TEEKESTRIAL MAGNETISM. 



21 



The foregoing quantities do not differ among themselves more than observations 

 at the same place by the same person will diflter at different times. 



It would seem, then, that, although the spheroid of magnetic force may have its 

 pole at latitude 52° 19' N., yet there is a high table-land, so to speak, extending 

 to the south shore of Lake Superior, where there is a "district" intensity. It fol- 

 lows from these observations, that the figurative spheroid of magnetic force would 

 be nearly level from the lake region to the above latitude; or, strictly, it would be 

 spherical instead of spheroidal, on that special line, with, perhaps, a slight depres- 

 sion between the two points. The comparison which I have here used suggests 

 the construction of a solid spheroid, on the surface of which the stations should 

 have their proper angular distances as represented spherically by latitude and lon- 

 gitude, but with distances from the centre proportionate to the intensity of magnetic 

 force. My line of observations along the north shore of Lakes Huron and Michigan, 

 presented in the following tables, will give an idea of the rate of decrease of mag- 

 netic intensity southwardly from Lake Superior; and with quantities formed by 

 taking means of groups of observations, judiciously selected at various points 

 towards Hudson's Bay, as ordinates, would exhibit an instructive curve, in which 

 the point at the south shore of Lake Superior would probably be prominent. 



Dip. 





Epoch of 

 commencing 

 vibration, 

 hrs. min. sec. 



Duration 

 of 500 

 vibra- 

 tions. 



II 



Calculated 

 duration 

 at 60°. 



Square of the 

 preceding. 



Horizon- Total in- 

 tal inten- tensity : 

 sity. hor. being 

 1000. 



Total inten- 

 sity: that at 

 Cincinnati 

 = 1000. 



182. Fort Wilkins.— Lat. 47° 28' N. Long. 88° 45' W. July 5, 1847. 



78° 03' 00" 





8 51 16.0 A 



1319.9 



71.5 



1318.9 



17394972100J712.14 



3439.3 



1149.6 



183. Horseshoe Harbor.— Lat. 47° 28' N. Long. 87° 57' W. July 8, 1847. 



78° 15' 05" 



5 

 6 



9 20 04.0 A 

 9 50 01.2 



1690.4 

 1686.0 



81.5 



88.5 



1688 

 1682 



284934400 



282912400 



Mean 



640.07 

 637.15 



638.92 



1339.7 



1046.9 



184. Eagle River Mine.— Lat. 47° 24' N. Long. 88° 25' W. June 30, 1847. 



77° 24' 00" 



4 

 5 

 6 



3 03 00 P 



4 00 00 

 4 40 00 



1465.93 

 1791.22 

 1774.43 



86.0 



87.5 

 86.0 



1463.45 



1787.84 

 1770.37 



21416859025 

 31963718656 

 31342099369 



Mean 



569.06 

 570.83 

 575.19 

 571.69 



3108.8 



1038.7 



182. Fort Wilkins. — Geology: Metamorphic conglomerate near to the trap. 



183. Horseshoe Harbor. — Geology: conglomerate. 



184. Eagle River Mine. — Range 31, township 57, section 30; United States survey. Locality: in 

 the ravine of the river, and at an angle opposite to an adit. Geology : trappean rocks. Weather clear. 



