166 Transactions of tee American Institute. 



miles of the earth's diameter. Observe that the illuminated circle, 

 containing the disc, has a diameter of eighteen feet ; let this suggest 

 to us a portion of celestial space which will extend everywhere 8,000 

 miles from the earth's surface. Now contemplate those curves sur- 

 rounding the magnet, and then lift up your minds and behold the 

 lines of terrestrial magnetic force. Only imagine this circle, 24,000 

 miles in diameter, instead of eighteen feet, and then these curves, 

 stretching thousands, yea tens of thousands of miles from the earth, 

 plot out for you, in celestial space, the earth's magnetic influence. 

 Thus, by a legitimate effort of the imagination, we will prove the 

 visible and tangible to the invisible and inapproachable ; yet, in this 

 case not altogether invisible, for these very curves you have all seen 

 traced in space by the light of the aurora's beautiful beams. That 

 these luminous columns of the aurora polaris lie in the curves of the 

 earth's magnetic force, and, therefore, have their lengths in the direc- 

 tions of the dipping needle, at the points over which they appear, is 

 a well established fact deduced from many observations and measures 

 made on them. Thus " the well remembered aurora of September 

 2, 1859, formed a belt of light encircling the northern hemisphere, 

 extending southward in North America to latitude 22^°, and reach- 

 ing to an unknown distance on the north ; and it pervaded the entire 

 interval between the elevation of 50 and 500 miles above the earth's 

 surface. This illumination consisted chiefly of luminous beams or 

 columns, everywhere nearly parallel to the direction of a magnetic 

 needle when freely suspended ; that is, in the United States, these 

 beams were nearly vertical, their upper extremities being inclined 

 southward at angles varying from 15° to 30°. These beams were, 

 therefore, about 500 miles in length, and their diameters varied from 

 five to ten and twenty miles, and perhaps sometimes they were still 

 greater."* 



The discovery that the auroral columns coincide with the curves of 

 the magnetic force was a noble one, and I have ever loved to dwell 

 upon the very words in which such truths are given to the world ; 

 for thus the inspiration of the discoverer infuses itself insensibly into 

 the soul of the reader, and leaves ineffaceable impressions of the sim- 

 plicity, the dignity, and the comprehensiveness of science. This 

 discovery, from its very nature, is the result of many accurate 

 measures made on these beams, and only when it had been clearly 

 shown that these measures agreed with the hypothesis of the aurora 



* The Aurora Borealis, or Polar Light, its Phenomena and Laws, by Elias Loomis. Smith. Kept., 

 1835, p. 220. 



