798.—The Tripod of the Heliograph TT′T″ consists of three circular mahogany legs 1-1/8 inches in diameter and about 4 feet 9 inches long. The legs are capped with sockets carrying collar-pieces which are attached to the tenon-pieces of the head. The head forms a box for the revolving apparatus and remains attached to it when the mirror apparatus and arm are removed. The tripod head is protected when out of use by a leather cap attached by a strap to one of the legs. The weight of the tripod is 6 lbs. In fixing the tripod for use it should have the legs extended nearly 60°, and the toes should be firmly pressed into the ground. At windy stations it is well to dig holes and sink the toes, or to have a heavy stone suspended under the centre of the head.
799.—The Case for the Heliograph is made of solid leather, with separate divisions for mirrors, arm, and sight. A spare mirror is sometimes packed in the same case that the instrument may not be made useless by accidental breakage. A strap is provided with the case to go over the shoulder. The instrument weighs 5 lbs. complete in its case. Great care should be taken to observe the arrangement and position of the parts of the instrument before taking it from its case, as it is always packed closely.
800.—To use the Heliograph with a Single Mirror.—In this case the reflection is direct. The instrument is approximately directed by looking through the mirror from behind, moving the arm L and the sight I to cut the distant station, and then clamping the screw C. After this is done the exact position is found by placing the head nearly in front of the mirror, with the back to the distant station with which it is intended to communicate. Then to adjust the mirror, if required, and move the eye until the distant station appears reflected in the exact centre of the mirror. After this, without moving the head, finally to adjust the sight vane I until the reflection of the sighting spot is brought exactly in line with the centre of the mirror and appears reflected upon the image of the distant station. The sighting spot is then in direct line between the distant station and the centre of the mirror, in whatever direction or inclination the mirror may be afterwards placed to reflect the sun's image. Care should be taken not to disturb the stand nor arm in future movements of the mirror.
801.—To Adjust the Mirror, stand behind the instrument and adjust the vertical screw R and the horizontal pinion A until the black spot, as it appears on the mirror from the reflection of the hole through it, is seen upon the centre of the point of the sight vane surrounded by a ring of bright reflection from the silvered surface of the mirror. The distant station will then receive the reflection, which must afterwards be kept constantly upon it by gently moving the screw R and pinion A, following the apparent path of the sun.
802.—To Use the Heliograph with Two Mirrors, which is necessary when the sun is shining towards the distant station and its image can only be projected by double reflection, the second mirror is placed upon the end of the arm in the socket S. This has a white paper vane cemented upon it, as shown at M. The mirror B is placed roughly facing the sun. The mirror M is turned towards the distant station upon which it is intended to direct the rays, being careful at the same time to observe that the two mirrors do not intercept each other's rays. Now from the back of the mirror M we look into the mirror B, moving the head until the centres of the two mirrors appear in a line with the eye. Then without moving the head, adjust the direction and inclination of M until the reflection of the distant station appears in the centres of the mirrors. Now clamp the mirror M in this position, from which it must not be moved so long as it is required to keep the same station in communication.
To keep the reflection following the sun a position is taken at the back of the mirror B, and this mirror is worked as before described, when it is used singly, by the milled heads, only that in the present case the paper vane M takes the place of the metal vane I.
803.—Telegraphing by the Heliograph.—The communication is made by the alternate pressure and release of the Morse key F, each pressure throwing the reflected image of the sun off the observing station. The Morse alphabet, which is universally used, consists of rapid touches represented by dots, and pressures of at least four times the time of a touch represented by dashes. The following arrangement forms the alphabet:—
| A | · – | N | – · |
| B | –··· | O | – – – |
| C | –·–· | P | ·– –· |
| D | –·· | Q | – –·– |
| E | · | R | ·–· |
| F | ··–· | S | ··· |
| G | – –· | T | – |
| H | ···· | U | ··– |
| I | ·· | V | ···– |
| J | ·– – – | W | ·– – |
| K | –·– | X | –··– |
| L | ·–·· | Y | –·– – |
| M | – – | Z | – –·· |
The time between the words is double that of a dash. Many other signs are commonly used for figures, etc., for which the reader may consult The Manual of Instructions in Army Signalling. The same system is used for signalling by flags; and by stopping off light of lamps this system is most valuable for the surveyor in new countries for information of forward ground and other matters.
804.—Lights for Observations by Night.—Under many conditions an observation of a distant station may be much more conveniently and accurately taken at night by observation of a luminous object of limited area. For this purpose the arc light, lime light, blue signal light and others have been employed. For the civil engineer where regular stations are not erected, as with geodetic work, oil lights or the burning of magnesium ribbon are the most convenient.