1895.] on Atmospheric Electricity. 467 



The formula agrees very well with observations in which the 

 vapour pressure varied between * 23 and • 95, and it is especially to 

 be remarked that it is the amount of vapour and not the humidity 

 which determines the electric force. Observations made by Mr. E. 

 Drory during a journey round the world fit in very well with 

 Exner's formula, and observations made at such widely different 

 places as Suez, Albany, Sydney, Colombo and Penang showed a fall 

 of potential practically identical with that calculated from the above 

 formula, though the same constants were taken and the vapour 

 pressure varied between ■ 8 cm. and 2*2 cm. 



Messrs. Julius Elster and Hans Geitel* have followed up this 

 research. Their investigations have shown a satisfactory agreement 

 with Exner's formula, if the mean values of a number of observations 

 in which the vapour pressure is approximately the same, is considered. 

 But individual numbers differ very widely from the mean, so that the 

 formula cannot be used to predict the normal fall of potential on any 

 particular day. There is, perhaps, nothing surprising in the great 

 divergence of such individual results if it is considered that we only 

 observe the moisture near the surface of the earth, but are ignorant 

 of the total amount of water in the column of air over the district 

 in which the observations are carried out. The same authors have 

 shown that an equally good agreement can be obtained if, instead of 

 the amount of aqueous vapour, we take the intensity cf active radia- 

 tion as the determining circumstance. The light might be supposed 

 to act on the general surface of the earth, as it does according to 

 Hallwachs' observations on a metallic body, dissipating a regular 

 charge. There are some difficulties in the way of this explanation, 

 the most serious being the absence of experimental evidence that 

 sunlight actually does act in the manner indicated on any substance 

 forming part of the earth's surface. It is impossible at the present 

 time to enter more fully into this subject, but attention must be 

 drawn to the very important indirect result, that there seems to be a 

 connection between ultra-violet radiations and the amount of aqueous 

 vapour present in the air. 



The phenomena of atmospheric electricity have been studied at 

 the mountain observatory established on the " Sonnblick," in Salzburg, 

 at a height of 3100 metres. 



The important result has been established that the electric force 

 is singularly constant. The great differences observed at low levels 

 between the electric field in summer and winter, or on dry and wet 

 days, seems to be completely absent, and these facts tend to support 

 the conclusion derived from balloon observation, that the positive ends 

 of the lines of force are situated at a height of something like 

 10,000 feet. 



Brief allusion must be made to some of the causes which alter to 



Wiener Akad. Sitzungsberichte,' vol. ci. 1892. 



