SiiO Intelligence and Miscellaneous Articles. 



tremely irregular and undulating, sometimes ascending and descend- 

 ing with a needle-like sharpness. He remarked, that if these di- 

 stinctive characters obtain so generally as to constitute a law, the 

 magnetical instruments will form a kind of " mineral or divining rod" 

 to the geologist. 



There was exhibited, also, a chart of the United States, showing 

 chiefly, the isodynamic lines consistent with the observations. These 

 lines form ovals around Lake Superior as an axis, the longer or 

 pointed ends jjointing towards New York, and in the opposite direc- 

 tion. The outer oval descending along the Hudson river, passing 

 through the city of New York, along the coast, to near Baltimore, 

 and, turning westward, traverses Maryland, Virginia, and Kentucky, 

 and crosses the Mississippi into Missouri about thirty miles below 

 St. Louis. 



The force along this line is 994, that at Cincinnati, in Ohio, being 

 1000 ; and an oval is delineated at every increase of 10, until, at the 

 axis at Lake Superior, it rises to from 1054 to 1060. But Prof. 

 Locke stated, more observations are needed to fix more precisely those 

 lines at remote points, and he expressed a desire that the observa- 

 tions of Major Graham, Prof, Bache, and Prof. Loomis, together with 

 his own, might be united in a suitable chart, such as might be pro- 

 duced by a convention of observers. Two other charts, explained 

 in the communication, were also exhibited ; the one a copy of a part 

 pf Col. Sabine's north polar chart, with additions, and with a delinea- 

 tion of the relative positions of the astronomical poles, the pole of 

 dip and convergence discovered by Ross, and the pole of intensity at 

 Lake Superior ; and the other a particular delineation of Copper 

 Harbour and of Porter's Island, where, in Prof. Locke's line of ob- 

 servations, he found the greatest intensity of force. 



Prof. Locke, in a second paper, continues his observations upon 

 the magnetic dip and intensity, made at different points in the 

 United States during the year 1844, and includes thirty-five sets of 

 observations made at twenty- four different stations. He then gives a 

 series of thirteen observations made at three stations, viz. Fort Lee, 

 N. Y., Snake Hill, and Patterson, N. J., intended to show the re- 

 markable changes which take place both in the dip and intensity, in 

 passing from rocks of the usual kind, to those belonging to the trap- 

 pean family ; which changes, so far as they regard the magnetic in- 

 tensity, he announces in the following terms : — "The intensity, which 

 ordinarily has a value along a line of moderate length not varying 

 beyond certain moderate limits, becomes at the base of a trappean 

 pinnacle, extraordinarily diminished, and at the top of the same still 

 more extraordinarily increased." These changes, as well as those 

 of the dip which follow the same law, are illustrated by diagrams, in 

 which the values of the dip and intensity are rej)resented as the ordi- 

 nates of a curve, the distances between the stations being taken upon 

 the line of abscissas. Prof. Locke believes these changes to be due 

 to the assumption of magnetic properties by the trappean rocks, the 

 axis of the magnet coinciding with the axis of figure of the hill, which 

 is generally vertical. 



