IGG 



♦ KNO^AT^LEDGE ♦ 



[Jdne 1, 1889. 



the true natuial form of a flash of lightnmg was more 

 coiTeetly represented by a crooked line as in fig. 2, and that 

 it occasionally assumed the forked or branched form indi- 

 cated in fig. 3. Any one who compares the.se woodcuts 

 (which were those used in Mr. Nasmyth's article, and have 

 been kindly lent by the British Association for the illus- 

 ti-ation of this paper) with the photographs of flashes of 

 lightning shown on the plates will at once see what a keen 

 observer Mr. Xasmyth must have been. 



Two years ago the Council of the Royal Meteorological 

 Society i.<sued a circular stating that they desired to collect 

 photogi'aphs of flashes of lightning, as they believed much 

 valuable information might be obtained from them respecting 

 some of the forms which lightning occasionally assumes. It 

 had at times been reported at the meetings that ball lightning 

 had been seen slowly moving through the air, occasionally 

 entering rooms and bursting, and it was hoped that these 

 photographs would throw some light upon the subject. 



Neiirly one hundred photogi-aphs from all parts of the 

 world have been received in answer to this appeal, but no 

 photographs of the ball lightning described have as yet 

 come to hand. The photogi-aphs received were first shown 

 at the annual exhibition of the E. ^Meteorological Society 

 in March, 1888, and subsequently at the conversazione of 

 the Royal Society, and on each occasion they attracted 

 much attention. 



In June last the Society published a report on these 

 photographs, in which a classification was made of some of 

 the various forms of hghtning flashes. The six most typical 

 forms are : — Stream, sinuous, ramified, meandering, beaded 

 or chapletted, and ribbon lightning. 



Some of the moi-e remarkable photographs are repro- 

 duced on the accompanying plates by kind permission of 

 the Council uf the Society. 



Plate I. contains six of the photographs taken in London 

 during the tenific and prolonged thunderstorm which burst 

 over the metropolis on the evening of August 17, 1887. This 

 storm is described as being the most severe that has occurred 

 in London for very many years past — the lightning was 

 exceedingly vivid and brilliant, and the thunder loud and 

 continuous. Certainly these sis photographs show that the 

 lightning assumed strange forms, and pursued very erratic 

 and meandering paths. We shall refer to these photographs 

 after describing those on Plates II. and III. 



No. 7 on Plate II. is an example of ramified lightning, the 

 flash dividing and branching ofi' in various directions. The 

 flash at the right hand of the photograph appears to have 

 twisted and turned about in a curious manner. This 

 photogra])h was taken by Mons. E. L. Trouvelot, at Meudou, 

 near Paris, on June 24, 188S. No. 11 is another example 

 of ramified lightning, in which little flashes branch out from 

 the main flash, though we have no evidence as to whether 

 these fibres branch ofl'from, or run into, the main flash. It 

 wUl be noticed that in most instances the lower or earth 

 end of the flash is brightest. 



No. 8 shows a grand display of sinuous flashes, the light- 

 ning also illuminating the thunder-clouds. This photo- 

 graph was taken by Mr. L. S. Clarke from the top of the 

 AUeghany Mountains on August 2, 1887. No. 11 is also 

 an example of sinuous lightning. In this photograph 

 several flashes appear to follow almost the .same path, 

 though, as the plate was exposed for some time, we have no 

 means of knowing whether the.^e flashes occurred almost 

 simultaneously or whether there was a considei-able interval 

 between each. Neither have we any knowledge as to the 

 distance of the lightning from the camei-a. Mr. H. C. 

 Russell, of the Sydney Observatory, has just sent over a 

 photogi-aph which shows three or four flashes of lightning 

 much closer together than those in No. 11. He was able 



to determine the position of the thunderstorm, and found 

 that the lightning was about twenty-six miles distant. The 

 flashes maj' consequently have been some distance apart. 



No. 9 is an example of the meandering and beaded forms 

 of lightning. The main flash seems to wander about in an 

 aimless sort of way, while in the central flash in the upper 

 part of the photograph there appear to be two bright spots. 

 It has been suggested that these bright spots are caused by 

 the flash at those puints moving directly towards, or away 

 from, the camera, and consequently giving a somewhat 

 longer exposure at these spots. 



Nos. 12 and 13 are examples of the ribbon form of 

 lightning, in which the flash has the appearance of a piece 

 of ribbon being waved in the air, the front edge being 

 brighter than the other part. Nos. 1 and 2 on Plate I. have 

 also the same appearance but in a more marked degree. 



Several suggestions have been made as to the probable 

 cause of this ribbon-like appearance. One suggestion is that 

 the second fainter image is formed by internal reflection 

 from the back of the glass plate. This duplication, how- 

 ever, is not confined to glass negatives, but occurs on paper 

 negatives as well. No. 13 was taken on a sensitive paper 

 film. Another suggestion is that the double image is formed 

 by the internal reflections of doublet lenses. A third sug- 

 gestion is that the lines are caused by the shaking of the 

 camera in the dii-ection of the folds of the ribbon. None 

 of these suggestions seem, however, to be borne out by a 

 close examination of the photographs. If the streaks were 

 due to reflections from the surfaces of the photographic 

 lenses, we should expect to find a symmetrical arrangement 

 of such ghosts with respect to the axis of the lenses or the 

 centre of the plate ; but there is clearly nothing of the 

 kind. It will be noticed that the delicate lace-like filaments 

 in photograph No. 1 are not quite parallel on difl'erent parts 

 of the plate, but the want of parallelism of the filaments 

 seems to be rather due to perspective than to a shift of the 

 plate ; this is even more noticeable on photographs 1 2 

 and 13. If the camera could be shifted rapidly enough 

 during the minute fraction of a second which the lightning 

 flash lasts to cause such traces on the plate, all the tracks 

 would be parallel to one another or tangential about a 

 centre round which the camera had moved ; but there is 

 clearly no such arrangement, and the brighter streaks do 

 not correspond with brighter spots on the edge of the 

 ribbon, as would be the case if they were due to a shift of the 

 plate. Recent experiments by Mr. J. Wimshurst with his 

 electrical influence machine have conclusively shown that in 

 the case of such sparks as can be produced in the laboi-atoiy, 

 a rapid motion of the plate will not give rise to strciiks. Mr. 

 Wimshurst has made a dark slide for his camera, to which 

 is fitted a train of clockwork carrying a disc, upon which is 

 an arrangement for holding the sensitive plate. When all is 

 complete for photographing a flash, the clockwork is wound 

 up, the sensitive plate then i-apidly acquires great velocity, 

 which at the maximum reaches 2,-500 revolutions per 

 minute, and the plate rotating at this speed, the spark is 

 photographed. The photographs thus obta,ined in no way 

 indiciite movement in the sensitive plate, the flashes being 

 quite sharp and distinct. 



It will be noticed that the brighter streaks proceed from 

 parts of the ribbon edge where there is a sudden change in 

 direction of the stream — possibly the appearance of in- 

 creased brightness may be merely an optiail illusion caused 

 bj' greater thickness of the stream at these points as seen in 

 projection. On this supposition the fla-sh would consist of a 

 bright sinuous band, with a broader and fainter band 

 running parallel with it. The flash in photograph No. 6, 

 when examined with a lens, is seen to be of the same 

 ribbon-like character, the plane of the ribbon remaining 



