The Red Cloud chief., 1879-04-17
MORNING MIRAGE.
brilliant Exhibition Of the sPhenomenon
in Minnesota.

I'm Minnesota.
From the -. Paul Pioneer-Pro-sl
We have had some still. clear, and
very cold nights this winter, followed by
brilliant dawns, which have been pecu.
liarly favorable TO the display Of the re-
markable phenomenon commonly
known here as ''mornipg mirage"
Dweilers on the prairies who are up be-
times, locking TO any point Of the COM
pass but the north, see oil such morn
ings shortly before sunrise a much more
distant horizon than usual. Miles and
miles Of country ordinarily cut off from
the view by rising ground or belts Of
timber will be raised, as il were, above
the obstacles. Towns and other promi-
nent objects 20 miles away are no longer
invisible, but are clearly revealed with
all that lies between them and the spec
tator. The windows may be counted
in houses which at other times can no
more be seen than if they were at the
antipOdes, and lear objects, usually
just within the range Of vision, seem TO
be brought much closer. As the suns
orb rises above the horizon the vision
sinks below it. While it lasted a man
might 'see through a brick wall" which
in the brightest noon will form an i'm
penetrable barrier 10 his view. An eX-
PLANTATION of this phenomenon is not, so
far as l am aware, to be found in u
textbooks on physics. The one gener-
ally given of the mirage of TIE desert
explains very little of what is seen there.
and is wholly inapplicable to the morn
ing mirage OF northern ATTITUDES. The
following explanation, which is based
upon laws observed in connection with
the wave theory of light, may, there
fore, be new and interesting 10 some of
your readers:
"MOining mirage" is what il call
Cd terrestrial refraction" extraordi-
narily increased by coincidence of con-
jitions only to be met with where the
atmosphere is very cold, dry and clear,
and the latitude is so far south tina, the
sun rises rapidly, even in winter. At
the end of a still night, with the Their
mometer some degrees below zero, the
atmosphere is not only at its greatest
density, but it probably retards the pas
sage of light in other ways unknown q
us at present. Optical effects observed
in the atctic regions favor the theory
that that this excessive retardation Is
due 10 lowness of temperature or some
accompanying condition of the atmos-
phere not indicated by the barometer.
But the rising sun raises the tempera-
ture and lessons the density of higher
regions of the air within our range Of
vision before it affects the lower, Just as
it gilds the mountain tops before its
beams reach the plain. The difference
of density and temperature increases
very rapidly, owing to the considerable
power of the sun, even within a month
of the winter solstice. The curvature
of the rays in territorial ~refraction is
to the fact that light generally
by the path which occupies the
least time, and therefore deviates toward
the rarer part Of the atmosphere, and
the morning mirage results from an IN
steep deviation. The appear
auce of sunrise is always accelerated at
this time of year nor than that Of sun
set is retarded; but on the mornings,
when, owing to the perfect calmness Of
the atmosphere, the increase of tempera-
ture takes place in horizontal planes, ter
restial objects considerably below the
horizon will seem to be raised above i,
until the lower strata of air being equal
y warmed with the upper, there is no
!onger any thing to cause the excessive
of the course of the waves of
light.