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the white and the red are by far the most common. The reddish 
tints are usually nearest the horizon. Comparing the colours of 
the Aurora with those of vacuum tubes, illuminated by an electric 
current, and containing different gases extremely attenuated or 
minute particles of metals, we find many points of agreement 
both in the form which is shown by the luminous clouds and in 
the colours produced. The spectroscope also has been applied 
in both cases. But while the spectra of the Aurora and those of 
vacuum tubes show that the phenomena of both often yield very 
similar results, there are many discrepancies and many problems 
yet which remain to be solved. Mr. Lockyer has experimented 
with meteorites, and concludes the Aurora is due to the electric 
discharge through meteoric dust in the higher regions of the 
atmosphere. ‘There seems to be a connection between Aurore 
and thunder storms. 
Prof. Christison affirms that the first great Aurora after the 
commencement of autumn, and following a period of fine 
weather, is usually a sign of a great storm of wind and rain on 
the second day ; and Mr. Prince remarks that displays of Aurora 
are almost invariably followed by very stormy weather after an 
interval of from ten to fourteen days. That the appearance of an 
Aurora is generally followed by considerable disturbance of the 
magnetic needle all are agreed. No scientific theory yet ad 
vanced as to the cause of the Aurora is entirely satisfactory. The 
one, however, which is most generally received is that the Aurora 
is caused by the passage of an electric current from the Equator 
towards the Poles, and in the resulting restoration of equilibrium 
between the positive electricity of the atmosphere and the 
negative electricity of the earth, acted upon, to a certain degree, 
by terrestrial magnetism. If this restoration of equilibrium take 
place near the earth, the result is the lightning flash, but if in the 
higher regions of the atmosphere, where the rarefaction is great, 
we shall no longer have the intense discharge of the lightning 
flash, but one more resembling an Aurora. 
The lecture was illustrated by a series of large vacuum-tubes, 
the electric discharge through which reproduced many of the 
beautiful phenomena of an Aurora. 
