110 Dr. C. Chree. Observations on Atmospheric 



PART IT. 

 Application of Results to Theories of Atmospheric Electricity. 



13. It seemed desirable to consider what bearing the special 

 experiments might have on the general facts and theories of atmo- 

 spheric electricity. In this investigation special attention has been 

 given to the possible influence of aqueous vapour on electrical 

 potential, on account of the important researches of Exner, and of 

 Elster and Geitel. 



Theories of Exner and of Elster and Geitel. 



14. Exner has advanced the view that the potential gradient in 

 the open, dV/dn in his notation, and the density q of aqueous vapour 

 simultaneously present in the atmosphere, are connected by a formula 



dV/dn = constant -4- (l-f &gr ), 



where k is apparently a constant, the same at all places and at all 

 seasons of the year. Exner, I believe, limited his observations, and 

 presumably the application of the formula, to days comparatively 

 quiet and free from clouds. To test the formnla he arranged his 

 observations in groups, according to the amount of vapour present, 

 and compared the mea.n vapour density measured in grams per 

 cubic metre with the mean potential gradient, measured in volts 

 per metre of height above the ground. In the ' Wien. Sitz.,' Bd. 99, 

 p. 618, he gives a table including results from Vienna, Wolfenbiittel, 

 St. Gilgen, and India, in which the vapour densities vary from 1*7 

 to 23'5. The table unquestionably shows a diminishing mean 

 potential gradient accompanying an increasing mean vapour density. 

 For values of q from 12'4 and upwards, however, including all the 

 Indian and most of the St. Gilgen observations the change in dV/dn 

 is somewhat small and irregular, An earlier, and somewhat similar, 

 but less extensive table by Exner will be found on p. 434 of ' Wien. 

 Sitz.,' Bd. 96. 



For information as to Elster and Geitel's work I am mainly indebted 

 to a long paper by them in the ' Wien. Sitz.,' Bd. 101, p. 703, 1892. 

 During 1888-91 they took an extensive series of observations on 

 quiet days at Wolfenbiittel. If I follow their explanations, they took 

 eye observations some ten times a day with an electrometer, in which 

 flame from a lamp acts as collector, and deduced the mean value of 

 the potential gradient dV/dn for the day. They compare these 

 potential gradients grouped according to the value of the vapour 

 density with Exner's formula, taken to be 



(dV/dn) (in volts per metre) = 1410/(l-fl-15g ), 



