Oct. 4, 1888J 



NA TURE 



547 



not by great rushes. But they could not assume that it would 

 always do so, and must prepare for the occurrence of a great 

 rush. The true character of lightning must be discovered by 

 observing lightning, and not by experiments in a laboratory. 

 The spark of one induction-coil at a considerable distance 

 would start another one sparking merely by its light. From 

 that he came to the conclusion that when there was a very 

 bright flash of lightning, it must involve very important con- 

 sequences. There was no doubt that it would cause discharges all 

 <i\xr the neighbouring area, and so he would say that areas of 

 protection were misleading, and if a flash had that effect, they 

 liad better be without it if possible. 



The Hon. Ralph Abercromby, who showed a number of 

 photographs of lightning flashes, said there was no absolute 

 evidence in the photographs of flashes of lightning following 

 each other rapidly on exactly the same path. There was, how- 

 ever, distinct evidence of the tendency of lightning- flashes to 

 occur parallel to each other. There seemed to be a tendency in 

 lightning flashes to be ramified, to give off threads all round the 

 main fla^h. Photography gave conclusive evidence that flashes 

 were not so instantaneous as was generally supposed. It 

 showed that the flash did not always jump from a cloud straight 

 to the earth, but sometimes went meandering through the air 

 and tying itself into knots, so that it could not be so instantaneous 

 as was imagined. He was of opinion that lightning-clouds were 

 generally more than 500 feet high, but ligh'ningwas rarely much 

 higher than 10,000 feet high. By this he did not mean that 

 lightning might jump 10,000 feet from the cloud to the earth ; 

 but that at an altitude of 10,000 feet on a mountain-side a 

 thunderstorm was usually Mow the observer. 



Lord Rayleigh said that, although some mathematicians were 

 unpractical, yet it was to mathematics one must go to find 

 the results of known causes under new circumstances. He had 

 no special knowledge of lightning-conductors, but from his 

 general acquaintance with electricity he should say that Prof. 

 Lodge's experiments could hardly fail to have a most important 

 practical application to lightning-conductors in the future. Mr. 

 Preece spoke of the development of energy by the condensation 

 of vapour into water, but the question was to find how some of 

 that energy came to take the electrical form. 



Sir \V. Thomson said that mathematicians never pre- 

 dicted that the Atlantic cable could not be laid, but a 

 ■celebrated engineer did so. He thought Prof. Lodge was 

 in the American stage of inertia and Mr. Preece in the 

 English stage. He believed that if Prof. Lodge proceeded 

 with his experiments he would confirm his discovery that iron 

 wire was a better conductor than copper. Self-induction was 

 in the air, and they were talking of nothing else. He thought 

 Mr. Abercromby's idea as to the duration was correct. It 

 ^eemed to him probable that it was the sound of one spark 

 which caused another rather than the light. There was the 

 photograph giving three parallel flashes. It would be well if 

 some experiments could be made to discover whether flashes 

 occurring like that were simultaneous or followed one another, 

 being started by the light or sound vibrations of the first. It 

 was rather startling to find that a lightning-rod had protecting 

 power over so small an area, and he would like to ask Mr. Preece 

 whether copper had been experimentally proved to be better 

 than iron. They could come to one conclusion from what they 

 heard — namely, that houses made of sheet iron would be the 

 safest possible places in a thunderstorm. The question of the 

 effect of self-induction on statical discharges was a very import- 

 ant one. He suggested as a class experiment the discharge of 

 a Leyden jar through a number of students (1) when they were 

 arranged in zigzag rows, so as to have no self-induction in the 

 path of the discharge ; and (2) when they stood in a circle, so 

 that the self-induction of the path was a maximum. The students 

 should stand on insulating material. He thought the result of 

 >uch an experiment would be to show that the students in the 

 middle of the chain would feel the effect of the discharge far 

 less in the second instance than in the first. With reference to 

 the reports as to the occurrence of globular lightning, he be- 

 lieved them to be much exaggerated, and expressed an opinion 

 that the whole effect might be a physiological optical delusion. 

 Reiss experimented some forty years ago on the question of 

 magnetism by jar discharges, and found that the direction of 

 superficial magnetization sometimes was the one to be expected, 

 sometimes the opposite one. IIe»suggested new experiments as 

 to the influence of the rate of oscillation on the result. The 

 mest efficient protection for gunpowder against lightning would 



be, he thought, to put it in a house whose exterior was entirely 

 of iron and to put no lightning-rod on it. 



Prof. Rowland observed that the conditions of Prof. Lodge's 

 experiments were scarcely the same as those of actual lightning, 

 and he pointed out that the length of the spark was no measure 

 of the resistance of the conductor. Further, he showed some 

 effects in Mr. Abercromby's photographs which were probably 

 due to the astigmatism in the lens of the camera. 



M. de Fonvielle, who spoke in French, observed that Sir 

 William Thomson had said most eloquently that Mr. Preece was 

 taking the English side of the question and Mr. Lodge the 

 American side, but he must say that Sir William Thomson him- 

 self had taken the French side, and he had proposed a revolu- 

 tionary system which consisted in the building of iron houses. 

 He took the liberty, though being a Frenchman, to disagree 

 with the great electrician, and to stand with Mr. Preece as an 

 English conservative, with reference to lightning-conductors. 

 Lord Rayleigh said that mathematicians and physicists should 

 unite together, but he supposed that Lord Rayleigh would agree 

 with him in remarking that Mr. Preece was realizing that 

 alliance in a very remarkable manner, for on the one hand he 

 dealt with a large number of experiments and observations of 

 natural facts, and on the other hand he introduced statistics, or 

 rather the calculation of probabilities, which was one of the 

 highest branches of mathematics. The experiments made in 

 laboratories were different from tho^e which were presented by 

 Xature only so far as they were conducted on very widely 

 different scales. On the previous day, in that hall, M. Janssen 

 had proved by his observations on the action of oxygen on the 

 composition of the electric light that in many phenomena there 

 was a coefficient behind. He congratulated them on the aid they 

 were now receiving from photography. He should advise the 

 meeting to delay its opinion for the time until the completion in 

 Paris of the Eiffel Tower, whkh would be the most extra- 

 ordinary lightning-conductor in existence, being 1000 feet high. 

 He must, moreover, ptate that Paris was practically free from 

 calamities produced by lightning. They had erected a sufficient 

 number of lightning-rods, according to the principles so admir- 

 ably advocated by Mr. Preece, and that was a strong evidence 

 that Mr. Preece was altogether travelling in the right direction, 

 quite irrespective of any mathematical or physical demonstration. 



Prof. George Forbes said thai Mr. Preece did not mean to 

 pay that mathematicians came to wrong conclusions when they 

 had all the right data, but that they sometimes came to a con- 

 clusion without taking all the data into consideration. Prof. 

 Lodge had come to say that if iron was not better than copper, 

 it was at least as good ; but they could not be quite prepared to 

 accept that, because the experiments might be tried in instances 

 more nearly approaching the natural conditions, and in that 

 case it was quite possible that copper would be found to be 

 the best. 



Sir f. Douglass said that his experience of lighthouses pro- 

 tected by lightning-rods covered a space of forty years, and v. as 

 comforting to the members of the Lightning- Rod Committee. 

 He never knew a rod fulfilling the conditions he prescribed to 

 fail in protecting the lighthouse and adjoining buildings. 



Mr. J. Brown suggested the use of a revolving camera in 

 taking photographs, in order to separate flashes, and thus see if 

 each is single or not. 



Mr. Sidney Walker said that anything which would cheapen 

 lightning-conductors would be gladly welcomed. In the cases 

 where damage had occurred, he believed that the result was due 

 to a defect in the conductor. He pointed out that iron would not 

 stand the weather so well as copper, and that, besides, it would 

 be affected by the gases at the top of a factory or similar place. 



Mr. G. J. Symons said he had investigated every accident by 

 lightning of which he could hear, and had so got valuable 

 experience. The conclusion left on his mind was that if people 

 would erect conductors precisely in accordance with the rales 

 laid down by the Conference, 1 and fulfilling all the conditions, 

 they would be absolutely safe. Where accidents occurred to 

 buildings with conductors, there was a reasonable explanation to 

 be found. Prof. Lodge's experiments were laboratory experi- 

 ments, and to get the real facts they must have something on a 

 much larger scale, perhaps by a series of interrupted conductors 

 on posts on the tops of some of those high hills where storms 

 frequently occurred. With regard to protected areas, there 

 were only two cases en record, and those doubtful, of anything 

 being struck within a protected area. 



1 Report of the Lightning-Rod Conference (Sp.n, 1882). 



