604 REPORT— 1888. 



the oscillatory discliargre in ordinary liyrlitiug in respect to the duration of the dis- 

 charge and in respect to the multiple flashes. He thought that Mr. Oliver Lodge 

 had distinguished these more or less from one another. There might be a slow 

 discharge. Mr. Abercromby had referred to a hall of lightning running about clown 

 the street, and so on. There was a curious description in Arago which Professor 

 Tait credited (although he must say for himself that he scarcely credited it) so far 

 as to quote in a lecture he gave at Glasgow some time ago, which described a ball 

 of lightning as coming in at the window, running about amongst the people, and 

 brushing up against their legs as if a kitten was about, and after that going out 

 up the chimney. This slow discharge by ball lightning had been described 

 very graphically by many people. He had never seen any description so minute as 

 that of Arago's, which he must repeat he scarcely believed. He thought in respect 

 to the duration that what INIr. Abercromby had called attention to was probably the 

 true explanation. When people see a ball, as it were, passing along the floor, 

 going up the wall and out at the window, he believed it was altogether a physio- 

 logical aflair. They had been looking in some direction or other when the flash 

 came ; at the instant that the flash came there was an intense action on the centre 

 of the retina, especially if they chanced to see the flash in the sky ; naturally after 

 such a startling incident the eyes are moved and the person after seeing the flash 

 looks about to see what has happened — looks on the floor, looks along the wall, 

 looks up at the window, and a spot of light follows, so that he believed this mar- 

 vellous ball of lightning could be seen by every person present going out of any 

 window that he happened to look out of 



He thought Mr. Preece was perfectly right in speaking of the multiplicity of 

 flashes of lightning. That gentleman gave some unmistakable experimental and 

 observational evidence which agreed altogether with what he (Sir William 

 Thomson) had noticed. The first lime he distinctly remembered noticing it was in 

 the year 1840, at Frankfort, where he had the pleasure of seeing a great many 

 thunderstorms. He then remarked what he did not remember noting before, namely, 

 triple and quadruple flashes frequently, but at such short intervals of time that one 

 could not but think they were somehow connected, and yet at certainly long enough 

 intervals of time to allow him quite distinctly to see that they were not one flash 

 but several flashes. It was impressed upon him then, and had been since — because 

 he had incessantly seen them since — that, as Mr. Abercromby had said, it did not 

 appear to be a repeated repetition of flash along one and the same course, but a 

 succession of connected flashes — a sort of breaking down all over the line — all over 

 the place. One flash causes ever so many others: it is something like setting up a 

 set of toy bricks or soldiers —tumble down one of them and there is a commotion 

 all along the line, with a very sensible time interval. He thought something must 

 be allowed for sound in this case. The light of one flash tends to produce another 

 flash. If there is a flash just ready to take place between two knobs here, a 

 flash there will cause it to pass, according to an experiment which Professor 

 Schuster bad given in his work, which had been a great deal referred to. As for the 

 fact of lightning tending for the moment to make air more easily broken by the 

 flash, it seemed that the very fine vibrations in light actually put the air into a more 

 disruptive condition, or rather a condition that is more easily disruptive than air 

 which is not agitated bj^ light. That would not account for the time interval between 

 the different flashes which were spoken of by Mr. Preece, and which he had himself 

 repeatedly noticed. He thought there must be something in the velocity of sound 

 — the velocity of the propagation of sound in the air. It seemed not improbable 

 — he would not say improbable, for the first flash does produce certainly a tre- 

 mendous disturbance in the air — a tremendous disruption of air probably causing a 

 very perfect vacuum in the place of the flash and the shaking together of the air. 

 He thought they could not account for it otherwise than by supposing a crack in 

 the air suddenly filling up and producing an exceedingly sharp elastic disturbance. 

 It is quite possible that that elastic disturbance coming at a time nearly equal to the 

 ordinary velocity of sound to another place where they are in a state of high tension, 

 ready to break down, causes it to break down. Thus it may be that one flash causes 

 considerable numbers of others to spark at an interval of a quarter of a second, or 



