236 The Rev. Dr. Robinson on the Aurora of 
approximately spherical, superimposed upon the general one, 
such that O shall be the projection of its vertical axis, and 
the dotted circle that of the circumference of its base. Then 
taking P as the projection of any proposed point in the par- 
tial elevation, we must suppose applied there, first, a tension 
(F) impressed on the mass generally perpendicular to D E; 
secondly, a tension (/,) in a direction passing through O (see 
p- 233); and thirdly, another tension /, perpendicular to P O. 
From these data the directions of the fissure through P, when 
the tensions become sufficient to produce it, must be deter- 
mined. And here we may remark, that since one lamina of 
our elevated mass will be similar to another, the tensions Ff, 
and f;, will be very approximately the same for each; and 
that consequently the direction of the fissure just mentioned 
will equally determine the horizontal direction of the fissure 
which shall pass through any point of which P is the projec- 
tion. The extensibility of the mass being assumed to be 
small, the intensities of the tensions F, f, f,, will be propor- 
tional to the extension each would produce in the mass at P, 
if it acted separately, or to the additional extension produced 
by each when acting simultaneously. The accurate determi- 
nation of these intensities would in most cases present great 
difficulties. In general, however, it will be sufficient to con- 
sider such tensions as f, and jf, (belonging to the partial ele- 
vation) merely as forces producing modifications in the effects 
of F, the nature of which can be determined with sufficient 
accuracy for practical purposes. 
[To be continued. ] 
XLV. On the Aurora of November 18th, 1835. By the Rev. 
T. R. Ropinson, D.D. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
ih R. STURGEON ’s notice of the aurora of § November 
18th (not 16th as misprinted,) induces me to send you 
the notes which I made of its appearance, as from his positive 
statement ‘ that he saw no appearance of aurora to the south 
of the zenith, though frequently looked for,” this seems to be 
one of the very rare cases where auroral phenomena can be 
proved to occur in a low region of the atmosphere. They 
are as follows, the time being reduced to Greenwich. 
‘Noy. 18. Sky strongly illuminated, but covered with 
clouds till 9» 20", when two arches were visible, which broke 
a 
