34 MR. C. C. CHRISTIE ON THE AURORA BOREALIS OF NOV. 18TH, 1835. 
notion, but, on the contrary, induced me to consider, whether the dark cloud might 
not be a substratum of matter differing in nature and density from the superincum- 
bent arch of light. The following are the facts which appear to favour this supposition. 
First ; every great outbreak of coruscations from the luminous arch produced a cor- 
responding disturbance in the part of the cloud immediately below. Thus, during 
the display at 9 h 15 m , the arch was gradually losing its regular form on the right, 
and at 9 h 20 ,n I noted it “ very irregular, with a large indentation on the eastern 
side,” while on the west, where the body of light was undisturbed, the arch remained 
perfect. Thus also, immediately after the western half had been in vivid corusca- 
tion, the whole of the arch Avas “ dilapidated,” and finally, “ entirely broken up.” 
Secondly ; neither the straight nor waving pencils appeared to proceed from behind 
the cloud, but always from the upper surface of the light. Thirdly ; when the arch 
was “dilapidated” (see Sketch IV.), it was not merely its upper surface which 
was of irregular form, but masses of it were lying in confusion, separated from each 
other by a boundary of light, not appearing in the least as if light behind were shin- 
ing through, but rather as if the substances of the arch and cloud had unwillingly 
interpenetrated each other, and refused to mix together more intimately, while at the 
same time the light above became more diffused and of diminished brightness. Lastly; 
when at 9 h 35 m the continuity of the arch is restored, it remains “ of irregular undu- 
lating form” (fig. B.), while fainter pencils continue to rise from every part ; at 9 h 55 m 
the cloud is “ much darker,” “ the pencillings very faint,” while its form is evidently 
becoming more regular (from fig. B. to fig. C.) ; and at 10 b 20 m it is nearly perfect 
in form (fig. D.), “strongly defined and steady,” the pencillings having entirely 
ceased. All these circumstances struck me as so closely resembling the disturbance 
of two fluids, the one superposed on the other, mutually repulsive, but compelled to 
mingle by forces, of whose action the vividness of the pencillings seemed to indicate 
the intensity, and requiring intervals of repose to re-collect their scattered energies, 
that I cannot but conclude the luminous matter of an aurora to be a superincumbent 
stratum, and, consequently, that its altitude is dependent on that of the dark mass im- 
mediately beneath. 
Bath, January Ath, 1836. 
