Mr William Sturgeon on the Aurora Borealis. 227 



of the rainbow-like arches of the aurora, above the earth's surface 

 is about 150 EngUsh miles." — (3Ieteorological Observations, &c.) 



If the aurora be an electrical meteor, as is now generally admit- 

 ted, the northern arch, which invariably assumes the white flame 

 colour, must necessarily be situated within the limits of the atmo- 

 sphere, for the reasons already stated ; the streamers also, which are 

 generally of the colour of the arch, are obviously within a similar 

 range from the earth's surface, and from the hazy condition of the 

 air through which they sometimes appear to progress, added to the 

 fact, that they often conform themselves to the figure and position of 

 certain formations of cloud, there appears much reason to believe, 

 that streamers are displayed within the regions of aqueous vapour. 



With respect to the apparent ascent of streamers, it is nothing 

 more than the effect of perspective, and ought not to be understood, 

 that those parts of the meteor rise to a higher region from the earth's 

 surface ; but, in the same sense as a cloud is said to rise, or the sun, 

 moon, or any other heavenly body is understood to rise, which im- 

 plies no inci'ease of distance between the earth and the body, but 

 merely an increase in the vertical angle, formed by visual ray from 

 the body, and the plane of the horizon ; it is therefore the elonga- 

 tion of a streamer towards the zenith of the observer, that causes 

 the appearance of ascent or shooting upwards, and no real increase 

 of distance from the earth's surface. When streamers pass the zenith, 

 their increase in length gives them the appearance of a doiunward 

 motion, which is also the case, whatever may be their course, pro- 

 vided the progress of elongation is from the spectator. 



From the observed influence of electrical forces in giving forms to 

 and producing intestine commotions in thunder-clouds, there is rea- 

 son to infer, that similar forces are productive of peculiar forms and 

 arrangements of aqueous vapour, in regions much higher than those 

 groups of heavy clouds ; and that electrical transmissions may be 

 accomplished through conductors which had been formed by electrical 

 forces, but such formations of conducting material could take place 

 exterior to tlie atmosphere, where none is in existence. 



It is a well-ascertained fact, tliat the electric fluid is more abun- 

 dant in the upper parts of the atmosphere than in the inferior strata ; 

 and that this is the normal state of the air when undisturbed by 

 clouds or other causes. Hence, were this normal state to remain 

 unruffled, there would be a steady equilibrium of electric forces 

 throughout the atmosphere, and an electrical tranquillity would 

 be permanently established in every part of it. Such a tranquillity, 

 however, cannot possibly exist in an atmosphere that is subject to 

 continual fluctuations of temperature, moisture, and consequent 

 winds ; hence the natural tendency to an electrical equilibrium is 

 ever being interrupted, and electrical commotions, of more or less 

 magnitude, are continually going on. 



When the air is highly charged with aqueous vapour, and suffers 



