Belmont chronicle. [volume] (St. Clairsville, Ohio) 1855-1973, 1881-05-19
The HeIiograph.

The heliOgraph, which was prominent
ly brought into notice in army signaling
IN the recent campaigns in Afghanistan
and South Africa, consists of a circular
| mirror, generally about AZ inches in di-
ameter, mounted on small but very
tirn folding tripod stand. 1t is SO
mounted as to be capable of motion
both horizontally and vertically. e., in
azlmnth and altitude. The horizontal
motion is impaired to it by means of a
slow-motion tangent screw, working
into worm thread cut round the hori-
zontal plate forming the base of the in
strument. The vertical arrangement,
however, is quite different; the mirroris
hung by couple of horizontal trunnions
to pair of brackets rising from the
horizontal plate. Under the mirror lies
a key similar in appearance to- tele-
graph signalling key, hinged at the for
ward end to the horizontal plate, and
kept upward by fat spring beneath. A
small rod connects the movable end of
this key or lever to the upper part of
the back of the mirror, so that when the
key is isdepressed, the mirror is tilted hp-
ward through small angle. This rod
is capable of being shortened or length
ened BY being screwedthrough the lump
by which it is attached to the mirroEs
This altering the length of the rod has
the effect of altering the inclination of
the mirror to the horizontal plane, i. e.,
OF giving 1t motion in altitude. The
mirror has a small portion of the silver
ing removed from its exact centre. For
directing the flash, movable arm or
bracket, twelve or. fifteen inches long, is
fitted to the head of the tripod, and is
capable of horizontal motion, and can
be clamped in any pUsiti0n. Through its
extremity slides p small vertical rDd, lit-
ted at its upper end with small ring
carrying a pair of stent cross-wires.
To work the instrument, it is set up
facing toward the point to be sigHalled
to; the arm and rod Carrying the cross
wires are now moved and adjusted until
the signaller, looking through the unsil-
Yered aperture in the mirror, sees that
the cross-wires intersect the point To he
signalled to. The arm is then clamDed.
The mirror is now moved round in alti- r
tnde and azimuth till, the signalling key
being depressed, the reflection of the
small unsilyered spot falls On the inter-
section OF the croSS-wireS. The signaller
then knows that his Aash-is- visible at
the station to be signalled to- The key
is then released, it springs up. and the
flash is depressed and rendered invisible
to the observer at the other station. The
Instruments now in adjustment, and
each time the key is depressed the flash
will be raised to the line of the cross
wires, and will be visible, and signalling
can te done exactly as with the Morse
telegraph instrument, long and short
flashes being sent. The motion of the
sun must be allowed for, and the alti-
tude and aZimuth screws altered so as to
keep the dark spot on the cross wires
while the key is depressed. Though not
always necessary, second mirror is
usually employed when the direction of
the sun is more than 12O degrees from
the line joining the stations, as the sun
is then rather behind the signallsr, and
it is difficult to send good flash with
ont second mirror. The flash is, in this
case, sent by double reflection. In clear
weather the signalling can be read up
to ten or twenty miles by the naked eye,
and up to perhaps fifty by the aid of
telescope. The. flash will penetrate a
fair amount of haze if the distance is
not too great.