The Abilene reflector. [volume] (Abilene, Kan.) 1883-1888, 1885-12-17
RANGE OF VISION.
The Influence of Refractfon and the The
DRY of the Mirage.

The Influence of Refrsctfon and the The
DRY of the Mirage.
IN perfectly clear weather the distance
from which an object of small size Ga
man for example) is visible to the naked
eye can not exceed about three and a
half miles, when the object is seen in re-
lief against white background. 1t
may be said that an object is only visible
when its movement is perceptible to the
/ naked eye.
From Cape Hode, situated near Havre,
! it is possible to perceive fisherman or
hunter walking upon the sand banks of
the Seine. From the valley of the Grin
delwald, in Switzerland, it is possible to
see tourist upon the Jungfrau or Mat
terhorn- This distance can be increased
by means of a simple opera glass. From
the Havre Jetty persons may thus be
seen, in very clear weather, upon the
beach at TrOuville, at a distance of eight
and Il half miles. / say see, and not rec
Ognize; and yet certain fisherwomen of
Villerville have found il possible to rec
OgRizc their husbands boat starting from
Havre. at six miles distance At the
last shooting match at Havre, Messrs.
Bigot and Pelot each made series of
bulls eyes at nine hundred yards dis
tance-a fact that. besides great skill,
supposes very strong sight.
In tropical seas, Captains agree in
saying that from the deck of their ships
the distance that is visible around them
is six leagues. In temperate zones the
distance is less. In many cases, the
earth's curvature presents an obstacle
to the range of vision; and yet this does
not always appear to be so. Captain
DucloS, of Havre, tells me that once,
while becalmed off Madeira, he could
not perceive the island during the day,
but in the morning and evening OBJ
served it at Il distance of twenty-five
leagues. Professor Morel, of Paris,
informs me that while at Nice,
he perceived the profile of the noun
tains of Corsica, and even made
sketch oft. myself, while near Dunge-
ness, England, saw houses that were
situated in the environs of BoulOgne-
sur-Mer, France.
In fine weather we can not see Lion
or Luc-sur-Mer from the Havrc ictty.
and yet there are days when both of
these localities become visible. ] have
found while on a steamer in ordinary
weather that the said localities become
visible at the middle of the route
On returning, the electric lights of the
Havrc jetty become visible mt the
same point. There are. then. days on
which, in certain weather, and despite
the curvature of the earth, the distance
of visibility is doubled or more than
doubled.
This phenomenon is explained to us
by well-known law of physics. If we
throw Il coin into an empty vessel, it
will be hidden by the edge of the latter.
at certain distance; but if we fill the
vessel with wafer, the coin will become
visible. We have here case of rcfrac-
ton. such as is explained in elementary
works on physics.
Every one also knows the theory of
the mirage emitted by the celebrated
Monge, who explains the phenomenon
;tS due to diminution of the airs
density in contact with the supcrheated
earth. A contrary theory will explain
the mirage at sea. While the sun is
making the atmosphere intensely hot,
the sea and the strata of air in contact
Herewith remain relatively cold, and
these strata become superpOsed in the
order of their density. A luminous
ray emanating from the sea will
pass from one stratum into an
other of less density, there will be a
refraction, and an object beneath the
horizon will be seen above it. This |
theory suppOses two conditions. tO-wit:
A heating of the atmosphere, while the |
sea and earth remain relatively cold. |
and calmness of it TO permit its strata |
to become superposed in the order of |
their density. 1t is precisely where
these two conditions are fulfilled that |
the phenomenon of the mirage occurs.
On such days, ships rise instead of de-
scending in measure rs they rccede. As
the horizon is perceived by direct WISH
bility, a ship is finally seen above the
horizon, as if it were suspended in the
air. 1t is the famous phantom ship
familiar to sailOrs-a visible ship in its
natural, upright position. If atmos-
pheric conditions were favorable, n sec
ond ship would be seen above the first,
etc. Certain mariners worthy of Gre
dencc have assured me that they have
seen rs many as seven superpOscd ships.