NATURAL PHILOSOPHY. 153 



impress upon them was, that, by attempting to adjust a transient influence 

 by a permanent influence, they were only aggravating error ; that captains 

 ought always to bear in mind this liability of their compasses to mislead 

 them two or three points ; that they should be always looking after their 

 correction and verification whenever the sun or a star was in sig^ht ; and 



D ' 



that, by keeping a compass aloft as far as possible from the iron of the 

 ship, they would always have a standard to which they would be able to 

 refer, and which he, in his Arctic voyages, had always found to be correct. 

 At the conclusion of Dr. Scoresby's paper, an animated discussion took 

 place, in which an opposite position from that taken by Dr. S. was 

 assumed by Mr. Grantham, of Liverpool. lie said, that the effect of the 

 arguments of Dr. Scoresby, if they made any impression at all, would be 

 to put a check to one of the greatest improvements of modern times. If 

 the idea should get abroad that wooden vessels only must be made the 

 medium of communication, and that iron ships were dangerous, the 

 mercantile marine of this country would be greatly injured. They all 

 understood and admitted that there was an immense amount of local 

 attraction disturbing the compasses of every iron ship, but he contended 

 that sufficient was done to correct these errors ; and he himself, from an 

 experience of twenty years, closely watching the subject, and employed 

 professionally to examine these very compasses, declared his opinion that 

 the impressions left upon the minds of many, where a single case, like that 

 of the Tayleur, was brought before their notice, were altogether erroneous. 

 Dr. Scoresby had alluded to the cases of the Tayleur and the Birkenhead 

 in depreciation of iron ships. He (Mr. Grantham) would ask him to take 

 equal pains to ascertain the scientific reasons which had caused so many 

 ships wooden ships to go ashore in the Irish Channel and in the Solway 

 Frith. If equal logic had been made use of to show how these losses 

 arose, instead of taking one or two isolated cases, and forgetting altogether 

 the total amount of human life lost in wooden ships, they would find the 

 losses of iron ships were very much less than the losses of wooden ships, 

 in proportion to their comparative number. Dr. Scoresby had instanced 

 the increased polarity of a bar of iron when its form was changed, or 

 when it was struck with a hammer. But this argument was founded on 

 the supposition that an iron ship was a single bar 'of iron, and that she 

 should receive a blow from a mighty instrument in a certain position. 

 There would, no doubt, be some alteration in the ship's compasses, if such 

 a thing was possible ; but he would ask the question how and when a 

 ship could get such a blow. Dr. Scoresby also maintained that, if a bar of 

 iron was bent, it would alter its polarity, but they must therefore suppose 

 the ship to be twisted and bent about. If she did so, every plate would 

 open, and she would inevitably founder. He called upon them to look at 

 the channel, full of iron ships, and those kind of ships most liable to 

 vibration that is, long ships, with most powerful engines on board of 

 them. If those present had, like him, looked over those vessels twice a 

 year, they would see how little they were affected, comparatively. One 

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