are
that the air must be still
and that the lower layers, heat-
ed by radiation from the under-
lying surface, must become less
dense than the overlying layers
and yet be unable to escape.
With the least disturbance of
these unstable conditions the
overheated air suddenly "spills"
upward and mirage disappears.
The conditions in a mountain
gorge such as that of Wilson
Creek east of Brown Mountain are
entirely unfavorable to mirage,
for as soon as the lower air
becomes warmed it may escape up
the surface slopes, and at eve-
ning there is likely to be a
downward draft of cool air from
the neighboring uplands. Yet Mr.
Martin, in seeking in air cur-
rents of different temperature
and density an explanation of
the light, has hit upon what the
writer believes to be a funda-
mental element in the problem,
as will be more fully explained
below.
10. Locomotive headlights: D. B.
Sterrett, of the U.S. Geological
Survey, who investigated the
light on October 11, 1913, noted
that the headlights of westbound
Southern Railway locomotives
could be observed from Brown
Mountain and that they were
brilliant enough to be seen in
the same straight line from
Loven's place, 6 miles beyond.
He checked on the train sched-
ules and concluded that loco-
motive headlights were "beyond
doubt" the cause of the Brown
Mountain light. Objection to
this view has been raised on the
ground that a locomotive head-
light casts a beam) which, like
that of a searchlight as fre-
quently seen, can be readily
identified. This objection is
considered under the heading
|
clear
sky. "Conclusions."
11. Automobile headlights: Powerful
headlights on automobiles have
been suggested as a source of
the Brown Mountain light. The
objection made to this sugges-
tion is similar to that made to
the suggestion that they are
caused by locomotive headlights,
and it fails for the same rea-
sons. When seen at long dis-
tances the two kinds of head-
lights behave in a similar man-
ner. Of the 23 lights recorded
by instrumental observation in
the investigation here reported,
ll were probably automobile
headlights.
THE INVESTIGATION IN 1922
METHODS EMPLOYED
After a conference in Morganton
with men who are familiar with the
lights the writer set out to take ob-
servations at the place near Loven's
Hotel and at other places from which,
according to reports, the lights
could be seen, Brown Mountain itself
being one of the places. The instru-
ments used consisted of a l5-inch
planetable (a square board mounted on
three legs), a telescopic alidade,
pocket and dip-needle compasses, a
barometer for measuring elevations, a
fieldglass, a flashlight, and a cam-
era, besides topographic maps of the
region.
In making the observations a topo-
graphic map was fastened flat on the
board, which was leveled and the map
turned to a position in which the
directions north, south, east, and
west on the map correspond with the
same directions on the ground. Sights
were then taken to known landmarks
with the alidade, which is essential-
ly a ruler fitted with a sighting
telescope, and corresponding lines
were drawn along the ruler on the
|