the case of the moon being near to the areas a\ a.> . . . a n the effect of the rays of which the pre\ 

 history is wiped out, will be of the same order as before, but now the moon may have an appreciable 

 effect on the rays which would otherwise have reached the earth. The moon's field in this case 

 act as a shield for the rays, and thus be able to diminish the effect of an already existing radiation. It 

 might be possible that the perturbations consisted in a diminution of a radiation which was constantly 

 being given out; effects of this kind are not impossible. But we cannot suppose that the great polar 

 storms here considered have been caused in this way. That the polar storms are due to something 

 positively occurring is evident from the connection with aurora borealis and sun-spots, and besides great 

 storms are found in the most varied positions of the moon. 



