152 



NATURE 



[June 12, 1890 



ably conclude that the discharge is partial and 

 incomplete. 



In beaded lightning there occur a number of bright 

 spots, giving the flash the appearance of an irregular 

 string of lustrous beads. This phenomenon is sometimes 

 well shown in photographs of the machine spark, 

 especially when the quantity of electricity passing is in- 

 creased by using very large Leyden jars. Under these cir- 

 cumstances the path of the discharge is often found to | 

 contain at irregular intervals certain small and abrupt V- 

 shaped indentations, and these, especially when seen 

 " end-on," appear to be more luminous than other portions i 

 of the flash. Probably, therefore, in a beaded flash the 

 quantity of electricity pa'^sing is more than ordinarily 

 great. 



Sometimes a lightning flash appears to take a very 

 circuitous and roundabout path, perhaps forming a 

 nearly closed loop, or even a complete knot. Such is j 

 what the Thunderstorm Committee of the Meteorological 

 Society have called " meandering " lightning. This re- 

 markable eftect is no doubt the result of an optical illusion, 

 and occurs when the general direction of the flash (or of i 

 part of it) is either towards or away from the observer. ' 



The diflerent parts of the flash which seem to approach 

 or to cross one another may in fact be miles apart. This 

 explanation may be simply illustrated by means of the 

 shadow of a properly bent wire. I have here a wire 

 which is bent in such a form as to imitate a common 

 type of flash or machine spark. When held transversely 

 to the beam of the electric light its shadow is seen to 

 represent fairly well the form of an ordinary sinuous 

 flash ; but if it is turned round so that its length is in the 

 direction of the beam of light, the shadow presents an 

 intricate appearance of loops and knots. Fig. i is from 

 one of the most remarkable photographs of lightning 

 flashes that I have seen. It was taken at Cambridge on 

 June 6, 1889, by Mr. Rose, of Emmanuel College, and I 

 am indebted for this copy to the kindness of Mr. W. N. 

 Shaw, who described it at a recent meeting of the 

 Physical Society. Among its many interesting features 

 I will at present only direct your attention to the compli- 

 plicated knot which occurs in the upper part of the 

 flash. 



Many photographs of lightning have a curious flat and 

 ribbon-like appearance. Such ribbons are sometimes 



NO. 1076, VOL. 42] 



broad and sometimes narrow. I have to thank Mr. 

 Clayden for an excellent specimen of the broad kind, 

 which was taken by himself last summer, and is repro- 

 duced in Fig. 2. The Thunderstorm Committee are of 

 opinion that this peculiar structure may possibly not exist 

 in nature at all, the effect being produced only in the 

 photographic camera. It is noteworthy that, in nearly if 

 not quite every case when broad ribbons have been 

 obtained, the camera was held in the operator's hand, a 

 fact which naturally suggests the idea that the widened 

 image of the flash may be due to unsteadiness. It may 

 be objected to this explanation that the duration of a 

 lightning flash is so exceedingly brief as to preclude the 

 possibility of any material movement during the time 

 that its image is upon the sensitive plate. But such an 

 objection is not unanswerable. It has often been observed 

 that a lightning flash may be followed by one or more 

 other flashes in rapid succession, all taking precisely the 

 same path as the first. If then the camera were in motion 

 a series of such flashes might impress themselves side by 

 side upon the photographic plate, being so near together 

 as to give the appearance of a single wide and flattened 

 flash.^ Moreover, though the true lightning flash is 

 practically instantaneous it sometimes has a phosphores- 

 cent glow along its track, which lasts for at least a large 

 fraction of a second. This phosphorescence would tend 



Fig. 2. 



to connect the separate images into a uniform whole, and 

 add to the ribbon-like appearance of the resulting picture. 

 Dr. Hoffert has been kind enough to give me a copy of 

 an exceptionally interesting photograph" which illus- 

 trates this explanation. The camera was held in the 

 hand, and moved horizontally to and fro at the rate of 

 about once in three-quarters of a second. The move- 

 ment was continued until a flash was observed, when the 

 lens was at once covered. The plate after development 

 showed no less than two triple flashes and one double 

 flash, eight in all, the whole of which, must have occurred, 

 Dr. Hoffert thinks, within a little more than a second, 

 forming a connected system of discharges which would 

 appear to the eye as one. The several sets of flashes in 

 the photograph are all joined together more or less per- 

 fectly by horizontal luminous streaks, which, though they 

 may not impossibly represent a continuous brush-like 

 discharge, are more probably due to phosphorescence of 

 the oxygen of the air, oxygen, especially in the form of 

 ozone, being a phosphorescent substance. If in taking 

 Dr. Hoffert's photograph the camera had been moved 



I It has teen pointed out by Prof. S. P. Thompson that the path of the 

 discharges might te shifted by the wind to a sufficient extent to produce the 

 hbbon-like effect, even if the camera were perfectly steady. , ., ., 



^ A good reprcduction of this photograph is given in the Phil. Mag. 

 (i£89), and in ihe Prcceedings of the Phys. Soc, vol. x. p. 176. 



