606 EEPORT~1888. 



thunderstorm. Here lie might say something which was perhaps ahsolutely wrong, 

 but he would hazard it. The suhject was full of pitfalls, and one could .<?carcelv 

 open one's mouth without putting one's foot in it in speaking of self-induction of 

 magnetisation of iron and of discharge, but he would venture to do so. He would 

 ssij that the magnetisabilitv of the iron seems in this case rather to prevent the 

 danger — rather to prevent the current coming into the inside than to facilitate its 

 coming into the inside. He thought he might quite safely say this, that within 

 an iron funnel — a long vertical iron funnel say — and within a copper vertical 

 funnel the interior would be protected in the case of the iron funnel by the mag- 

 netic susceptibility of the iron ; whereas in the interior of the copper funnel there 

 would be no such protection, although in each case there would be a very potent 

 protection from the conductivity of the metal. However, he thought that with 

 what Dr. Lodge had put before them they knew quite enough about iron to say 

 that an iron building will be as safe as safe can be — as safe as science can make it. 

 He thought it was rather an important thing for powder magazines that the rule 

 .should be no lightning-conductors at all, but iron roof, iron walls, iron floor: 

 wooden boards over the floor naturally to prevent people setting fire to the gun- 

 powder by walking over sheet iron, but a complete surrounding of iron ; and then 

 that might be placed on a dry granite rock, it might be placed on glass, it might 

 be placed on anything they pleased, it might be placed over water — no matter 

 what the surroundings are, the interior will be safe. He thought that was a 

 somewhat important practical conclusion that might safely be drawn from these 

 electrical oscillations and mathematical calculations of which they had heard so 

 much. 



Professor Eowland, referring to Professor Lodge's experiment and the photo- 

 graphs exhibited by Mr. Abercromby, said the question seemed to be whether that 

 experiment actually represented the case of the lightning. For himself he was 

 very much disposed to think that it did not. In Professor Lodge's experiment the 

 whole of the circuit between the condensers was a material of copper and iron, 

 whereas in the case of the lightning the greater portion of the flash was in the air, 

 and therefore it might be an entirely different phenomenon in the case of the light- 

 ning from what it was in the case of Professor Lodge's experiment. Furthermore, 

 he was not entirely disposed to think that the length of the spai'k was an index of 

 the resistance of the conductor in that case, for the reason that they did not know 

 what the length of the spark was when there was an oscillating electro-motive force 

 acting on it. He was not disposed to say that the discharge would take place at 

 the first moment ; there might be considerable oscillation, and finally the air might 

 break down. The oscillation might take place before the spark went, in which 

 case the length of the spark might not be an index altogether of what they might 

 call the temporary resistance of the conductor to the passage of the spark, and the 

 presence of iron might change the time of the oscillation in some way, so that it 

 might give a difft?rent length of spark in that case from what it did in the copper. 

 But he thought the fact should be remembered that in the case of the lightning the 

 greater portion of the spark was in the air which was a very bad conductor, and 

 that therefore the discbarge of the lightning might not be oscillatory, but might 

 be of the nature of a swing, in which case a solid conductor or a conductor of 

 copper would probably be worse than that of iron. "With regard to the photo- 

 graphs he noticed many curious phenomena, which he thought should be guarded 

 against. He was happy to say that the best one exhibited, which might form the 

 text of his remarks on the rest of them, was an American photograph, in which 

 case they had the whole plate exposed and the Ughtning in the centre of it. He 

 did not suppose that all those flashes were taken at once, but that, as Sir William 

 Thomson had remarked, the plate might have been exposed some time and have 

 several discharges upon it. As he looked at that he noticed near the centre the 

 lines of discharge were perfectly sharp. As he wandered off towards the edge he 

 saw some phenomena which were very similar to tho.se that he saw upon those 

 other photographs, which therefore he would say were due to the astigmatism of the 

 lens. For instance, some sparks were merely an astigmatic image of the true fact, 

 and in the corners and off" on the side he saw sparks which were very similar. 



