Viewed as an electric discharge. 315 
Whatever may be the material constituting the auroral arch, 
it does not seem capable of penetrating the denser portions of 
the atmosphere but rather glides over them with a horizontal 
motion. Its observed form and motions may perhaps be mos 
readily explained by supposing it to originate as a horizontal 
stratum of cloud, of a circular form, having its centre vertical 
over the north magnetic pole. Such a cloud, if repelled by this 
pole and attracted by the south magnetic pole, must, in com- 
mencing its motion southward (if there be any coherence between 
its parts) be converted into a ring, which would glide over the 
upper surface of the atmosphere, its diameter constantly increas- 
ing, like that of a circular ripple in water, as it moved towards 
the magnetic equator. In this case the ring itself would always 
occupy the position of a magnetic parallel of latitude; and the 
part of it visible from any place on the surface of the earth 
would appear as an arch with its ends resting upon the horizon 
and with its highest point on the magnetic meridian. Other 
_ similar clouds successively formed over the pole and then im- 
pelled southward would present the same phases, and when 
they were sufficiently near to each other, an observer would see 
several concentric arches, as is the case in some auroral displays. 
The material composing the arch seems, in the steadiness 
and mildness of its light, its rolling motion, and cloudy appear- 
iki e “glow” which i tl 
h 
ble in some forms of the “stratified discharge.” In the “ Pro- 
b 
describes many varieties of the “ stratified discharge.” In some 
instances several luminous cloud-like concentric envelopes sur- 
nee as platinum being insufficient to resist it; and the great height to which 
auroral streamers are seen to extend renders it probable that their visibility is due 
to matter so transported from the auroral arch—and since they are illuminated in- 
stantaneously throughout their whole length, the velocity of the particles of matter 
carried up must approximate to that of electricity itself. 
angular veloci ‘ : ogous 1 
the tails of comets shot out from the nucleus with inconceivable velocity and main- 
taining their rectilinear form while sweeping around the sun, in perihelion, notwith- 
: > an nts can transport particles of our atmosphere to 
distance of several hundred miles beyond its limits we may readily conceive that 
Similar forces may carry portions of the extremely rare ma composing the en- 
Yelope of a comet to a distance of even millions of miles from the nucleus. 
7 8 ‘1D . . ° 
_ Suspicion may therefore be indulged that the tail of a comet simply indicates the 
a Position of a stream of electricity rendered visible by its illumination of particles of 
