Electricity at the Keiv Observatory. 



Ill 



where q is measured as above in grams per metre. They give an 

 abstract of the results on their p. 742, in the shape of a table which 

 I reproduce. 



Elster and Geitel's Table III (loc. cit., p. 742). 



It would appear that Elster and Geitel, like Exner, found large 

 departures from the mean dV/dn of a group amongst its individual 

 members. 



15. Elster and Geitel next proceed to investigate a possible con- 

 nexion between the potential gradient and the intensity of that species 

 of solar radiation which dissipates a negative charge on an insulated 

 sphere of polished zinc. If I understand them rightly, they measured 

 the mid-day intensity J of this radiation, and compared the potential 

 gradient with several formulae in which the variable was either J or 

 J/, where / is a " Beleuchtungsf actor," equal apparently to (possible 

 hours of sunshine) /12. Taking a formula dV/dn = 110 + 360a~ J , 

 where log a = O'OIOO, they give the following comparison of the 

 results of observation and theory : 



The agreement seems better than in the case of Exner's formula, 

 and Elster and Geitel seem strongly inclined to regard ultra-violet 

 radiation as the direct cause of variations of potential on normal qniet 

 clear days. They consider apparently that there are only two defective 

 links in the chain of evidence, viz. : 



(1) absolute proof that the earth is electrified negatively; 



(2) proof that there is a sufficient supply at the earth's surface of 

 materials susceptible to the influence of ultra-violet light. 



There are, of course, numerous other theories of atmospheric 

 electricity, but none, so far as I know, admits of numerical com- 

 parison with observation. 



VOL. LX. 



