Royal Society. 443 



made at the Paris Observatory. According to the former, the ba- 

 rometer is highest at new and full moons, and lowest at the qua- 

 dratures, the extent of the fluctuations being 0.08 of an inch : ac- 

 cording to the latter, the contrary is the case, and the extent is 

 only 0.05 of an inch. 



Lastly, the author endeavours to ascertain how far the barometer 

 is affected by the direction of the wind, and gives, in the form of 

 tables, the mean results of observations bearing upon this point. 

 The fluctuation, he observes, due to this, is much greater than that 

 due to any other cause. The barometer is lowest, as might be 

 expected, when the wind is in the rainy quarters of S.W. and 

 W.S.W. There are not yet sufficient data for any general conclu- 

 sions with regard to the influence of electrical phaenomena on the 

 weather. 



April 21. A paper was read, " On the Errors in the Course of 

 Vessels occasioned by Local Attraction, with some remarks on the 

 recent loss of His Majesty's ship Thetis." By Peter Barlow, Esq. 

 F.R.S., &c. 



The author observes that the errors arising from the deviation of 

 the compass produced by the attraction of ships, were formerly 

 much less considerable than at present, from the comparatively 

 small quantity of iron existing in the vessel. The increase of this 

 disturbing force in a modern ship of war is easily accounted foi by 

 the immense proportion of iron now employed in its construction, 

 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 may be 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 



3 L 2 cargo. 



