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where its equilibrium becomes disturbed by external forces, 
mechanical or chemical, as after mentioned. 
Let us consider the nature of these forces, and, first, the 
vast mechanical forces continually acting upon this medium 
by the earth’s orbital passage through it. As the earth 
advances her force will compress the medium in front of her 
path, but its equilibrium must be instantly restored by its 
reaction in her wake. This reaction explains the slight 
retarding force of the medium on the periodic motions of the 
planets. Now, the disturbing forces of these orbital move- 
ments, along with the axial rotation of the earth, serve to 
generate continuous currents in the medium, and which 
constitute what we call magnetism, or the magnetic currents, 
such as flow from the central action in the earth’s path 
towards the north and south poles. In addition to the 
invisible currents, shown by the magnetic phenomena, we 
may fairly assume that this mechanical action of the earth 
generates those luminous currents in the medium that are 
known, at certain seasons, as ground lightning, and at others 
as auroras or corruscations of polar lights. 
We have next to consider the frictional action of the sur- 
faces of the sun and earth upon that portion of the medium 
which extends in a right line between the sun and the earth, 
and which, acting at the ends of the intervening column, 
excites the medium into those undulations or vibrations that , 
act on our senses, and give the impression of white light 
when seen through clear air, and of coloured light when the 
undulations are disturbed by clouds or vapours. But, to 
receive the impressions, we must be within the intervening 
column, or n ear it, as in twilight. N ow, although this medium 
is only perceptible as light, yet the medium itself is at all 
times present, equally in the night and in dark rooms as in 
daylight, — for when its action as light is cut off by opaque 
bodies, still the ambient element continues to act on our 
feelings and bodies as sensible heat, and this in different 
