34) Instructions Jbr Observers of the Aurora Borealis. 



different theories of the Aurora : if the first be admitted, it 

 can scarcely be an atmospheric phenomenon, as, if air exist at 

 that height, its attenuation must be so great as to make its influ- 

 ence insensible, while the second would place it in the ordinary 

 region of clouds. It is, however, possible that both may be cor- 

 rect, and that electrified clouds may assume luminous appear- 

 ances in the lower regions of the atmosphere, (see the Report on 

 Meteorology in the last volume of the British Association Reports, 

 p. 9^55 ;) while a far greater height and different origin must 

 be attributed to those faint nebulous bauds, which are less fre- 

 quently seen traversing the sky in a direction nearly at right an- 

 gles to the magnetic meridian, appearing in the N. W. and (when 

 they move) drifting to the S. E. with a slow regular motion. 

 If one arch be visible at two distant places, as, for example, 

 Edinburgh and Manchester, even without calculation we know 

 that its height is very great, but it is asserted, that different ob- 

 servers are, in this case, observing different arches ; on the other 

 hand, it is replied, that if this were so, the intermediate arches must 

 be visible, while, in the memorable instances of March 29. 1826, 

 described by Dr Dalton, and September 29. 1828, observed by 

 Mr Davies Gilbert, Captain Kater, Mr Harvey, &c., one only 

 was seen in the sky. By the establishment of numerous stations, 

 the Committee hope to obtain a decisive answer to this import- 

 ant question. 



Sound of the Aurora Borealis. — It has been stated, that a 

 sound accompanies brilliant displays of Aurora, especially in 

 high latitudes ; but here also, the evidence is of the most con- 

 tradictory kind. In this country we can scarcely expect to re- 

 solve the doubt, as, except when the meteor; is in our zenith, its 

 distance is too great, on any hypothesis of its elevation, to allow 

 a weak noise to be audible. It is, however, possible that this 

 notice may reach observers more favourably situated. To ob- 

 viate the influence of imagination, which has been supposed to 

 be the principal agent in this observation, it may be mentioned, 

 that, even where the meteor is but one or two miles distant,^r>^ 

 or ten seconds must elapse between the corruscation and the 

 sound supposed to be produced by it. Most writers, however, 

 speak of them as contemporaneous. If the height of the Aurora 

 exceed a few miles, sound can neither be excited in, nor trans- 

 mitted by, the attenuated air. 



