PHOTOGRAPHING ELECTRICAL DISCHARGES. 309 



be distinctly traced on its journey to the lightning conductor of 

 the cupola of the Academy of Sciences. 



The most interesting part of this photograph is the shadow of 

 the cupola on the wet and foggy heavens. The intensity of the 



Fig. 4. Traces of a Lightning Stroke. 



light produced by the lightning is the cause of this peculiar effect, 

 which I believe has never been obtained before or since. It is 

 analogous with the phenomenon commonly known as the Specter 

 of Brocken. 



At a meeting of the Academy of Sciences, held a short time 

 ago, Prof. Zenger read a most interesting paper upon the subject 

 of Electricity as a Vortex Motion, in which he endeavors to prove 

 that electrical discharges, no matter what their origin, produce a 

 vortex motion on matter lying in 

 the electric field. He makes the 

 following experiment: A spark 

 from a Ruhmkorff coil or a Wims- 

 hurst machine is discharged with- 

 in the receiver of an air pump, 

 under which has been placed a test 

 tube containing diluted ammonia 

 and another containing hydro- 

 chloric acid. At the instant of the 

 discharge there will be visible 

 eddies or little whirlwinds, ren- 

 dered visible by the tiny crystals of ammonium chlorid suspended 

 in the air. In circling about they are condensed into peculiar 

 shaped veins which fall upon the plate of the air pump, arrang- 

 ing themselves in somewhat the same manner, says Zenger, as 

 did the debris from the roofs and trees during the cyclone in 

 the valley of Roux in Switzerland i. e., they form lines of elec- 

 tric force. 



Prof. Zenger sends me some very interesting photographs in 



Fig. 5. Discharge of a Wimshurst 

 Machine. 



