4 
PHYSICS: E. THOMSON 
It was observed by me in the Adirondacks, and served to confirm the 
inferences drawn from the earher displays. 
But on August 28 there occurred a simple type of aurora which I had 
only seen on two former occasions. It may be said to contain a single 
element only. It consisted of a single band a belt of light, estimated 
at from 1^° to 2° wide extending across the sky due east and west, 
from horizon to horizon and passing through the zenith. It lasted for 
an hour or a little more. At the zenith and for 2 or 3° east and west 
thereof, the band was broken into patches which shifted, faded and were 
replaced by others of different shapes and positions. A friend observed 
the same appearance in Maine hundreds of miles to the east on the 
same night. 
The explanation of this simple type of aurora is not difficult when 
made in accordance with the ideas here presented. No streamers were 
seen as such. At the zenith they were directly over the observer, and 
though present were seen only on end as small patches of light constantly 
changing: while in the extensions or narrow bands to east and west 
from the zenith the luminous streak was a composite of super-posed 
streamers in the same latitude as the observer, which streamers over- 
lapped each other all the way down to the apparent horizon along the 
overhead arch east and west. They were like the palings of an extended 
fence located far above the observer. He would see the separate pal- 
ings, directly above him but only on end. Otherwise they overlap and 
obscure each other or one another. To appreciate fully the reality, 
the observer is better placed if the auroral arch when, as in this case, 
very narrow is not directly overhead. The usual auroral arch is the 
location of the lower ends of the vertical streamers extending upward 
from it. This location would appear from the work of Stormer and 
others to be in a layer of atmosphere about fifty miles high. This 
layer on which the feet of the streamers may be said to rest is prob- 
ably a conducting layer like the partial vacuum in a Geissler tube. 
The so called auroral arch appears as an arch or curve merely because 
of perspective vision, just as a searchlight beam or a long straight 
cloud appears bent when crossing the sky, or as a cloud layer, known 
to be horizontal appears above us as a great inverted bowl. 
The streamers in the aurora of August 26, were, as in the aurora of 
1883, directed towards the zenith crown from all sides. The appearance 
of the whole aurora when at its height, and looking upwards towards 
the zenith was as if one were looking into a truncated hollow cone from 
below its base, with the slant sides composed of ribs of light; these ribs 
as well as the blunt apex, constantly changing, shifting, fading, and 
