128 THE SCIENTIFIC PAPERS OF 



laid, depending upon double action, one contact being established 

 between the ships and the torpedoes, and the second contact on 

 land. If both contacts were established the mine would explode, 

 but if one or the other was not in contact it would not explode. 

 He agreed with Captain Dawson that it was altogether a very 

 difficult subject ; but a great deal would no doubt be done, and 

 submarine mines would form an essential point in modern warfare, 

 not only for defence, but for attack. 



In the discussion of the Paper 



"ON LIGHTNING AND LIGHTNING CONDUCTORS," 

 By MR. W. H. PREECE, 



THE PRESIDENT* (Mr. C. W. Siemens) rose to move a vote of 

 thanks to Mr. Preece for his valuable paper. The paper dealt 

 with two subjects one a purely theoretical, and the other a 

 practical telegraph subject. 



With regard to the first, the distribution of atmospheric electri- 

 city, they had heard a very able discussion, and he thought they 

 had all learned a great deal respecting that most difficult question. 

 Practical observation on lines in countries where atmospheric 

 electricity abounds, indicates that its distribution is perhaps more 

 local than most electricians suppose. 



For instance, if they erected a line with posts, each of them 

 carrying a lightning discharger, even that might be an insufficient 

 protection, seeing that between pole and pole there might be an 

 accession of electricity in the atmosphere and discharge into the 

 wire. The only absolute protection to a land wire would be 

 probably to make all the post conductors, either iron posts or 

 wooden posts, with lightning conductors, and make them carry at 

 the top a wire connected with earth all the way ; but that would 

 be expensive, and would involve many practical difficulties ; there- 

 fore, the next best thing was to protect the station itself. He 

 thought it was not sufficient to protect the coil of the instrument, 



* Excerpt Journal of the Society of Telegraph Engineers, Vol. I. 1872-73, p. 378. 



