668 Prof. S. Chapman : A Note 



only, provided the explanation afforded is equally good in 

 respect of points which can be compared with observation. 



6. On my stream hypothesis the particles entering the 

 earth's atmosphere are mainly of one sign of charge: those 

 of the opposite sign which also enter will be neutralized by 

 the flow of the most mobile ions present, but this I regard 

 as probably a secondary and unimportant part of the phe- 

 nomena, and as detracting, though only slightly, from the 

 storm-producing capacity of the preponderant charges. The 

 latter give rise to the storm in the course of their escape 

 from the earth : their motion would be radial, under the 

 influence of their mutual electrostatic repulsion when brought 

 to rest in the earth's atmosphere, did not the earth's mag- 

 netic field deflect them on their escape as at entry. But the 

 inward and outward paths differ, because of the difference 

 in distribution and velocity of the charge on entry and 

 at escape. The charge spreads over the earth very rapidly 

 and nearly uniformly, and is deflected sideways round 

 the earth : and the magnetic storm I regard as due to this 

 horizontal component of the outward flow of electricity. 

 On the cloud hypothesis the two kinds of charge need not 

 escape, but have simply to coalesce : and since both sets enter 

 the earth near the poles, their mutual approach and neutrali- 

 zation would seem to be a simpler task, and one less likely 

 (on consideration) to produce the world-wide magnetic field 

 of a magnetic storm, than in the alternative case, where the 

 escaping particles must redistribute themselves and also travel 

 outwards and round the earth for great distances while making 

 their escape. 



These considerations (to be supplemented by detailed calcu- 

 lations later) are the chief ones which, though not urged as 

 conclusive against the cloud hypothesis of Prof. Lindemann, 

 yet seem to me to render that hypothesis less probable than 

 the one I originally proposed. 



7. In conclusion, though the matter has no direct relation 

 to the foregoing, I will indicate briefly my reasons for having 

 come to the conclusion that magnetic storms and aurora3 are 

 produced by negative charges. When writing my iC Outline 

 of a Theory of Magnetic Storms," I was of the opinion that 

 charges of either sign would serve equally well to explain 

 those phenomena, considering that, when they became en- 

 tangled in the atmosphere, the charged layer of air rose 

 bodily, and produced horizontal E.M.F.s and electric currents 

 by thus cutting across the earth's horizontal magnetic field. 

 The mean free path of an ion at auroral heights is, however, 

 such that the ions have far less " grip " on the air molecules 



