98 



KNOWLEDGE. 



[May 1, 1898. 



sensation in the joints, as if they had been violently 

 twisted, a sensation which lasted for a few hours. 



Many of the effects of atmospheric electricity depend 

 upon that kind of discharge known as the brush and the 

 flloit-, both of which, according to Faraday, consist of a 

 charging of air, the only difference being that the glow 

 has a continuous appearance from the constant renewal of 

 the same action in the same place, whereas the brush 

 ramification is due to a momentary independent and 

 intermitting action of the same kind. In other words, a 

 continuous discharge to the air gives the yhnc, an interrupted 

 one produces the brush, with a kind of subdued roaring 

 noise, while in a more exalted electrical condition we have 

 the spark. 



The brush discharge may often be seen playing on 

 pointed bodies, as on the spears of Cssar's legion, and in 

 this mild form it did not produce the terror of the more 

 energetic display already referred to. In later times, when 

 it appeared at the extremities of the masts of ships, it was 

 hailed as a good omen, and was known to the French and 

 Spaniards as St. Elmo's or Helmo's Fire : to the Italians as 

 the Fires of St. Peter and St. Nicholas : to the Portuguese 

 as Corpos Santos, which the English sailors have corrupted 

 into Comazants. 



Such a phenomenon is referred to by De Saussure in 

 the lighting up of the rocks at night, and the motion of 

 electricity over the prairies of America. It has been 

 compared to a miniature lightning discharge, resulting 

 from the electrified cloud brushing over the earth, and 

 discharging itself by thousands of sparks. In Switzerland 

 the discharge sometimes produces loud rattling or crepita- 

 tion of the stones. On the summit of Piz Turley De 

 Saussure experienced pricking and burning sensations, and 

 heard sounds like simmering water, emitted by sticks 

 laid against the rocks, or by the vibrations of the alpen- 

 stocks. Professor James Forbes, while on one occasion 

 on Mont Cervin, at an elevation of about 9000 feet above 

 the sea level, noticed a curious sound, which seemed to 

 proceed from his alpenstock. He asked the guide whether 

 he heard it, and what he thought it was. " The members 

 of that fraternity,' he remarks, " are very hard pressed 

 indeed when they have not an answer for any emergency. 

 He therefore replied with great coolness that the rustling 

 of the stick no doubt proceeded from a worm eating the 

 wood in the interior." On reversing the stick, the worm 

 was already at the other end. He next held up his fingers 

 above his head, and they yielded a fizzing sound. There 

 was only one explanation : the party was so near a thunder- 

 cloud as to be highly electrified by induction. All the 

 angular stones near them were hissing like points near a 

 powerful electrical machine. 



It is not necessary, however, that a thunder - cloud 

 should be present to account for a similar excited state of 

 the atmosphere when Livingstone was on the borders 

 of the Kaluhari Desert. During the dry seasons that 

 succeed the winter and precede the rains a hot wind 

 occasionally blows from north to south. This wind is in 

 such an excited state that a bunch of ostrich feathers held 

 for a few seconds against it becomes as strongly charged 

 as if it had been attached to a powerful electrical machine, 

 and clasps the advancing hand with a sharp crackling 

 sound. When this hot wind is blowing, and even at other 

 times, the peculiarly strong electric state of the air causes 

 the movement of a native in his kaross to produce a stream 

 of small sparks. "The first time," says Livingstone, 

 " that I noticed this appearance was when a chief was 

 travelling with me in my wagon. Seeing part of the fur 

 of his mantle, which was exposed to slight friction by the 

 movement of the wagon assume quite a luminous appear- 



ance, I rubbed it smartly with the hand, and found it 

 readily give out bright sparks, accompanied with distinct 

 cracks. ' Don't you see this ? ' said I. ' The white man 

 did not show us this,' he replied ; ' we had it long before 

 white men came into the country, we and our forefathers 

 of old ! ' " 



Another form of low-lying electrical meteors is known as 

 (itohular or hall lightning {eclairs en boule). A celebrated 

 case of this kind was reported in the Philosophical Trans- 

 actions of the Royal Society about the middle of the last 

 century. Mr. Chalmers states that being on board the 

 Montague (seventy-four guns), on November 4th, 1749, he 

 observed a large ball of blue fire roUing along on the 

 surface of the water as big as a mill-stone, at about three 

 miles distant. Before they could raise the main-taclj the 

 ball had reached within forty yards of the main chains, 

 when it rose perpendicularly, with a fearful explosion, and 

 shattered the main topmast in pieces. So also on the 

 Malvern Hills, in June, 1826, what was called " a ball of 

 fire " was observed to roU along the hill towards a building 

 where some people had taken shelter ; here it exploded, and 

 killed two of them. 



These luminous balls seem to be the result of a brush 

 discharge on a large scale. In the case of the Montague, 

 the ball was seen rolUng on the surface of the water 

 towards the ship from to windward. The discharge was 

 produced by some of the polarized atmospheric particles 

 yielding up their electricity to the surface of the water. 

 On nearing the ship, the point of discharge became 

 transferred to the head of the mast ; and the striking 

 distance being thus diminished, the whole system returned 

 to its normal state, that is to say, a disruptive discharge 

 ensued between the sea and the clouds, producing the usual 

 phenomena of lightning and thunder, or as it was described 

 by the observers, " the rising of the ball through the 

 mast of the ship." The case on the Malvern Hills is 

 another instance of the same kind. 



Another case of glow discharge was reported by Mr. Jabez 

 Brown in a letter to the 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. The light was 

 seen by the sailors in the harbour coming in from the sea, 

 and passing up the valley hke a low cloud." 



Some electricians doubt whether these balls of electric 

 fire have ever been seen within doors. Such cases, how- 

 ever, have been reported on good authority. Thus M. 

 Trecul, in a note to the Comptes Rendus, states that on the 

 18th August, 1876, while writing at an open window 

 between 7 and 8 a.m., he observed simultaneously with 

 some loud thunder, small luminous columns descend 

 obhquely on his paper, about two metres long, and half 

 a decimetre broad at the widest part ; obtuse at the further 

 end, but gradually thinning towards the table. They had 

 mostly a reddish-yellow tint, but near the paper the tints 

 were more intense and varied. In disappearing they left 

 the paper with a slight noise like that produced by pouring 

 a little water on a hot plate. 



Arago relates a case of globular lightning within the 

 walls of a building as reported by Maffei as occurring in 

 September, 1713, in the territory of Massa-Canara, in Italy. 

 He took refuge from a storm in a chateau, where he was 

 received by the mistress of the house in a room on the 

 ground floor. Suddenly they saw a bluish-white flame rise 

 from the floor ; it was agitated, but had no progressive 

 motion. After gradually acquiring a considerable volume 

 it suddenly disappeared. Maffei felt in hia shoulder a 



