OF THE AURORA BOREALIS ON THE MAGNETIC NEEDLE. 
113 
fitfully, within the limits of about 50°, in the most extreme case, on each side 
of the ordinary magnetic line. 
5 min. past 9 p.m. — With a very lengthened fringe in the plane of the dip, 
varying much in intensity of light from end to end ; the needle on the steel 
point shifted to 32° W. ; I then made trial of the intensity with the needle, and 
observed the arc of oscillation, at the conclusion of 50 oscillations, much less 
than I have uniformly before observed it. Time of 50 oscillations 224". Dropt 
the needle on steel point, and found it now (a few seconds only after conclu- 
ding the observation) 25' E. Repeated the trial of the intensity immediately, 
and again found the arc of oscillation at the conclusion greatly diminished. 
50 oscillations in 224".25. Needle instantly dropt on the steel point now 
returned to 34' W. 
Having examined the needle on the steel point for some considerable time, 
and the extremely irregular nature of its movements, under the influence of 
succeeding fringes coming into the plane of the dip, I conceive the true expla- 
nation of the irregularity in the extent of the arcs occurred to me. The arc 
of oscillation being bisected by the line of the magnetic force, when this line 
changes its direction suddenly, as is indicated by the needle on the steel point, 
the arc is either enlarged or diminished suddenly at the same time, accord- 
ing as the line shifts from or towards that of the needle, at the moment when 
it has completed an oscillation, to return in another ; and the time of the oscil- 
lations is thus sensibly changed. 
This conclusion might indeed have been drawn from the previously observed 
phenomena, but did not occur to me till the experiments of this evening 
brought it into view. 
Besides changes in declination, the needle on the steel point often exhibited 
a slight vertical movement and tremor, indicative of a change of the dip, and 
showing thus another impediment to an accurate determination of the intensity. 
At a quarter to 10 p. m. the region occupied by the Aurora was extensively 
obscured by a haze, preventing further good observation. 
Several circumstances occurred this evening confirming the views that the 
observations of the 20th December last had led to ; viz. “ That the Aurora is 
dependent on the condensation of vapour.” It is familiar to every one, resident 
in a mountainous country, that clouds, even during hard gales, continue appa- 
MDCCCXXX. 
Q 
