AURORA BOREALIS. 
139 
It was a law in electricity that bodies in different electrical con- 
ditions attract each other; this held good whether they were 
electrified conductors or electrified non-conductors. The air was 
a non-conductor, and served to insulate the masses of condensed 
moisture, such as clouds, &c., but was able itself to take on and 
retain electrical conditions. Two clouds in different electrical states 
thus attracted each other, and approached until they were near 
enough to permit a restoration of equilibrium by the ordinary 
lightning discharge. 
The lecturer proceeded to exhibit by experiment the phenomenon 
of lightning, and explained the nature of the apparatus for explor- 
ing the electricity of the atmosphere. The cause of the different 
appearances of lightning and the influence of the pressure of the 
atmosphere in modifying them, were exhibited, and protection from 
damage by lightning briefly alluded to. The atmospheric con- 
ditions of the Polar regions were shown to be such as to favour the 
production of electrical discharges on a grand scale. The moisture 
carried up into the higher regions of the atmosphere at the tropics 
contained a large supply of electricity ; this was transferred by 
natural currents to the Poles. Here the moisture was condensed, 
and the electricity, as it were, expelled and left to reside in the 
non-conducting medium — the air, which thus became positively 
charged. 
The attenuated condition of the atmosphere of the higher regions 
would not permit the retention of these electrical accumulations 
beyond a given point. As the change of atmospheric currents 
which occurred in the Polar regions gave rise to a change of elec- 
trical condition, the upper and lower strata frequently differed in 
their electrical polarities, and the discharges from one stratum into 
the other without the intervention of conductors, gave rise to the 
appearances recognized as the Aurora Borealis, and which were so 
correctly imitated by our electrical apparatus under similar con- 
ditions. 
The light of these continuous electrical discharges spread far and 
wide through the upper regions of tlie atmosphere, and by the 
known laws of refraction, were visible at a greater or less distance, 
according to their intensity. The lecturer considered the variations 
in colour to arise chiefly from the different densities of the air in 
which the phenomena occurred, the crimson colour being developed 
where the attenuation was greatest, precisely analagous to the 
