60 Analyses of Boolcs, [July/ 



that modification of cirro-stratus, which, descending low in the 

 atmosphere, produces a hazy continuity of cloud over-head, or 

 a fo£j bank in the horizon. Indeed, I am inclined to infer, that 

 •Uie Aurora Borealis is constantly accompanied by, or imme- 

 diately precedes, the formation of one or other of the various 

 forms of cirro-stratus. On the 13tb of November, and 18th of 

 December [1820], its connexion with a cloud intermediate be- 

 tween cirrus and cirro-stratus is mentioned ; but the most vivid 

 coruscations of the Aurora were observed when there were 

 only a few attenuated shoots of cirro-stratus floating in the 

 air, "or when that cloud was so rare that its existence was 

 only known by the production of a halo round the moon. The 

 bright moonlight of December was peculiarly favourable for 

 observations of this kind. Had the nights been dark, many of 

 the attenuated streaks of cloud hereafter mentioned would have 



een totally invisible.'' P. 597. 



" I think I have on some occasions discerned," Dr. Rich- 

 ardson continues, " a polarity in the masses of cloud belong- 

 ing to a certain kind of cirro-stratus, which approaches to 

 cin'us, by which their long diameters, having all the same di- 

 rection, were made to cross the magnetic meridian nearly at 

 right angles. The apparent convergence of such masses of 

 cloud towards opposite points of the horizon, which has been 

 frequently noticed by meteorologists, is of course an optical 

 deception, produced when they lie in a plane parallel to that 

 on which the observer stands. These circumstances are here 

 noticed, because if it should be hereafter proved that the 

 Aurora depends upon the existence of certain clouds, its appa-^ 

 lent polarity may, perhaps, with more propriety, be ascribed 

 to the clouds themselves which emit the light ; or, in other 

 words, the clouds may assume their peculiar arrangement 

 through the operation of one cause (magnetism for instance), 

 while the emission of light may be produced by another, a 

 change in their internal constitution perhaps, connected with 



a motion of the electrical fluid Generally speaking, the 



Aurora appeared in small detached masses for some time before 

 it assumed that convergency towards the opposite parts of the 

 horizon, which produced tne arched form. An observation 

 that I would connect with the previous remarks, by saying 

 that it was necessary for the electric fluid (or the Aurora, if 

 they are the same) to operate for some time before the polarity 

 of the thin clouds, in which it has its seat, is produced." 



" An electrometer, constructed upon Saussure's plan, placed 

 in an elevated situation out of doors, exhibited no signs of a 

 charge from the atmosphere at any time during the winter^ 

 The electricity of our bodies, however, at times was so great, 

 that the pith balls instantly separated to their full extent upon 

 approaching the hand to the instrument ; and our skins were in 

 the middle of winter so dry, tliat rubbing the hands together 



