198 Magnetism of the United States and its Vicinity. 



Beering's straits, including the great northern lakes and a con- 

 siderable part of Hudson's bay. The isodynamic line of 1-85 is 

 a smaller oval included within the former, and passing nearly 

 through Fort Mackinaw. The isodynamic line of 1-875 is an 

 oval 446 geographical miles in length, and 170 in breadth. Its 

 centre is in latitude 52° 19' n., and longitude 92° w. ; where the 

 intensity is 1-878. These are the results which accord best with 

 all the observations of Lieut. Lefroy and Prof. Locke. But it 

 must not be understood that the observations agree perfectly 

 among themselves. Many of the observations present unexplain- 

 ed anomalies, which, in our ignorance of their cause, we ascribe 

 to local attraction. Thus while the great mass of observations 

 assign to the magnetic focus above mentioned an intensity of 

 1-878, and indicate that from this point the intensity slowly 

 diminishes in every direction, a few of the observations indicate 

 an intensity considerably greater. In three instances, Lieut. Le- 

 froy observed intensities greater than 2, viz., in lat. 48° 46', Ion. 

 87° 40', intensity 2-099 ; lat. 51° 44', Ion. 96° 48', intensity 

 2-031; and lat. 47° 37', Ion. 85° 11', intensity 2-016. And in 

 sixteen instances he observed intensities greater than 1 -878. Prof- 

 Locke also observed intensities greater than 1-878 in four in- 

 stances, one of them rising as high as 1-950 in lat. 47° 28', Ion. 

 88° 1'. This point he inferred to be the point of maximum in- 

 tensity for this continent ; but Lefroy observed still higher in- 

 tensities, and each of these cases was doubtless the effect of some 

 local cause of limited extent. Within less than one mile's dis- 

 tance of the point where Prof. Locke observed the intensity 

 1-9d0, he found the intensity reduced to 1-842. The cause then, 

 whatever it may be, of the high intensity on Porter's Island is 

 extremely circumscribed in its influence. Prof. Locke has made 

 some important observations to determine the laws of local at- 

 traction. At Patterson, N. J., on the summit of a trap rock one 

 hundred and fifty feet high, he found the dip to be 75° ; while 

 at the bottom of the rock the dip was only 72° 17'; makings 

 difference of 2° 43' within a distance of a few hundred yards- 

 Near Fort Lee, about nine miles north of New York, on the 

 ridge of the palisades, the magnetic intensity at two stations only 

 forty feet apart, was found to vary by nearly one-twentieth of its 

 whole value. Prof. Locke infers that trap rocks become magnet 

 by terrestrial induction, diminishing the magnetism of the e»rtl 

 at their bases, and increasing it at their summits. 



Lieut. Col. Sabine, in his magnetic survey of Scotland, men- 

 tions an instance of local attraction even more remarkable than 

 the preceding. In the northwestern part of Scotland, near U& 

 ^cavig, on a rock intersected by trap veins, he observed a dip J 

 7b 10'. On the other side of the harbor, the dip was only 7* 

 being a difference of more than five degrees. These cases o 

 local attraction deserve a particular examination. 



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