228 Mr William Sturgeon on the Aurora Borealis. 



a sudden depression of tempei'ature, dense clouds are formed with 

 amazing rapidity, and the electric fluid being condensed in them to 

 a higher degree of intensity than they can retain it, liberates itself 

 from this aqueous imprisonment in the shape of lightning. In the 

 upper regions of the air, whei'e the insulation is njuch less perfect, 

 no lightning cloud can possibly be formed ; because the electric fluid 

 finding but little I'esistance to its movements, however suddenly it 

 may be distubed, by change of temperature, flows from one part to 

 another before its intensity gets sufficiently high to form lightning 

 or dischai-ge itself in a close compact body ; hence any sudden dis- 

 turbance of the electric fluid in the upper regions of the atmosphere, 

 instead of producing lightning, would cause it to move in waves, or 

 as a diffused ocean, in those directions oflfering the least resistance; 

 covering an extensive area in its transit ; such an electric tide, occur- 

 ring at night-time, would be visible, and partake of all forms of the 

 conducting media through which it passed. 



Now, it being a well-established fact that attenuated air is a 

 better conductor than air of greater density, and that a vacuum is a 

 better conductor then attenuated air ; and as the attenuated regions 

 of the atmosphere are more highly charged with the electric fluid 

 than the dense air below ; analogy v/ould lead to the inference, that 

 the electric fluid is still more abundant exterior to the shell of 

 air than anywhere within it ; an inference which will readily be 

 conceded by those who allow that the aurora boi'ealis is an electrical 

 phenomenon displayed at elevations far beyond the reach of the at- 

 mosphere. But here it is that an insuperable difficulty presents 

 itself in finding the disturbing agent. Within the atniosphei'e, elec- 

 trical disturbances are easily accounted for by the influence of well- 

 known agents ; but at the distance that some philosophers have 

 placed the aurora from the earth, such agents are not known to 

 exist. 



The electrical theory of the aurora borealis, as it exists at the 

 present day, is considerably alloyed with the magnetism of the earth. 

 Halley appears to be the first on the list of those philosophers who 

 have called in terrestrial magnetism to assist in explaining the cause 

 of the aurora borealis ; but the circulating magnetic cjfluvia of this 

 eminent philosopher appearing insufficient for the views of Dalton, 

 the latter invented " an elastic fluid partaking of the properties of 

 iron, or rather of magnetic steel," which he placed in the upper re- 

 gions of the atmosphere, in " the form of cylindrical beams," which, 

 when illuminated by the electric fluid, become the beams or streamers 

 of the aurora boveiolis ; and " the rainbow-like arches," says this 

 philosopher, " are a sort of rings of the same fluid, which encompass 

 the earth's northern magnetic pole, like as the parallels of latitude 

 do the other poles.* 



* Meteorological Essays, p. 169. 



