319 



needle was deteriorated, by touching it with the pole, of its own 

 name, of a magnet, a deviation then arose ; and communicating .the 

 facts to Mr. Barlow, he proceeded to investigate the laws and amount 

 of the deviation so arising. 



He distinguishes the action into three several cases : 



1st. When the needle is on any part of the magnetic meridian of 

 the ball. In this case there is no deviation caused by the primary 

 of the shell, nor any secondary deflection produced by deteriorating 

 one pole of the needle. 



2nd. When the needle is in the magnetic equator of the ball. In 

 this case he observed, that whichever end of the needle is weakened 

 approaches the ball. 



3rd. In every other position, one branch of the needle is nearer to 

 the centre of the ball than the other. Here it is observed, that if 

 the near end is deteriorated, the needle approaches its natural mag- 

 netic situation ; but if the more distant, then the reverse takes place ; 

 and this represents the general law of the phenomenon. 



From this law, Mr. Barlow is led to the explanation of the facts, 

 on the principle of induced magnetism in the shell ; for in the second 

 case above enumerated, the equilibrium of the needle is produced by 

 equal and opposite repulsions on its equal and equally magnetized 

 ends. When, therefore, the repulsion on one end is weakened by 

 deteriorating that end, that on the other obtains the advantage, and 

 the deteriorated end is drawn towards the ball. 



In the general case, the reasoning is equally simple ; and Mr. Bar- 

 low shows that its results are precisely those which observation 

 affords. 



These results he considers as decisive in favour of that theory 

 which regards the magnetism of an iron shell as induced in it by the 

 action of the earth. 



He concludes this paper with an account of some experiments in- 

 stituted for the purpose of determining numerically the amount of 

 the secondary deflections, arising from a given extent of deterioration 

 in the needles. This was estimated by the increase of the times of 

 oscillation in the needle, freely suspended, and the deflections esti- 

 mated by making the deviations produced by the shell in a series of 

 situations all around it, in various planes ; the results of which are 

 stated in a table. 



On the Difference of Meridians of the Royal Observatories of Green- 

 wich and Paris. By Thomas Henderson, Esq. Communicated by 

 J. F. W. Herschel, Esq. Sec. R.S. Read May 17, 1827. [Phil. 

 Trans. 1827, p. 286.] 



Mr. Henderson, in going over the calculations of the observations 

 made by the Commissioners on the part of the British Board of Lon- 

 gitude and the French Ministry of War, for determining this element, 

 in July 1825, detected an error of one second in the reduction of 

 the observations made at the Royal Observatory at Greenwich, from 



