DEVIATION OP THE COMPASS.] NAVIGATION-NAUTICAL ASTRONOMY. 



1119 



permanent magnets ; so that if the principle of adjust- 

 ment hail been correct, they should not either of them 

 have changed or differed from the other. 



Trusting to the compass near the helmsman, the captain 

 hart the idea firmly impressed upon his mind that he was 

 sailing fairly down almost mid-channel ; at all events, in 

 a good position for navigating the Irish Channel. The 

 other compass indicated a difference of about two points. 

 The captain, however, judging from certain indications 

 which he had noticed previously, assumed that the wheel 

 compass was the correct one. 



In the course of a few hours, about half-past eleven 

 o'clock on the same morning, the wind having increased, 

 and a heavy sea setting up the channel, the ship made 

 rather a rapid progress, when they suddenly came in 

 sight of laud on the lee beam, in such a position that 

 there was necessarily a great difficulty in this case 

 (according to the measures pursued) an insurmountable 

 difficulty in avoiding the land. An attempt was made 

 to wear the ship round. This failed : and then an attempt 

 was made to use the anchors to bring her up. Both the 

 cables snapped on the occasion, and the ship was then left 

 helpless, driving broadside upon the rocks of Lambay 

 Island. The result was the fearful catastrophe namely, 

 the loss of about 290 lives : out of 100 females who were 

 on the ship, only three escaped upon that melancholy 

 occasion. 



Investigations into the cause of the calamity were under- 

 taken ; and the Marine Board of Liverpool, after stating 

 that C:ipt:iin Noble hail given very great attention to the 

 ascertaining of the correctness of his compasses, and 



ving their action on different occasions report that, | 

 " notwithstanding these precautions, it appears to this 

 Board that the Tnylenr was brought into the dangerous 

 position in which the wreck took place through the de- 

 viation of the compasses, the cause of which they (the 

 Marino Board) had been unable to determine." 



To those important matters Dr. Scoresby has given 

 much thought and attention ; and he finds, from numerous 

 experiments, some of which are very simple, that me- 

 chanical violence has a very considerable influence upon 

 the magnetic condition of iron. When an iron bar, 

 entirely neutral as to its molecular magnetism, if held in 

 an upright position, or inclined in the axial direction of 

 the earth's magnetism, was subjected to percussion or 

 other mechanical violence, not only did its magnetism 

 become much more powerful than that of simple in- 

 duction, but it strongly exhibited augmented polarity, 

 when placed in tho east and west equatorial position ; and 

 however it might be moved about and swung round, its 

 polarity remained the same. Dr. Scoresby applies these 

 facts to iron ships, and points out that, in consequence of 

 the percussive action to which the material is exposed 

 while the ships are in course of construction, it became 

 as intensely magnetic as it is possible for malleable iron 

 to be. This augmented magnetism, however, is not per- 

 manent or fixed, but, under different circumstances, as to 

 the relative directions of the ship's magnetism and that 

 of the earth, is easily changeable, and liable necessarily 

 to be changed. The magnetism developed by mechanical 

 violence can be readily neutralised or changed, under a 

 proper change of conditions, by other processes of me- 

 chanical violence. 



The general result of his experiments went to the 

 establishing of the fact, that besides the two denomina- 

 tions of magnetism ordinarily received that of simple 

 te -restrial induction, and that of permanent independent 

 in i_;neti3m there is another denomination corresponding 

 with neither ; not being absolutely controllable, like the 

 former, by terrestrial influences, nor capable 1 , like the 



latter, of resisting all kinds of mechanical violence. To 

 this third denomination he gives the name of Retentive 

 Magnetism, and which he proves to be a fluctuating 

 quality, though hitherto considered as permanent. On 

 the contrary, the long-continued vibration of a ship under 

 steam, and, much more so, the straining of the vessel 

 in a heavy sea, under the circumstances when the terres- 

 trial induction might be acting in a very different 

 direction from the original axial polarities of the ship, 

 would be sufficient to change the direction of the mag- 

 netism originally developed in the course of her con- 

 struction. Hence, he observes, much would depend, in 

 respect of the mechanical action of the sea, on the posi- 

 tion in which the ship had been built. In the case of the 

 Tai/Ieur, when he first heard of the catastrophe and had 

 read the evidence, he stated to some friends at Torquay, 

 that he would venture to affirm that she had been built 

 with her head northward ; he found, on inquiry, that she 

 had been built with her head nearly north-east. Hero 

 then, he adds, were the precise circumstances for ex- 

 pecting a change in the ship's magnetic distribution. 

 Having been- built with her head to the north-east, she 

 had a certain magnetic distribution accordingly ; and 

 when she began to strain, with her head to the south- 

 west, that distribution was necessarily changed, and the 

 first effect of it hail been to produce a great difference in 

 the two compasses adjusted by fixed in lunets. If the 

 captain had been aware of the changes which might, and 

 most probably would, take place when the ship began to 

 strain in a different position from that in which she had 

 been built ; if he had known that the compasses, having 

 so largo an original deviation as 60, might vary as much 

 as two, three, or even four points, he would have known, 

 of course, that he must place no relianco upon them. 



It is most important, therefore, continues Dr. Scoresby, 

 for safety in n:ivi^atin^ those vessels, that captains should 

 be made aware of the liability of the compasses to din 

 and so to mislead them ; that they should know the cir- 

 cumstances under which, in accordance with natural 

 laws regulating and applying tho earth's inductive action, 

 changes were most likely to occur ; that they should bo 

 always watchful of opportunities for determining tho trne 

 magnetic direction, with reference to their compasses, by 

 observations of the sun and stars ; and that by providing 

 a place for a standard cum: KISS aloft, as far from tho 

 deviating influence of the body of the ship as possible, 

 they might have guidance sufficient, with some small 

 allowances, for steering a correct magnetic course. With 

 such precautions, Dr. Scoresby did not doubt that the 

 difficulties in respect of compass guidance, in the naviga- 

 tion of iron ships, might be mainly and practically over- 

 come. 



These remarks and suggestions, from an experienced 

 navigator so well acquainted with his subject, deservo 

 the serious attention of mariners ; and there is no doubt 

 that, even in wooden ships, such a locality for a standard 

 compass, as he here recommends, would prove of service. 

 In iron ships, as sufficiently shown above, any compass- 

 adjusting apparatus, applied at the outset of a voyage, 

 becomes of little or no avail when the vibration and strain 

 of the vessel are thus known to change its magnetic 

 condition; and any confidence placed in such .adjust- 

 ments is likely to beget a feeling of security, and to 

 allay apprehension, even in situations of the most immi- 

 nent peril. 



For a full account of Dr. Scoresby's views, and of those 

 of Mr. Towson, another very competent authority, tho 

 reader is referred to The Proceedings of the Twenty-fourth 

 Meeting of the British A saociation for the Advancement nf 

 Science. (See, also, on this subject, p. 24C. ante). 



