44 



by the use of iron ballast and iron tanks, of iron knees, iron cables, 

 and above all, of iron capstans, besides various other articles made 

 of the same material, forming altogether a very large and powerful 

 magnetic mass. 



The direction and intensity of the deflecting forces thus produced, 

 vary in different latitudes and on different sides of the equator ; 

 being greatest in the highest latitudes, where the dip is consider- 

 able, and when the ship's course is east or west : and in high south- 

 ern latitudes, being the reverse of what it is in high northern lati- 

 tudes. In His Majesty's ship Gloucester, which maybe taken as an 

 example, the deviation of the compass in the east and west points 

 was found to be, in the British Channel, 9° 30' : so that after run- 

 ning ten miles, the vessel would be more than a mile and a half to 

 the southward of her reckoning, and so on in proportion as the di- 

 stances increased. An error of this magnitude, occurring in a nar- 

 row channel and in a dark night, were it unknown or disregarded, 

 might lead to the most fatal consequences ; and the disaster might 

 perhaps be erroneously ascribed to the prevalence of a powerful 

 current, the existence of which was before unknown. 



The Thetis sailed from Rio Janeiro, in December last, with a 

 million of dollars on board, in the finest weather, directing her 

 course to the S.E. The next day, thinking they were clear of land, 

 they tacked, and were sailing at the rate of nine knots, when the 

 first intimation they had of being near land, was the striking of the 

 jib-boom against a high perpendicular cliff, which broke the bow- 

 sprit short off, and sent all three masts over the side ; thus in a mo- 

 ment bringing utter destruction on this fine vessel and her valuable 

 cargo. The author shows that the deviation of the compass arising 

 from the attraction of the vessel, was exactly of the kind which was 

 likely to occasion this great mistake in the ship's reckoning : for 

 the distance run by the Thetis being about eighty miles, if the 

 local attraction of the vessel had been equal to that of the Glou- 

 cester, she would have passed five miles nearer to Cape Frio than 

 her reckoning, — an error quite sufficient to account for the fatal ca- 

 tastrophe. The author hence infers the importance of bestowing 

 more attention than has hitherto been given to the influence of the 

 local attraction of vessels, and to the application of the proper 

 means of correction. 



April 28. 



HIS ROYAL HIGHNESS THE DUKE OF SUSSEX, K.G., 

 President, in the Chair. 



James Henderson, Esq., His Britannic Majesty's Consul at Bo- 

 gota, was elected a Fellow of the Society. 



The following Presents were received, and thanks ordered for 

 them : — 



