Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

CHAPTER XVII.

STATIONS OF OBSERVATION—PICKETS—FALSE PICKET—PERMANENT STATIONS—REFERRING OBJECT—HELIOTROPE—HELIOSTAT—HELIOGRAPH—SIGNALLING—MORSE ALPHABET—NIGHT LIGHTS—OIL LANTERNS—MAGNESIUM LIGHT.

779.—Stations of Observation vary materially according to the extent of the survey and its purpose. For geodetic works stations are raised at great expense, often in masonry or solid woodwork. For ordinary local or civil surveys the stations are commonly formed of single poles set up vertically, which vary in dimensions according to the extent of survey and the difficulties which may be encountered by various obstructions to direct visions by woods, lakes, marshes, etc. The apparatus that may be useful in the work of the civil engineer in ordinary practice will only be considered here.

Fig. 383.—Ranging pole or picket.

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780.—Pickets or Ranging Poles, Fig. 383, as the name indicates, are used for ranging a direct line through a district, either by a series of poles sighted from one to the other or by being placed in position convenient for triangulating by the theodolite where the country is open, or free from many buildings, trees, or other convenient landmarks.

781.—The picket (Fr. piquet) is a straight, slightly tapering pole shod with wrought iron or steel. It is generally made of about 1-1/8 inches diameter, and is painted in alternate feet red and white with an enamel paint that will not soil the hands or take dirt from them. The shoes should be made with strap-pieces, so that the picket, which is generally made of yellow pine for lightness, should not be liable to break off at the shoe in use. Fig. 381 represents the lower part of a picket as made by the author: B black, W white, R red. It is usual to have six pickets at least out in use with a theodolite in open country.

Fig. 384.—False picket.

Fig. 385.—Spur-shod picket.

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782.—False Picket.—For the placing of a picket it is usual to clear the sod with a small spade where possible, so as to suspend the plummet from the theodolite into the hole made by the picket to triangulate from its position. In marshy lands and under many conditions this is not easily done. It will generally be found more expeditious to carry about one of the author's false pickets, to place directly in the hole from which the picket is removed, which saves the trouble of removing the grass. This is shown in Fig. 384. It consists of a wooden peg, upon the top of which a cross is sawn to represent the axis. This cross is filled in with a veneer of ebony, and the whole is polished over to keep it clean. It will be readily seen that any picket accidentally broken will make a false picket. In setting up the theodolite over it the plummet is brought to verticality with the centre of the cross. In moving the false picket the original one is easily replaced, if required, in the same position for continuing the work.

783.—Spur-shod Picket.—Much stouter poles than may readily be pressed in by hand, as for instance, of 2 inches diameter, may be driven into the ground by having a spur or cross-bar of steel, about 7 inches long and about 3/8 inch diameter, placed through the pole, say at 1 foot distance from the point, a form which is much used on the Continent. This picket may be jerked down for a certain distance by pressure of the foot on each side, and then jerked home to the ground by standing upon it, to make a 10-feet or 12-feet pole stand sufficiently rigid for temporary work, Fig. 385.

Fig. 386.—Socket for station pole.

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784.—Permanent Stations are commonly constructed upon hilltops or other commanding positions. A very general way is to set up a long pole of fir or other wood at command, from 10 to 20 feet in height, according to the circumstances. Occasionally it is desirable to remove the pole and place the theodolite centrally over its vertical position. A very good way to do this is to have a slightly tapered wooden socket, Fig. 386 S, constructed of stout boards, say 1½ inches thick, made into a hollow square with a cross of boards, WWWW fixed to it. The socket is placed in a hole dug out entirely below the ground, and is rammed in and fixed as an ordinary gate post. The pole P is squared at the end to fit the tapered socket up to shoulders which are formed by leaving the other part of the pole round. The socket for a 15-feet pole should be 18 inches deep; for a 20-feet one, 2 feet deep. Where these poles are properly prepared they may be jointed together in two or more parts for portability. Bunting flags, red and white, about 18 inches by 9 inches, may be fixed at the tops of the poles. In fixing the socket the pole should be erected in it to be able to keep it constantly vertical during the ramming. A plummet suspended at arm's length, at a distance from the pole in two positions at about right angles to each other from the centre of the pole, will provide a means of keeping it erect during the fixing of its socket. The socket hole, upon lifting the pole out, forms the centre for erecting the theodolite over its position.

785.—Referring Object.—It is desirable that all arcs taken by the theodolite from an important station should contain one point in common, for which the best defined object to be found at a distance may be selected. Colonel Clark, of the Ordnance Survey, recommends as a referring object two rectangular plates of metal placed with their edges parallel to each other in the vertical plane, at such a distance apart that the light of the sky seen through the opening appears as a vertical line of about 10 in width. The best distance for this object is from 1 mile to 2 miles. Two pieces of board, fixed a small distance apart by ledges screwed thereon, answer the same purpose. The description fully conveys the method without illustration.

Stations Visible at Great Distances are formed by means of reflection of the sun's rays or by artificial light.

Fig. 387.—Stanley's heliotrope.

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786.—Heliotrope, or heliostat as it is sometimes called, may be any form of mirror to throw the sun's ray in a constant direction or to a distant station at a time of day fixed for making observation. The instrument is uniformly constructed with a small glass mirror having a plane surface. The angle of divergence of the extreme rays in the reflection is the same as that subtended by the sun's diameter at the position of the mirror, that is, of about 32 minutes of arc. This divergence is sufficient to render the reflector visible at a great distance. The plan upon which the author has constructed this instrument is shown in Fig. 387. It consists of a reflector M formed of a plain glass mirror of about 5 or 6 inches in diameter, placed in a metal tray. The mirror is centred vertically upon an axis to which a worm wheel B is attached upon one side that works into a tangent screw which is moved by a milled head so as to place the mirror at any angle to the horizon. The mirror and its vertical adjustment just described are carried by a fork which is erected from the base board of the instrument upon a socket joint which permits the mirror to be turned about. Upon the lower part of the fork above its socket another worm wheel is constructed centrally to the axis. This works into a tangent screw attached by fittings to the base board. The tangent screw has a long shank leading to a milled head A. By means of the milled heads the mirror may be set to any position, so as to throw the reflection of the sun in any required forward direction. A small hole is cut through the silver in the centre of the mirror to sight the position to which the sun's reflection is directed.

787.—The Base Board is of ¾ inch mahogany about 20 inches by 10 inches, and is supported upon a very firm tripod stand, like that described for a plane table, art. 700. At one end of the board a sighting screen of mahogany, 10 inches by 10 inches and ¾ inch thick, is hinged, so as to be held erect by means of a stay bar E. In the centre of the screen an opening is turned out 3½ inches diameter, and a frame-piece of half circle only is placed over this. The frame piece is grooved out at the back so as to hold discs, shown abc in the figure.

788.—The Discs abc, are of thin brass and have openings respectively ¼, ¾, and 1½ inches wide, so as to reduce the width of the line of light which appears through them when the reflection of the sun is thrown from the back. These have each a fine wire stretched across them to indicate the centre. A fourth disc, not shown, has a double cross of wires to indicate the centre only.

789.—To Pack the Instrument, the screen is turned down the index frame, falling into the opening F; the mirror with its fork is lifted out and secured to the surface of the base board by buttons; and the whole apparatus is put in a pine case. Its weight without tripod stand is 8 lbs.

790.—To Use the Heliotrope, the station on which the sun's light is to be thrown is sighted by looking through the small hole in the centre of the mirror, and adjusting the base board until the station appears in the centre space of the disc opening. The mirror is then turned towards the sun by means of the milled heads until its image, reflected upon the back of the screen, appears central with one of the discs which is intended to be used. All parts of the stand and fittings being made quite firm, the attendant moves the milled heads, as required, to follow the apparent motion of the sun, at intervals of five minutes or less. It must be observed that the centre of the slit in the disc represents the station visible to the observer. This point must therefore be plumbed to the station point in setting up the instrument. A part of the screen at P is cut away to admit of the suspension of a plummet.

791.—The heliotrope was much used in India for the great trigonometrical survey. Colonel H. Thuillier states from experiment that "A heliotrope of 9 inches diameter answers for 90 to 100 miles. For nearer distances it is much too bright to be observed through a telescope, and the light must be diminished in the following proportion. For distances of 2 or 3 miles (the usual distance of a referring mark) an aperture of 0·25 of an inch will answer, and for longer distances about 0·1 of an inch of aperture per mile of distance will suffice, viz., an inch for 10 miles, 2 inches for 20 miles, and so on, provided always the apparatus is carefully adjusted and the man who works is alert and skilful."[56]

Practically the discs here described will give all the variation required. In less favoured climates than India more opacity will be found in the atmosphere, and larger apertures required than those just stated.

Signalling with the Heliotrope.—A thin wooden bat D is moved over and off the outside front of the open disc aperture, following the rule of Morse signals, which will be presently described for the heliograph.

792.—Heliostat.—Is a smaller instrument than the heliotrope, in which the mirror or mirrors are moved by clockwork, so as to keep the sun's reflection in a uniform direction throughout the day. This instrument is delicate and not generally well adapted to field work.

Fig. 388.—Heliograph.

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793.—Heliograph.—This instrument is the invention of Sir H. C. Mance,[57] since improved by Major Macgregor, Colonel Bonham, and others. It is used for a military signalling apparatus, but it is also employed, on account of its portability in place of the heliotrope for surveying, where great precision by limiting the area of light reflection is not required. The construction of the instrument is shown in Fig. 388. B is the back of a plain circular mirror of 5 inches diameter, supported upon pivots on a fork frame J, the lower part of which forms a socket. The socket is furnished with a thumb-screw to secure the mirror and its frame when placed upon a cone projecting from the apparatus connected with the base plate formed on the top of the tripod head. The cone is erected upon a disc or wheel cut at its edge in teeth and centred upon the axis of the tripod head. The wheel is revolved by means of a pinion connected with a milled head A which moves the mirror and the entire apparatus above in horizontal revolution.

794.—The Sighting Arm L is attached to a collar fitting projected from the tripod head. This may be fixed in any horizontal direction by means of the tangent clamping screw C. The arm L has a supplementary extension by the piece Sj, which is jointed at the position of these letters and also by a socket fitting into the arm. The termination of the extension is a sighting point I formed of a thin blade of metal. The arm and its fittings permit the sighting point I to be set in any direction or elevation to follow the inclination of the land.

795.—The Sighting Vane is a piece of white metal upon which there is placed a black dot termed the sighting spot. A small circle, about 1/5 inch diameter, is left unsilvered in the centre of the mirror, which does not reflect the sun's rays. It therefore causes a small disc of shadow in the centre of the reflection of the mirror, termed the shadow spot. The shadow spot is made to appear upon the sighting spot when the instrument is adjusted to throw the sun's image upon a distant station.

796.—The Supplementary Mirror M is similar to that already described, centred also on pivots and placed in a forked frame. This is mounted on a cone S' which fits into a socket at S, when the extension arm J is removed. This mirror is intended to receive the image of the sun when placed towards the back of the pointing of the instrument to throw the sun's image from the mirror M to B, to signal by double reflection, when the sun is at a forward angle to the distant station. The coincidence of reflection is taken with this mirror by the reflection of a piece of paper pasted on its centre of the same form as the index I.

797.—Telegraphing Apparatus, called technically flashing apparatus. This consists of a rod R hinged to the top of the mirror at its upper end and also to a lever which forms a Morse key at the lower end. The rod is formed of a screw of about half its length, which passes into a female screw tube so as to shorten or lengthen it as required to direct the reflection of the sun's rays by turning the milled head above R, which forms a part of the tube. The Morse key is hinged at J to the stem of the instrument, and is kept up to a fixed stop by means of a spring P extended by an arm from the stem of the instrument, so that pressure upon the disc F moves the key down to its stop P, and also tilts the mirror to throw its reflection off the observing station during the pressure. The flashing described by the jar of its action is liable to displace the mirrors. The use of the bat, shown at Fig. 387 D, is more certain for signalling words.

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.

805.—Oil Lanterns.—In the great trigonometrical survey of India large reverberatory lamps were used, which were furnished with Argand burners with circular wicks about 2 inches in diameter. The back arc of rays was reflected by a parabolic reflector 12 inches in diameter and 4·9 inches in depth. The lamp was enclosed in a strong box with a plate-glass face 12 inches in diameter, with apertures to admit sufficient air and chimney to carry off fumes. The box was constructed to form a packing case for conveyance of the apparatus.[58]

806.—The oil lantern which will be found most convenient for the civil engineer will be one of the same form of construction as the bull's-eye lantern, but much larger—6 inches square is a good size. This may be made to go on the same tripod as the heliograph, and will take its place for signalling by night, or telegraphing by the Morse signals by the hand or bat shown Fig. 387, D. A 6-inch bull's-eye lamp with treble wick may be seen well in clear weather 5 miles to 10 miles off. A railway signalman's hand lamp forms a very good signal, or even an ordinary 4-inch bull's-eye is very useful in working over new countries.

807.—Magnesium.—The intense light given by burning ribbon magnesium, and the extreme lightness in weight of this material, render it of especial value for night signalling. Magnesium ribbon is now sold at a very low price (about two shillings per oz.), and 1 oz. will give a continuous intense light, visible at 30 miles, for over an hour, whereas for a night signal arranged to be given at a stated time, fifteen minutes is amply sufficient for a single observation. Great difficulty is often found in lighting magnesium ribbon when this is slightly oxidized from exposure to air. The best method is to employ the flame of a portable spirit lamp, made for the purpose. Under any condition the burning ribbon should be shaded from wind. A common plan is to hang a straight slip of ribbon from the centre of a tripod which can be readily shaded by a pocket handkerchief. Where expense is not the object to be considered, lamps may be had for burning the wire. Tin cases are made for soldering up and storing the ribbon in for use abroad.

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