Intelligence and Miscellaneous Articles, 6S9 



nature and as to the laws they obey. Various quotations are given 

 from authors who have been struck with this analogy, and who have 

 pointed it out more or less circumstantially in the narratives of their 

 travels in the neighbourhood of Etna and Vesuvius ; to which the 

 author adds some of his own observations on the lava from these 

 mountains, which throw further light on the subject. 



LXXIX. Intelligence and Miscellaneous Articles. 



TERRESTRIAL MAGNETISM. 



A COMMUNICATION on Terrestrial Magnetism, by Professor 

 Locke, the result of several years' observation in various parts 

 of the United States, was read at the American Philosophical So- 

 ciety, April 5, 1844, and referred to a Committee, consisting of 

 Prof, Frazer, Mr. M'Euen and Prof. Bache. 



Prof. Locke gave a brief history of the circumstances which in- 

 duced him to undertake these researches, in which he made a grate- 

 ful acknowledgement for the kind and able instruction, especially ia 

 the practical manipulations, received from Prof. Bache of this Society, 

 while in London and Paris. He read a part of the correspondence 

 between himself and Col. Sabine on the subject of the point of maxi- 

 mum intensity at or near to Lake Superior, and suggested, as the 

 probable result of his researches and of those of Lieut. Lefroy, 

 briefly alluded to by Col. Sabine, that the region of maximum inten- 

 sity is either at Kewenon peninsula, south side of Lake Superior, or 

 along a line extending thence to Rainy Lake — the last opinion having 

 been expressed by Col. Sabine in his correspondence. To determine 

 this point more precisely. Prof. Locke suggested that observations 

 should be multiplied in various parts of the region of the lake. The 

 observations were prefaced by a popular explanation of what is meant 

 by the four elements of terrestrial magnetism, declination (" varia- 

 tion ") dip or inclination, horizontal intensity of magnetic force, and 

 total intensity of the same force. These explanations were followed 

 by the observations of six successive years, commencing in 1838, 

 and extending more or less over the region included between the 

 middle of Kentucky on the south, and the north side of Lake Supe- 

 rior on the north ; and between Cambridge, Mass., on the east, to the 

 middle of Iowa on the west. His observations had been accompa- 

 nied throughout by remarks on the geology at each station, and he 

 expressed the opinion that those observations justified the conclusion, 

 that the general character of the rocks, especially as regards the di- 

 stinction between the aqueous and the igneous formations, could be 

 ascertained by the elements, as indicated by lines of observations ex- 

 tended along any given region. He exhibited a chart illustrative of 

 this subject, on which the two quantities of dip and intensity were 

 represented by curves passing through ordinates standing on hori- 

 zontal distances, generally along lines of latitude. These curves, 

 along regions of aqueous rocks, such as those along the Mississippi, 

 were remarkably uniform, while those over igneous rocks were ex- 



