and certain Electrical Phenomena. 485 



thunderstorms on the Nevada de Toluca, in Mexico, and on 

 the Piz Surley, in Switzerland. In the former locality he 

 alludes to the loud rattling or crepitation of the stones, due to 

 electrical discharges. On the summit of Piz Surley he notices 

 certain pricking and burning sensations, and sounds like that 

 of simmering water emitted from sticks laid against the rocks, 

 and the vibrations of the alpenstocks. This humming of the 

 mountains is by no means rare. It seems to indicate a flow 

 of electricity from the ground into the air. 



In a former paper I noticed * that M. Arago was sceptical 

 as to the existence of ball-lightning. Faraday was equally so. 

 He says : — " That the phenomena of balls of fire may appear 

 in the atmosphere I do not mean to deny ; but that they have 

 anything to do with the discharge of ordinary electricity, or 

 are at all related to lightning or atmospheric electricity, is 

 much more than doubtful." t The cases given in my former 

 paper seem to resolve bail-lightning into an exalted case of 

 brush-discharge. Thus, Mr. G. Ambrose Pogson, British 

 Vice-Consul at Hamburg, states in a letter to the ' Times ' of 

 June 16th, 1880, that a series of thunder-storms had passed 

 over Hamburg, and that during the 11th inst. the air was 

 densely charged with electricity. " The storm broke about 

 10.15 p.m., lasting until 11 p.m., during which time at very 

 short intervals from my station, about 1200 yards distance 

 from the copper-roofed tower of the church known as S. 

 Jacobi, about 300 feet high, I saw this phenomenon [which 

 he had previously named St, Elmo's fire\ apparently resting 

 about thirty feet from the summit of the steeple. The colour 

 was a reddish purple and reminded one somewhat of burning 

 potassium. From repeated comparisons with other objects 

 during the lightning-flashes, I judged these fire-balls (two 

 were several times visible) to be from 4 feet to 6 feet in dia- 

 meter. The longest duration that I timed was 42 seconds." 

 The appearance and disappearance of these globes of electri- 

 city were counted some twenty times : the colour was referred 

 to the glare of the copper roof. 



The following also seems to be a case of glow-discharge, 

 described by Mr. Jabez Brown in a letter to the l Times/ 

 dated Boscastle, December 1st, 1858. He says: — " Last night, 

 at fifteen minutes to nine, ascending one of the sharp hills of 

 this neighbourhood, I was suddenly surrounded by a bright 

 and powerful light, which passed me a little quicker than the 

 ordinary pace of a man's walking, leaving it dark as before. 



* See p. 120 of this volume. 



t l Experimental Researches ' (1641). 



Phil. Mag. S. 5. Vol. 26. No. 163. Dec. 1888. 2 L 



