Proceedings of the British Association. 375 



eluding three series of observations — the dedination, inchnation 

 and intensity. Prof. Loomis* had extended his observations of 

 inchnation over great part of Ohio, Indiana, Illinois and Missouri. 

 These, and numerous other observations and surveys in the States, 

 would connect the northern British survey with the determina- 

 tions of Capt. Barnett, of the Thunder, in the Gulf of Mexico. 

 As to observatio?is at sea, by Mr. Pox's instrument, the inclina- 

 tion and dip of the magnetic intensity might be measured with 

 all the precision requisite for every use to which observations at 

 sea could be turned, for the purpose of tracing out the isodynam- 

 ic and other magnetic curves in portions of the globe covered by 

 water. To extend and facilitate the use of this valuable instru- 

 ment, the set of instructions drawn up by Col. Sabine, had been 

 printed by order of the Admiralty, as a general circular, with some 

 statements of the mode of using it practised on board the Erebus 

 and Terror, and the hope was expressed that this method would 

 be followed, not only in exploratory voyages, but by ships pursu- 

 ing ordinary tracks, so as to furnish data for complete magnetic 

 sea-charts. For these important observations, as well as the de- 

 clination, it was necessary to eliminate the influence of the ship's 

 iron, — an evil increasing from the greater quantity of iron now 

 used. After mentioning the observations of Capt. Belcher, of 

 the Sulphur, on more than twenty islands in the Pacific Seas, 

 which had arrived in England, and would be published, and the 

 important results deduced from M. Erman's journey in Siberia, 

 the report noticed the subject of magnetic disturbances, respect- 

 ing which Gauss remarked, that one of the results of this great 

 British enterprise was that the existence and extension of these 

 disturbances over the whole of the globe had been ascertained. 

 As a physical fact, deeply connected with the general causes of 

 terrestrial magnetism, this was indeed a result of the first magni- 

 tude, and considering all the circumstances, how it was modified 

 by distance and locality, was eminently calculated to lead to the- 

 oretical truths. It distinguished what was local from what was 

 general, and traced individual shocks from observatory to obser- 

 vatory, and station to station, till they were so far enfeebled as 



* See Prof. Loomis's paper on the Magnetic Dip and Variation in the United 

 States : this Jour. Vol. 43, p. 93-116. Also his paper on the Magnetic Intensity 

 at several places in the United States : Trans. Am. Phil. See. Vol. 8, N. S. p. 61. 



