Crawford avalanche. (Grayling, Mich.) 1879-19??, 1892-08-25
THE HELIooRAPH. /,
How Communication Between Distant
Points Is Reached by Rejecting the
SUMS Rays.

How Communication Between Distant
Points Is Reached by Rejecting the
SUMS Rays.
Photography is the name applied to the
method of communicating between dis
tant points in which usual signals are
obtained by rejecting the rays Of the
sun from a mirror or combination of
mirrors in the right direction. As
matter of course, the method can only
be employed where the sky is free from
clouds and the atmosphere clear for con-
siderable periods of time and the fact
that no place is free from sudden atmo-
spheric changes makes the establishment
of permanent photographic stations unad-
visable. In military signalling heliog-
raphy has been found to possess great
advantages over other methods, particu-
larly on account of the portability of the
apparatus, and in coast and geodetic
surveys it has been found invaluable.
To those who have not studied the
phenomena attending the reflection of
sunlight it is quite astonishing, under
favorable atmospheric conditions, at
what great distance the flash can be seen,
bright and distinct as a planet, which in
the clouds and intervening obstacles adj
NIT the possibilities of photography.
Equipped with the proper telescope and
suitable mirrors, and stationed at the
requisite elevation, with favorable con-
ditions of atmosphere, and there is prac-
tically no limit to the distance at which
signaling can be carried on. A little
consideration of certain facts will make
the truth of this statement plainly appar-
ent. The suns rays reach the
earth after travelling $4,000, 600
miles. Striking a mirror simply al
tiers their course, but makes very little
difference in their energy, and hence it
does not seem remarkable that they
should preserve sufficient vital force to
carry them hundred or two hundred
miles further. The moon, which is not
perhaps the best reflector, projects the
suns rays to us from distance of 232, 900
miles. The discovery of the principle in
volved in photography is as old as the
time of Alexander the Great, whose feet
is said to have been piloted along the
Persian Gulf by mirrors. The aborigines
of North America long ago adopted the
policy of sun-fashing from polished sur
faces, and the Russians made use of sum
telegraphy for important signaling at the
siege of Sebastopol.
The two instruments generally used In
photography are the heliograph and the
heliotrope. The latter is sometimes con-
founded with the heliostat, but hello
stat proper is an instrument operated by
clockwork in such way as to reflect the
suns rays in fixed direction. BY the
use of this instrument, the mirror is SO
adjusted that, ct particular hour of the
day. arranged beforehand, the light of
the sun is reflected to che surveyors sta
ton. The heliotrope, however, is ope-
rated by hand, and il is the instrument
which, in its more or less improved
forms, is used by Prof. Davidson in coast
survey work. Heliotropes are usually
placed upon a stand or table and adjust
ed to the distant signal station by the
sights over the uprights. The centre
ring is trifle larger than one at the
end, so that when the shadow of the
centre rim shows upon the rim of the
outer ring, the flash may be said to be in
focus, and when the sun is in front of
the operator, that is all there is to it.
When the sun is in the rear, however it
is necessary to make use of another mir
ror, placing it where it will reflect the
sun on to the heliotropic mirror and
thence through the rings.
This creates steady light which has
already been described as like a bright
star, but in order to keep it continuous,
the operator has to adjust the mirror
every moment or two with his hands to
conform to the motion of the earth. In
triangulation the operator usually begins
work at sunrise, and keeps his heliotrope
in focus until the days work is practically
over. The trianguator, having that
bright daylight star for his basis, has by
that time been able to lay out considerable
drawing, and he signals the heliotropist
to shut up shop by passing his hat over
the rays of his own instrument given
number of times. Each party on survey
work prearranges its own signals. There
is no regular code made use of in this
work, but in the army it is quite differ
net.
The heliotrope had been in use nearly
half a century when, in the autumn of
1869 at the Jack telegraph station in
Beloochistan, Henry G. Nance, now Sir
Henry, adapted the principle of the
heliotrope, or heliostat, to the modern
instrument since then known as the hello
graph. He conceived the idea of con-
verting the fascicle of reflected suns rays
which had previously been regarded, in
a signaling sense, as entirely passive,
into active speaking agents. This he ac-
complished by adapting to mirror the
means of imparting to the reflections the
character of pulsations of varied dura-
tons, in accordance with the Morse code.
Nance's Photograph, as now perfected for
field signaling, is the one now used in
the United States army. The mirrors are
not always of the same size. The greater
the mirror the more powerful the flash.
There are usually two tripods,
one bearing the mirror to reflect
the sun to the signaling mirror
when necessary. The stand for the sig
mailing mirror has a sighting bar, by
means of which perfect alignment can
be made by looking through hole made
by scraping off little of the silver from
the centre of the back of the mirror.
The obstruction is obtained by giving
small angular motion to the screen mirror,
or shutter, causing the reflected beam to
travel through twice the angle and COM
pletely disappear from the distant point.
Its working is made very similar to that
of Morse key. Several important inn
provements on the Photograph have been
made by Lieut. John p. Finlay, United
States army, now in charge of the weather
bureau in this city, all of which are in
,nractical uea in the army SIGNAL corne