VI. ON THE POSITION OF THE ISOGONAL LINES IN IRE- 

 LAND, DEDUCED PROM THE OBSERVATIONS OF 

 CAPTAIN SIR JAMES CLARK ROSS, R. N. 



From the Proceedings of the Royal Irish Academy, 1850. 



IN the year 1835 I laid before the British Association, then as- 

 sembled in Dublin, a Eeport on the Direction and Intensity of the 

 Terrestrial Magnetic Force in Ireland, based upon observations 

 made by Lieut.-Colonel Sabine, Sir. James C. Boss, and myself. 

 In these observations Mr. Eobert Were Fox and Professor Phillips 

 afterwards took part ; and the survey was subsequently extended 

 to the whole of the British Islands. The details of this extended 

 survey are given in a Memoir on the Magnetic Isoclinal and Iso- 

 dynamic lines in the British Islands, drawn up chiefly by Lieut.- 

 Colonel Sabine.* 



The observations contained in these Reports are limited to the 

 Magnetic Inclination and Intensity. Observations of the Declina- 

 tion, as well as of the other two elements, were indeed made by 

 Sir James Boss ; but they have only lately been given to the public 

 in a Memoir by Lieut.-Colonel Sabine, on the lines of Magnetic 

 Decimation in the Atlantic, f In this Memoir, the observations 

 referred to are combined with a large mass of other materials, and 

 the position of the isogonal lines inferred from the whole by a 

 graphical process. The Irish portion of these observations is, how- 

 ever, so distinct, and so complete in itself, that it seemed to me 

 desirable that they should be discussed by the same method which 

 had already been applied to the observations of the other two ele- 



* Eighth Report of the British Association for the Advancement ofScimet. 

 t Philosophical Transactions, 1849, Part ii. 



