TEANSACTTONS OF SECTION A. 593 



accurate measurements, he believed ; and during the past summer, amongst the 

 mountains of North Wales, he saw several thunder clouds, and those thunder clouds 

 seemed at a height of about 600 feet, and he felt almost certain that the length of 

 our flashes of lightning did not average more than 500 feet. In consequence of 

 the researches of Sir AVilliam Thomson and also the measurements made by Dr. 

 Warren De La Rue and Hugo Miiller the striking distance across the layer of air 

 for steady currents in one direction was pretty well known. It was something 

 like ci0,000 volts per centimetre. Now if a flash of lightning were 500 feet long 

 and it varied directly as the striking distance, then Oliver Lodge's measurements 

 given in his papers would be fairly accurate. But there was every reason to believe 

 that the striking distance did not vary directly as the distance, especially for alter- 

 nate currents that were modifying their conceptions in every direction of the action 

 of electricity. There was a Mr. Acheson in America who had been making some 

 very careful observations ou the striking distance across air when currents were 

 produced by transformers ; and there he made out that the striking distance varied 

 not directly as the electromotive force, but as the cube of the electromotive force. 

 If that be so then it would follow that the electromotive force forcing a current of 

 electricity through the air and producing a flash was very much less than they 

 hitherto had considered it. And again a flash of lightning always went along a 

 path that had been prepared for it. All photographs, as they would see presently, 

 gave indications that wherever lightning flowed there some preparation had been 

 made for its approach. 



Next there was an assumption which he considered a fallacy, and that was 

 that a flash of lightning was instantaneous. Now they all knew experiments 

 that were made to show revolving wheels ; the spokes of the revolving wheel 

 suddenlj' appeared before them whenever a spark passed, and they most of them 

 believed that a flash of lightning was really instantaneous. There was no proof of 

 its instantaneity. What they knew was that the flash was merely a flash of light 

 that indicated the path. How long that flash lasted they could not tell. But 

 there were dark and invisible flashes, if they might so call them. There were dark 

 and invisible charges of electricity, and there were cases on record where people 

 had been struck and killed underneath trees where no light of any kind whatever 

 had been seen. But he argued on the non-instantaneity of lightning flashes from 

 their effects on telegraph wires. Now of course we had spread all over this country 

 an enormous network of telegraph wires. Whenever a lightning flash took place 

 anywhere some wire was sure to be affected somewhere, and wherever lightning 

 took place it might not hit the wire. It sometimes did. Whether it did or whether 

 it did not, it did not apparently make very much difference in the effects observed. 

 But there they always got currents of sensible duration. For instance, on board 

 ships, ships had often been struck, especially when their lightning-protectors 

 were a little defective, and there invariably it had been observed — or rather he 

 should not say invariably, but frequently it had been observed — that compasses 

 were turned about, and they went playing about wherever the ship passed through 

 an electric storm. Now it was quite impossible that the compass of a ship whicb 

 had a very slow rate of vibration could be deflected or turned round unless there- 

 was a considerable duration in the flash. However that was a point on which the 

 photographs taken gave a considerable amount of information, and during the dis- 

 cussion he hoped that Mr. Abercromby would give them a sight of some of those 

 beautiful photographs that he had collected. 



Now he went to perhaps the hardest nut of all in the questions before them to 

 discuss — one that he approached with a certain amount of diffidence because it was 

 supported by very high authorities, and that was the assertion that a flash of 

 lightning was oscillatory in its character ; that was, that it did not flow straight 

 from the cloud to the earth and disperse ; that it went slashing backwards and 

 forwards, somebody said a million times in a second ; but lie was not going to discuss 

 that — at any rate that that flash went flying backwards and forwards with 

 considerable frequency. Now he might say at once that that assertion of the 

 oscillatory character of a lightning flash was based more on mathematical reasoning 

 than it was on absolute observation. Well, he was not going to say much on the 

 1888. Q Q 



