66 



MAGNETISM. 



loc.,,1 



Attraction. 

 33' 

 7 

 30 

 27 

 22 

 36 

 43 

 30 



Ship. Commander. Pit 



Conway Capt. Basil Hall Portsmouth 



Leven . Capt. Owen , . Northfleet 



Barracou a Capt. Cutfield . Do. 



Hecla . Capt. Sir E. Parry Do. 



Fury . Capt. Hopner . . Do. 



Griper . Capt. Clavering . Nore . . 



Adventure Capt. King: Plymouth 



Gloucester Capt. Stuart . . Channel . 



giving a mean of 8 44' at the east and 

 west points in these latitudes. 



(260.) The last of these ships, the 

 Gloucester, was reported as being ' in- 

 variably drawn, in consequence of this 

 deviation, to the southward of her in- 

 tended place, notwithstanding the great- 

 est care being taken in steering her/ 

 Had it not been ascertained, by taking 

 an observation, that this error was al- 

 together the effect of local attraction, it 

 would probably have been ascribed to 

 the influence of an unknown current. 

 The real deviation, estimated in dis- 

 tance, would occasion the vessel, after 

 running ten miles, to be more than a 

 mile and a half to the southward of her 

 reckoning ; and so on in proportion as 

 the distance increased. An error of 

 this magnitude, occurring in a narrow 

 channel and in a dark night, were it 

 unknown or disregarded, might lead to 

 the most disastrous consequences. The 

 wreck of his Majesty's ship Thetis, 

 which lately happened on the coast of 

 Brazil, has been ascribed, with some 

 probability, to a mistake of this kind. 

 The following is the account given of 

 this accident in the * United Service 

 Journal : ' ' The Thetis sailed from Rio 

 Janeiro on the 4th of December, with a 

 million of dollars on board, besides 

 other treasure, and every prospect of a 

 fine passage, stretching away to the 

 south-east. The next day, the wind 

 coming rather favourable, they tacked, 

 thinking themselves clear of land ; and 

 so confident were they, that the topmast 

 studding-sails were ordered to be set, 

 the ship running at the rate of nine knots ; 

 and the first intimation they had of being 

 near land, was the jib-boom striking 

 against a high perpendicular cliff, when 

 the bowsprit broke short off, the shock 

 sending ail three masts over the side ; 

 thus in a moment bringing utter de- 

 struction on this fine vessel and her 

 valuable cargo.' Mr. Barlow shows, in 

 the paper above referred to, that the 

 deviation of the compass, arising from 

 the attraction of the vessel, was exactly 

 of the kind that was likely to occasion 

 this great mistake in the ship's reckon- 

 ing : for the distance run by the Thetis 



being about eighty miles, if the local at- 

 traction of the vessel had been equal to 

 that of the Gloucester, she would have 

 passed five miles nearer to Cape Frio 

 than had been calculated" upon; an 

 error quite sufficient to account for the 

 fatal catastrophe. 



(261.) It' is obvious that, when the 

 cargo of the ship consists chiefly of iron, 

 the error in the reckoning may be even 

 more considerable than what has been 

 now stated. The most fatal conse- 

 quences might arise in a few hours to a 

 vessel in the Channel, under these cir- 

 cumstances, in a dark and blowing 

 night, having for its only guide a com- 

 pass, subject to an error of fourteen de- 

 grees in opposite directions at. east and 

 west, the very courses on which she 

 would be endeavouring to steer. How 

 many of the mysterious wrecks that 

 have taken place in the Channel might 

 not be traced to this cause ! The loss 

 of the Thames Indiaman is given as an 

 example by Mr. Barlow *. This vessel, 

 besides the usual appointments of guns, 

 &c., had a cargo of more than four 

 hundred tons of iron and steel. The in- 

 fluence of such an enormous magnetic 

 mass would alone be quite sufficient to 

 explain the otherwise unaccountable cir- 

 cumstance, that after leaving Beachey- 

 head in sight at six o'clock in the even- 

 ing, the "ship was wrecked upon the 

 same spot between one and two o'clock 

 in the morning, without the least appre- 

 hension of being near the shore. 



(262.) The practical application of 

 the principles above established to the 

 correction of the actual deviations of 

 the compass in a ship, being, as we 

 have seen, of such great importance 

 in navigation, Mr. Barlow bent his 

 mind to the discovery of a method of 

 effecting so desirable an object. His 

 first idea was, that since the guns and 

 other iron of a vessel must produce 

 exactly the same deviation of the needle 

 as a smaller mass of iron placed in a 

 similar situation, but as much nearer as 

 its mass is smaller, it might be possible 

 to place such a body of iron aft of the 

 compass, as would exactly counter- 

 balance the action of the guns, &c., for- 

 ward, and consequently leave the needle 

 as free to move, as if no such action 

 existed: but he soon found that, for 

 this purpose, the position of the compen- 

 sating ball of iron would require to be 



* Essay on Magnetic Attractions, p. 36?. 



