66 Instructions for Observers of the Aurora Borealis^ 



gage, as co-operators, any friends that may be situated a few 

 miles to the north or south of his residence, as their observations^ 

 compared to his, are alone available for low Aurora ; while his, 

 combined with those made at more distant stations, will deter- 

 mine that of greater elevation. He will instruct them in the 

 approximate methods of observation which follow. 



Should Aurora be seen, among other objects of attention may 

 be pointed out, its connection with the clouds, if any are visible, 

 and its distance relative to them : the nature of the black mass 

 sometimes seen under luminous arches, whether mere contrast 

 with the enlightened part, or of a cloudy nature ; its transparen- 

 cy or opacity, and the way in which it vanishes. If streamers 

 appear ; their breadth, apparent velocity, as determined by stars, 

 their lateral motion, and their relation to arches or clouds, should 

 be carefully observed. The southward motion of arches should 

 also be determined, by observing the time between their passing 

 over different stars ; and any changes which may occur in their 

 form noted, or any defect of symmetry. The determination of 

 elevation can scarcely be applied to the streamers of Aurora, ex- 

 cept when some sudden incurvation or change occurs, which may 

 happen to be noticed at two stations ; but the arches are of a less 

 evanescent nature. For them the altitude above the horizon, 

 and the azimuth with respect to the true meridian, are required. 

 It must be observed, that the Aurora is not visible in telescopes, 

 and therefore these elements are not susceptible of such precision, 

 as to make large instruments necessary or even useful. A small 

 theodolite, or one of the little altitude and azimuth circles, invent- 

 ed by Captain Kater, in which the telescope is replaced by a tube 

 having two cross wires at one end, and a sight hole at the other, 

 is all that is requisite. Having levelled the instrument, inter- 

 sect with the wires the lower edge of the arch (which is always 

 the best defined) at its summit, and read both circles. If the 

 index corrections are not known, reverse and repeat the obser- 

 vation. The azimuth is the difference between the reading of the 

 horizon circle, and that shewn when the sights are made to in- 

 tersect a mark placed due north of the instrument's place, by 

 any of the well known methods. It will nearly coincide with 



on the 17th of September 1833, that the streamers had attained their greatest 

 brilliancy, and the arches reached their highest elevation, before 9 p. m. 



