600 REPORT— 1888. 



this effect must have very important consequences. He thought there was no 

 doubt that once a flash occurs the lifrht of it must make all conductors in the 

 neighbourhood, all the air in the neighbourhood, very easily able to break down, 

 and so may give rise to a multiplicity of flashes from all the neighbouring points. 

 That was why he said areas of protection were misleading' ; if one flash caused a lot 

 of others they had better not have the one if they could help it. Therefore he 

 said, do not run up these great long rods to attract flashes near powder magazines 

 or anywhere where it is dangerous, but try to avoid them if you can. If there must 

 be a flash they must be very careful indeed to have all the other conductors 

 arranged so that sparks along them do not matter, because if one occurred they 

 were very likely to get the other. 



The Hon. R. Abekcromby said he wished to be allowed to contribute to the 

 discussion some facts which had been brought out by looking at about 90 photo- 

 graphs of lightning flashes in different parts of the world which had been collected 

 by a Committee of the Royal Meteorological Society, of which he had acted as 

 secretary. A few of the most remarkable of the photographs he had brought with 

 him for the inspection of the meeting. In doing .so he proposed to conflne his 

 remarks to such facts only as bore on the discussion they were having that 

 morning, instead of ranging over the whole theory of lightning flashes. 



The flrst point was, is there any evidence in the photographs of a duplication 

 of flashes, that is, of one flash fallowing rapidly after another? He thought the 

 answer to that must be that there is no certain evidence He had a very remark- 

 able photograph with him, which might at first seem rather to negative that. 



Here there was one very bright flash. In one place there were no less than 

 three lines which were sorts of doubles of that bright line. In another p'lrt of the 

 same picture they had a thin flash with another flash exactly parallel to it, only a 

 little bit fainter. For reasons which it was diflicult to give without a close 

 inspection of the original picture, there was reason to suppose that that ett'ect was 

 due to a secondary reflection from the back of the glass. Although he was far 

 from denyinsr the alleged phenomenon, he thought they had often seen flashes 

 come very nearly after each other, yet so far there is no photographic evidence of 

 a flash coming exactly along the same path; but there is most decisive evidence of 

 the tendency of flashes to be parallel to each other. He now exhibited a most 

 magnificent photograph which had just been sent to the Committee from Massa- 

 chusetts. In that instance the whole air is filled with threads of lightning coming 

 down like the roots of a tree from the sky. He thought it was very much a question 

 where the area of protection would be when the whole air seemed to be pouring 

 lightning down upon you. There were other photographs in which appeared two or 

 three flashes apparently following pretty much in the same path. Then, connected 

 "with this, there is the inveterate tendency of a lightning flash to ramify. Some- 

 times thej^ got in a photograph a comparatively smooth line ; at other times it was 

 like that photogi-aph which he had already shown the meeting. The main flash 

 seems to throw out threads in every sort of direction. That would be observed in 

 an influence machine; instead of a spark always coming from one to the other, 

 they very often saw it begin to go oft' at right angles, and then jump away to the 

 other. 



Connected with that there was one thing of which he had not been able to find 

 any notice in any of the discussions or publications on lightning-conductors, and 

 that is, what is the efl'ect of rain or cloud on the passage of electricity ? All 

 laboratory experiments are taken, of course, in dry air, but every experimenter 

 knew that the dust in a laboratory has a very considerable effect on the discharge 

 of static electricity. His impression was that when lightning comes down through 

 rain or hail, or even through cloud, the continuity of what are called potential 

 surfaces, and all that sort of thing, must be very much disturbed. The Committee 

 of the Royal Meteorological Society had collected for this year of very frequent 

 thunderstorms an enormous amount of material relating to them, including cases 

 where much damage had been done to buildings. So much did he believe in the 

 disturbance he bad mentioned that when the Committee met again he should 

 suggest to them to send out a circular with the object of finding out, if possible. 



