﻿EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY. 19 



2. '' Notwithstanding this increase in the power of discharge of light, 

 we [Elster and Geitel] did not succeed in establishing with certainty 

 any new actinometrically active surface. Even perfectly freshly fallen 

 snow as well as dry rock were not appreciably discharged by light." 



3. " Waterfalls can produce negative falls of potential in a valley and 

 even to considerable heights (1,600 feet), and it may be assumed that. 

 this remarkable phenomenon is not produced by friction, but by the in- 

 fluence of the normal positive fall of potential on the finer pulverulent 

 water which detaches itself from the large masses of water ; and it may 

 perhaps be assumed that in a rain-cloud the process of self-induction 

 increases to high values the originally feeble negative charges of a layer 

 of air dust at the foot of a fall." 



4. " In July, 1890, on three days, which were almost cloudless until 

 1 p. m., the normal positive fall of potential on the top of the Sonnblick 

 was apparently constant. The morning maximum, which in the Plain 

 and Alpine valleys occurs with great regularity between 7 a. m. and 9 

 p. m., was not observed at a height of 10,168 feet." 



5. " Before the outburst of the storms which we observed on the 16th, 

 18th, and 20th of July, the positive fall of potential within the cloud, 

 which sent only a small quantity of rain, went slowly down to the value 

 zero and remained there for a long time." 



6. " In storm clouds the atmospheric electricity usually changes its 

 sign after a discharge of lightning, as with storms on the plain." 



7. " Saint Elmo's fire was found to be a constant accompaniment of 

 storms. It was not found that negative Saint Elmo's fire was more in- 

 frequent than positive." 



8. " The observation that negative Saint Elmo's fire follows bluish 

 lightning and positive follows reddish lightning was frequently confirmed 

 by us. The direction, then, of the electrical current which traverses the 

 atmosphere in the form of lightning aj^pears to have an influence on the 

 color of the lightning." 



All of the above results may be found in the " Phil. Mag.," May, 1891, 

 and " Wien. Berichte," November, 1890. We shall now proceed to com- 

 ment upon them. 



It is of some interest to know whether our mountain peaks, taking as 

 they must a negative electrification from the earth, are discharged more 

 rapidly when illuminated by the more refrangible rays. Since the above 

 experiments were made the entire question of the capacity of the air 

 and the disposition of a charge has been brought forward for discussion 

 by Kelvin's paper on the subtraction of vapor from air and the conse- 

 quent change in electrification. J. J. Thomson has shown in his recent 

 paper on the " Electricity of Drops " that a small amount of impure 



