December, 1906] 



KNOWLEDGE & SCIENTIFIC NEWS. 



601 



article, for the ramifications in each case are all directed 

 cloudwards and not earthwards. 



It may be useful to show dia^'-rammalically (Fig. 2) the 

 two main conditions mentioned aljove, namely, {a) when 

 the cloud is positive and the earth negative (the common 

 case); and {b) when the signs are reversed (the excep- 

 tional ca,se); the arrows indicate the direction of travel 

 of the current in each case. 



It will then be seen that the g'eneral trend of the 

 branches indicates the direction of the discharge, and 

 this can be determined as easily by the eye as it can be 

 recorded by the photographic plate. The reader's at- 



-A liKhtinf; flash fr 

 Julyji, 1904. 



the liiffel Tower to ; 



cloud. Paris, 



tention will, however, here be restricted to photographs 

 alone, for they can be examined at leisure. Although 

 I have examined some two hundred piiotographs of 

 ligiUning flashes, I ha\e only come across two which, 

 in my opinion, are, without doubt, earth-to-cloud dis- 

 charges, one of them, which shows two such flashes, 

 being a nearly perfect example. The third photograph 

 represents also, I believe, an upward discharge, but it 

 may be an exceptional effect of perspective upon a 

 cloud-tcvearth flash. One has to Ih' exceedingiv careful 

 in differentiating lutweeii photographs of true earlh- 



to-cloud discharges, and those which appear like them, 

 but are really due to an effect of perspective. Several 

 photographs which I have examined appeared to indi- 

 cate an upward trend of the ramifications, but in nearly 

 every case the directions of the branches could be ex- 

 plained without difficulty in the above manner. 



It is, therefore, important to treat as typical cases 

 of earth-to-cloud flashes only those photographs which 

 show the earth and cloud ends of the flashes in the field 

 of view of the camera. This means, therefore, that 

 such photographs can only be secured when the flashes 

 are some distance away from the photographer. 



A case of a very probable perspective effect of a 

 cloud-to-earth flash is that shown in Fig. 3. This ex- 

 cellent photograph was taken by Mr. James Crosbie, oi 

 l'>ith, in 1894. The flash commences in a cloud some 

 distance away behind the trees, and comes towards the 

 observer. The single stream, which, in the first in- 

 stance, is thin, because the flash is so far distant, splits 

 up into two branches, as the observer is approached; 

 each of these becomes more intense, and, therefore, 

 broader, because they are getting nearer the observer. 

 The flash probably reached earth somewhere behind the 

 photographer. The ramification towards the right of 

 the photograph has also a direction inclined rather 

 towards the camera, but it was apparently dissipated 

 before it reached earth. 



Coming now to the first of the earth-to-cloud photo- 

 graphs, we have an excellent example in the interesting 

 flash shown in Fig. 4. This was taken by Monsieur 

 li. Mesmer at Paris on July 31, 1904, at iih. 45m. 

 p.m. The upper portion of the picture was the actual 

 photograph secured, but owing to the faintness of the 

 landscape in the original negative, a similar view was 

 taken by him the next day from the same position with 

 the same camera. This view has been placed exactly 

 below in the illustration, so that the top of the tower 

 should be imagined to be at the lower extremity of the 

 Hash. 



The flash itself emanated from the top of the Eiffel 

 Tower. It made a sinuous path upwards, and then 

 j\ idently met a stratum of air through which it could 

 not easily pass; this caused it to alter its direction (to- 

 wards the left in the photograph). The flash then split 

 itself into two, each of the branches becoming fainter 

 and eventually discharging themselves in the clouds in 

 the upper air. 



It seems to me more easy to explain the peculiarity 

 of this flash as an earth-to-cloud discharge than to 

 imagine the probability of two flashes meeting together 

 in the air and pursuing the same track to the earth. 



It may be of interest to the reader to know that when 

 1 suggested to Monsieur Mesmer that this flash was of 

 a i>cculiar nature and I felt bound to consider it as an 

 carth-tcv-cloud discharge, he wrote : — 



"Your remark on the subject of the flash with an 

 upper bifurcation appears to me quite correct. It is 

 manifest that this discharge ought to be directed from 

 the luffel Tower towards the clouds. I had not re- 

 marked this peculiar character of this flash, and Mon- 

 sieur Fmile Touchet, to whom I have .sent a print of 

 this photograph at the same time as the others which 

 ha\e been pul)lish('d, has also doubtless not perceived 

 it, as he did not mention it to me. What did appear 

 remarkable to me about this flash was that it was 

 visible in the whole of its development (from beginning 

 to end), and struck, or rather flowed, from a well- 

 known terrestrial object. . . ." 



( To be continued, j 



