6oo 



KNOWLEDGE & SCIENTIFIC NEWS. 



[December, 1906. 



of such an observation as subjective, as can be gathered 

 from the following extract : — 



" A remark is made very commonly in thunderstorms, 

 which, if correct, is obviously inconsistent with what I 

 have said as to the extremely short duration of a flash. 

 Even if we supposed the flash to be caused by 'a 

 luminous body moving along, like the end of a burning 

 stick whirled around in a dark room, it would pass with 

 such extraordinary rapidity that the eye could not possi- 

 blv follow its movements. Hence it is clear that when 



Fig. 2.— Diagram to Illustrate the direction of tlie ramifications from 

 tlie main flash when the discharge Is from (A) cloud to earth 

 and (B) earth to cloud. The main discharge is In both cases 

 from + to -. 



people say they saw a flash go upwards to the clouds 

 from the ground, or downwards from the clouds to the 

 ground, they must be mistaken. The origin of the 

 mistake seems to be a nibjcctivc one, viz., that the 

 central parts of the retina are more sensitive, by prac- 

 tice, than the rest, and, therefore, that the portion of 

 the flash which is seen directly affects the brain sooner 

 than the rest. Hence a spectator looking towards 

 either end of a flash very naturally fancies that end to 

 be its starting point." (Thunderstorms, p. 9.) 



It will be gathered from the above extract that Prof. 

 Tait only regards the observation of the direction of 

 the flash as impossible, but in no way suggests that 

 flashes from earth to cloud do not occur. 



In my own case I have never been fortunate enough 

 to observe an earth-to-cloud lightning flash, but I have 

 very carefully observed a great number of flashes which 

 I regarded as travelling from cloud to earth, in spite of 

 the fact that in many instances my eye was directed 

 towards the horizon; these flashes, according to Prof. 

 Tait, should have been noticed by me as ufnvard dis- 

 charges. 



Further, I have watched numerous ramified flashes, 

 that is, discharges \\hich have branches or tributaries 

 like a river, and a strong impression has been formed 

 in my mind that these ramifications were not quite simul- 

 taneous with the main stream. This observation can, 

 however, be easily explained, as most probably the 

 brightest part of the flash, the main stream, would 

 effect the retina before the fainter portions, namely, the 

 ramifications. 



The examination of a large numlx-r of photographs 

 of ramified flashes taken with moving cameras has con- 

 vinced me that my impression of the apparent lateness 

 in the appearance of the ramifications was incorrect. 

 The photographs Indicate that they occur simultaneous 

 with the main stream, since they are joined to, and not 

 separated from, the chief channel of discharge. .\n 



example of such a photograph will be seen in the left- 

 hand flash shown in Fig. 7 (this figure will appear in 

 the second part of this article), which was taken by a 

 camera rapidly moving from left to right horizontally. 



One cannot, however, be too careful in checking eye 

 observations and impressions by photographic methods 

 whenever possible, and it is in this respect that the 

 camera is such a valuable aid to science. The eye gives 

 us onlv a temporary view, which is gone for ever, but 

 the photographic plate provides us with a lasting re- 

 cord, which can be studied at leisure. 



Assuming, therefore, that the eye is not able to de- 

 tect the direction in which a single flash is proceeding, 

 owing to the extreme rapidity of travel of the electric 

 current, a way is open to us, I think, of determining by 

 photography whether a flash is going from earth to 

 cloud or from cloud to earth, w-ith the proviso that the 

 discharge has ramifications. 



If we turn our attention for a moment to the action 

 of an electrical machine, we find that when a spark is 

 made to pass from one pole to another, the ramifications 

 from the main spark are always in a certain direction, 

 namely, from the positive to the negative pole, i.e., in 

 the general direction of the main discharge. In fact, as 

 Prof. Silvanus Thompson states in his excellent " Ele- 

 mentary Lessons in Electricity and Magnetism " (Edi- 

 tion 1895, p. 303), " the branches always point towards 

 the negative electrode." 



If anvone makes a studv of the direction of ramifica- 

 tions in photographs of lightning, he will find that the 

 ramifications are directed earthwards in about ninety- 

 nine cases out of every hundred. The accompany- 

 ing illustration (Fig. i) gives an example of such a dis- 

 charge, and is a reproduction from a photograph taken 

 by Mr. J. W. Bridges in .August, 1906. The deduction 

 to be made from this fact is that in most cases the 



Photo by Mr, James Crosbie. 



A discharge which has the appearance of a flash from earth to 

 cloud. This Is, however, \ery probably only an effect of 

 perspective. Erith, 1804. 



discharge is a cloud-to-earth one, and, therefore, the 

 cloud is the positive and the earth the negative pole. 



It can, and docs, happen, however, that clouds may 

 be negatively charged, and this has often been made 

 apparent to us bv flashes of lightning passing between 

 cloud and cloud. 



The question now arises. Can the earth become posi- 

 tive, or, in other words, can a lightning flash be an 

 earth-to-cloud discharge? This question, I think, is 

 answered bv the few photographs which accompany this 



