Extract
from 'The Sun Telegraph' by Colonel C Cooper King, FGS, RMA..
[Notes
by Chris Long]: Wemight
boast of modern DX records with lasers and exotic photodetectors, but
the all-time distance record for Morse transmission by light was set one
hundred and nine (109) years ago in 1896, using the
human eye as the receiver! It was achieved by the United States Army
Signal Sergeants, from Uncompaghre Peak, Colorado; to Mount Ellen in
Utah - a distance of 183 miles. The best modern amateur laser MCW
communication has only spanned three-quarters of that distance. Is
this a function of increasing worldwide atmospheric air pollution, or
have we just not tried hard enough?
The
extract below appeared in Volume 3 of Cassell's popular anthology
"Science For All" (ed: Robert Brown, London, n.d., c.1899,
pps 133 - 139), under the title of "The Sun Telegraph". Its
author, Colonel C Cooper King, was based at the most famous of
British military training establishments, the Royal Military College
at Sandhurst. The techniques described were used extensively in South
Africa in the earlier part of the Anglo-Boer War (1899 - 1902). The
Mance Heliograph can be seen in operation in the still frame from the
Mutoscope Company's film (1899), above, where British troops are
using it to communicate across Boer lines into the besieged city of
Ladysmith. The simple but effective optical alignment techniques used
may be of special interest to laser experimenters today. Let's review
the 'forgotten' 19th century technology of 'heliography'. Over to
Colonel King ...
"The
principle of using a reflective mirror for signalling is neither new
nor confined to civilised modern communities alone. Mr Galton notes
that the North American Indians transmitted information to each other
from a lofty hill range to points beyond adjacent forests by means of
flashes from a piece of glass. The fleet in which Alexander the Great
coasted the shores of the Persian Gulf was said to be directed in its
course towards India by flashing signals from the shore.
The
'Times' of July 11, 1853, states in its Crimean
correspondent's letter that "a long train of provisions came
into Sebastopol today, and the mirror telegraph, which works
by flashes from a mound over the Belbeck, was exceedingly busy all
the forenoon." This evidently indicates telegraphic
communication by some form of sun-signalling.
But
the actual construction of an instrument specially designed for such
work is of comparatively late origin. First of all, the Heliograph
must not be confounded with the Heliostat, though the names are often
used indifferently. The latter is used on great surveys to fix the
points of the greater triangulation of the area, and is merely a
mirror which throws a flash steadily in a certain known direction,
but which is provided with no means for obscuring the reflected ray.
It has been in use by, the Ordnance Survey of England for nearly
fifty years. In the Cashmere survey it was used between points as
much as 100 miles apart, and often when intervening clouds or haze
shut out the view of the light from the observer, it could still be
detected by the use of a powerful telescope. Of course this
instrument could be easily used for signalling, but ordinarily is not
fitted with the means for making regular and definite short or long
flashes of light. The Heliograph in most general use is that of Mr
Henry Mance, of the Government Persian Gulf Telegraph
Department; but an "improved" instrument has been
constructed by Captain Begbie, of the Madras Engineers, and another
very similar to his by M. Leseurre, in France.
They
are all alike in principle, that is to say, the dash or dot is
produced by a long or short exposure of the reflected ray. But in
Mance's instrument, and in the "Field Heliograph"- the most
portable form for field service - the obscuration of the ray is
effected by moving the mirror itself; while in the two others the
reflection is shut off from the distant observer by a screen
interposed between him and the mirror. In the French Heliograph,
moreover, this screen is like the shutter apparatus before referred
to, being constructed of thin slips of metal on pivots, which can all
be turned together, by a connecting rod and handle, so as either to
expose or interrupt the ray of reflected light. The object of this
arrangement of the screen is to increase the rapidity with which the
exposure or non-exposure of the ray can be made.
It
will be seen at once, however, that, putting aside the many things
that interfere with sun-signalling, or "heliography," such
as clouds and haze, and strata of unequally-heated air, which produce
irregular refraction, the single-mirror instrument is useless when
signalling directly away from the sun. At early morning and towards
sunset, moreover, the quarter of the horizon opposite the sun can
receive only extremely feeble reflected light. Hence it is that in
both the Begbie and Leseurre instruments a second, or "sun-mirror,"
is added, which can be turned in any direction, and which receives
the sun's rays directly, and then reflects them back on the
signalling mirror. In the Begbie apparatus this is supported on a
separate tripod, as also is the screen interposed between the
signalling mirror and the observer; but in the French instrument the
sun-mirror is attached to the stand on which the signalling mirror is
fixed, and a separate stand for it is not thought necessary.
It
does not seem, from experiment, that the size of the mirror affects
the size of the reflected image; but by increasing its dimensions
greater intensity of light is produced, and therefore the reflected
ray has greater penetrative power. For long distances, therefore, the
size of the mirror is usually increased. One 3 inches square can
signal to points 10 or 12 miles off ; one of 4 inches, 15 to 20
miles, as a rule, and up to 40 miles in exceptionally clear weather.
The
only difficulty in these methods of visual signalling by sunlight, is
to ensure that the pencil of light shall be thrown on the point where
the opposite observer stands. How this is effected will best be
understood by a description of the instruments themselves.
The
Mance instrument, or "Field Heliograph," is shown in Fig.
6. It consists of a small 4.5 inch mirror, supported or pivoted in a
light frame on a tripod stand. In the centre of the mirror a portion
of the glass is left unsilvered, so that the signaller can look
through it and direct its axis towards the required spot. In order to
set it at the required angle, the mirror is fixed in a frame which
admits of horizontal or lateral adjustment: by means of a tangent
screw. Vertical movement is obtained by means of a steel rod, which
passes through a nut in the top of the mirror, and which is also
provided with a tangent screw; but it terminates in a sort of handle
or key, which is kept in position, somewhat, by a spring. Thus when
the handle is pressed down, movement is imparted to the mirror, and
on withdrawing the pressure the spring restores the handle to its
former position. About 10 yards in front of the signalling stand is
placed a "lining-rod," having on it a sliding stud, and
also a small cross-piece, with a stud in the centre. The former is
for aligning the centre of the mirror with the object. The latter is
intended to show if the reflected ray is wandering to the right or
left of the true line.
A
complete set of apparatus consists of two mirrors, a lining-rod, and
separate cases to contain the mirrors and their frame. It is,
therefore, possible to work in any direction, by utilising one of
these as a "sun-mirror," to throw a reflection on the
"signalling mirror."
The
mode of operation is simple. The pressing back of the tangent screw,
which imparts the horizontal movement to the glass, leaves the frame
free to be moved by hand roughly into the required direction; and the
operator then looks through the unsilvered part of the glass and
aligns the sliding stud on the lining-rod with the distant station.
This is first found by moving the mirror so as to throw a ray in the
direction of the place, and waiting until the answering flash
determines its exact position. The cross-bar on the lining-rod is
adjusted at right angles to the rod, about a foot below the sliding
stud; or, rather, it should be so placed that the ray of light should
rest on the stud in its centre when the instrument is not moved, and
should rise to and rest on the sliding stud when the key or handle is
fully pressed down. Movement, for signalling, can now be given to the
mirror by pressing the handle; and as the position of the sun alters,
which would be shown by the ray passing to the right or left of the
stud in the cross-bar of the lining-rod, a slight touch of the
horizontal tangent screw will bring the ray back to the central
position.
The
disadvantage of this instrument is said to be that the movement of
the handle, or key, disturbs the glass, and has a tendency to alter
its position. This would especially be possible on sandy or soft
sites.
The
Begbie arrangement (Fig. 7) is designed to meet this difficulty. It
consists of two mirrors and screens, each upon a tripod stand. Each
mirror has a hole in the centre, in which can be fixed white or black
" guiding discs " (the use of which will be seen
hereafter), and has a slow-motion screw on its frame for vertical
adjustments. Small horizontal adjustments are produced by a
slow-motion screw at the head of the tripod. From the signalling
mirror projects a horizontal bar, which can be moved in any
direction, and clamped when in its proper line. At its extremity is a
small circular frame, with cross-wires and a sighting-hole in the
centre, which is fixed on a rod that moves up and down in a socket at
the end of the bar, and which can be clamped by a screw. The
sun-mirror is 5 inches, the other 4 inches square. Finally, the
screen, which is in one piece, and unlike, therefore, the
venetian-blind sort of arrangement of Leseurre's instrument, is
provided with a key, or handle, by which it can be made to obscure or
expose the ray of light from the signalling mirror. Its position,
therefore, is just in front of the cross-wire frame, and the
reflected ray should fall on its centre.
Of
course the machine can be worked like the Mance pattern, that is to
say, when the position is favourable the sun-mirror can be dispensed
with. In this latter case, then, the mode of operation is as
follows:- While an assistant stands at the end of the bar to move it
horizontally, or else raise or depress the wire frame as required,
the observer looks through the hole in the mirror and aligns the
centre of the cross-wires with the object. The arm and wire frame are
then clamped, and a white disc is placed in the hole in the latter,
and a dark one in the hole in the glass. The mirror is then unclamped
at the tripod-head and moved till the ray of light rests near the
sighting arrangement, and then being re-clamped, the small-motion
screws are used to bring the reflection of the black disc of the
mirror on to the white disc of the cross-wires. As, owing to the
motion of the sun, the black disc shadow passes over the white one,
it is brought back by the small-motion screws.
In
using the second or sun-mirror, the tripod on which it is fixed is
placed on one side of, and about a foot in front of, the signalling
glass. After aligning the latter with the object, as before
described, a white disc is placed both in the cross-wires and the
signal mirror. Then looking through the hole in the sun-glass, the
operator should alter the position of the other glass until the
reflection of the hole in the cross-wires coincides with the white
disc on it. Next, by the small-motion screws, the position of the
sun-mirror should be altered, until the black disc, which now fills
the hole in it, is reflected on the white disc in the other glass.
Then the screen tripod is placed in position, and the signalling
mirror should not be moved again, any alteration rendered necessary
by the sun's movement being remedied by the sun-glass.
Though
this obviates the possible disadvantage of moving the signalling
glass itself, it has still disadvantages. First, the quantity of
apparatus is greater, and the weight, 12 lbs., as compared with the 5
lbs. of a simple Mance instrument, is therefore greater. Next, it
requires two operators to manipulate the signals - one at the screen,
the other at the mirror. Lastly, the working of the screen through an
arc of 90° takes a relatively longer time than moving the mirror
itself through 2°; that is to say, the Begbie instrument is
slower in operation than that introduced by Mance.
At
Ekowe, in the campaign in Zululand, the whole apparatus had to be
improvised, a common looking-glass belonging to one of the staff
being utilised. The great difficulty experienced was in directing the
flash on to the position occupied by the signalling party on the
hills above the Tugela. A common gun-barrel resting on bags of
"mealies" {Indian corn), and provided with brightened
bullets for sights, was first tried, but it was not very successful.
Then two wires with the upper parts bent into the form of a
semi-circle, and with cross-wires uniting the bent end with the
upright part, were tried. The sights were composed of brightened
bullets in the centre of the cross-wires, and when these rod~ were
set in the ground and aligned with the flash, no difficulty was
experienced in communicating (Fig. 8).
In
India, in some of the Cashmere surveys, the native assistants
dispense with the sighting-rod, using only a piece of straight bamboo
with a piece of cotton wound round it at the proper altitude. And
there are many ways in which means of aligning the ray, which is the
only practical difficulty, could be improvised.
Sun-signalling
has great advantages over all other methods of visual telegraphy.
Messages can be transmitted to great distances, and the clearness
with which the signals can be made renders background of but little
importance. In flag-signalling [(semaphore)] the distinctness of the
signal depends materially on this question, and one of the first
points that requires the operator's attention in this case is that he
should place himself in the exact line in which he is going to
telegraph, and then turning his back upon the distant station, notice
against what materials his signals will be shown. On this depends his
choice of the flag for the work. Unbroken backgrounds are better than
more varied ones; thus trees and the sky would represent the darkest
and lightest classes. And this kind of signalling can be most
favourably carried out when there is a clear atmosphere and a clouded
sun. Heliography, on the other hand, is naturally most effective on a
clear day and with a bright sun. Given these conditions, there seems
to be no practical limit to the distance over which a signal can be
transmitted. The ray of sunlight is not lessened materially in
intensity by reflection from the mirror's surface, for it is merely
bent or altered in direction. But for great distances a considerable
altitude for the signal station is essential, and this is not always
obtainable under ordinary conditions of ground.
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