[49] See British patents--Winter, 1760, _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.

Nugent, 1794, No. 1980; Wright, 1796, No. 2081; Cook, 1796, No. 2087;

Roxby, 1822, No. 4695; Glover, 1839, No. 8256; Lane, 1857, No. 1669;

Rahill, 1860, No. 1845, etc.

[50] Adams' _Geometrical Essays_, p. 264, 1803.

CHAPTER XV.

GRAPHIC SURVEYING INSTRUMENTS AND APPLIANCES CONNECTED

THEREWITH--PLANE TABLES--ALIDADES--TELESCOPIC ARRANGEMENTS--SUBTENSE

MEASUREMENTS--VARIOUS DEVICES FOR HOLDING THE PAPER--CONTINUOUS

PAPERS--ADJUSTMENT OF TRIPOD HEADS--METHOD OF USING--EDGEWORTH'S

STADIOMETER--SKETCHING PROTRACTOR--SKETCHING CASE--CAMERA LUCIDA, ETC.

697.--=Plane Tables.=--These instruments have been used for filling

in the greater number of topographical surveys in all countries. They

possess the merit that any intelligent, untrained person can be readily

brought to comprehend their manipulation in the work to be performed,

as angles of position of objects are taken directly by drawing lines

pointing to them from a point upon a sheet of paper stretched upon a

table. In new countries natural objects without very marked outline are

conveniently defined for position. The objection to this method, from

a point of view of the practical surveyor, is that the work which can

be done with equal facility in a comfortable office from the field-book

is with this instrument performed in the open air, under risk of rain,

dust, and other atmospheric discomforts affecting both the person and

the material on which he works. But for countries where the climate

can be depended upon, the facility with which surveyors with little

experience can map details for filling in superior triangulations made

with the theodolite, its use has gained much favour. Natives can

be easily taught to use it, and the check on their work through the

previous triangulation is perfect. The subject of plane tables will in

these pages be considered only in its general aspect, with the examples

of a few good instruments, referring the reader who cares to follow

the subject further to an excellent paper by Mr. J. Pierce, Jun., read

before the Institute of Civil Engineers, February, 1888.[51]

698.--_The Plane Table_ in its simplest form consists of a small

drawing-board mounted upon a firm tripod stand, and is shown at Fig.

313.

[Illustration: Fig. 313.--_Simple plane table._]

A rule termed an _alidade_, with sights placed at its ends, gives

the direction of any object from a given point on the sheet of paper

stretched upon the table, to which a fine line is drawn by an HH pencil

to point the direction. The alidade sometimes carries a trough compass

fixed upon it, but this is generally a separate instrument which is

placed against its _fiducial_ or ruling edge to give a magnetic north

to south line, to which all other lines are assumed to take angular

direction. A loose spirit level is also provided, by means of which

the board may be set level by shifting the legs of the tripod.

699.--=Plane Table with Telescope.=--Where greater refinement of

observation is required than is possible with sights, a telescope is

mounted on the alidade, which moves in the vertical plane upon an axis,

so that it may be directed in a linear direction with the fiducial edge

of the rule to any point in azimuth. The telescope sometimes carries a

level, so that the table may be set level by means of the alidade.

[Illustration: Fig. 314.--_Plane table._]

[Illustration: Fig. 315.--_Tripod stand._]

700.--A class of plane table which meets all necessary refinement

for ordinary filling in of field work is shown in the illustration,

Fig. 314. This nearly resembles those made by the author for filling

in details of the great trigonometrical survey of India. The drawing

surface of this table consists of a loose panel which stretches the

sheet of paper by pressing it into its frame, where it is afterwards

held by a pair of ledges which fit at their ends into long slots. The

panel of the board, shown in detail Fig. 316, is mounted upon a firmly

braced tripod stand. The head of the tripod stand, shown Fig. 315, is

secured to the board with a central screw (not shown) which permits the

board to be set in any direction, it being the rule that the edge _W_

should always take a north to south direction. Three screws _sss_ at

the corners of the triangular head can be raised or lowered by milled

heads from the under side. These screws permit about 15° of adjustment

to the table without any unsteadiness, as the centre screw clamps it

finally hard down upon them when all adjustments are made. A small

trough form of magnetic compass a is placed upon the rule to strike

the magnetic north to south line, to which all angles are referred in

transposing the work of the plane table. The diaphragm of the telescope

is provided with a platino-iridium point fixed vertically at the

mutual focus of the object-glass and the eye-piece. A pair of points

to subtend an angle to measure a staff for distance, Fig. 319, is a

convenient addition.

[Illustration: Fig. 316.--_Panel board of plane table._]

701.--_The Telescopic Arrangement_ of the alidade is varied in

different countries. In some cases it is placed near to one end, which

is perhaps better than in the centre of the rule, as it is more easily

read. In the modern French military alidade a prismatic eye-piece is

used, so that observation is made by looking directly down upon the

eye-piece of the telescope. In the Prussian alidade adjustment is made

to the standard of the telescope so as to bring the horizontal axis

upon which it moves level, that the telescope may move in azimuth,

however irregular or uneven the surface of the paper on the board may

be. This is necessary for any great degree of refinement in the plane

table, as the surface of a piece of wood upon which the paper is

stretched will be almost certain to warp if exposed to all weathers,

and this, added to the small width of the alidade, can scarcely retain

the axis in exact horizontality, placed as it is high above the

surface of the table. Some plane tables made by the author for General

Robinson for Indian service were of papier-maché to remedy the defect

of warping, but even this material warps upon exposure. Plane tables

have been made in Germany of metal and of glass, but in this case the

weight is a great objection. The author has found surfaced slate very

good, but it has the same objection of too much weight for a portable

instrument.

[Illustration: Fig. 317.--_Stanley's plane table._]

[Illustration: Fig. 318.--_Alidade to plane table._]

702.--_Lateral Adjustment to the Alidade._--The author's plan of

obtaining this is to increase the practical width of the rule by giving

it an extended point of support on one side so as to set the telescope

in azimuth. For this construction the telescope is mounted upon a

plate with an arm extending outwards upon the back of the rule. This

has a milled-headed screw placed at the near extremity of the arm. The

screw is inserted in a deep bush for wear; this attachment is shown in

section Fig. 318. The adjusting screw _A′_ has a collar fixed upon its

point which is centred upon a tight screw tapped into the milled head.

This collar, as it does not turn with the milled head, does not abrade

the surface of the paper by contact with it. A small cross level _B_

is put upon the arm between the milled head and the standard of the

telescope. The under side of the rule is cut away or placed obliquely

to the surface, so that it bears on the outer ruling edge only. The

milled-headed screw being at its normal position and the table level,

less than half a turn one way or the other will bring the small cross

bubble to its centre in a few seconds for any average irregularity of

the surface of the table, and by this means cause the telescope to move

correctly in azimuth.

[Illustration: Fig. 319.--_Subtense points._]

703.--_The Telescope Arranged for Subtense Measurements._--Where a

stadium, art. 556, is to be used for estimating distances from station

to station, when an ordinary telescope is used, the author places two

platino-iridium points vertically from top and bottom of the diaphragm,

and adjusts these by a screw until a subtense angle upon the stadium of

1 foot cuts the point at a distance of 100 feet, or according to the

measurements to which the land is taken. In this case it is necessary

to have an altitude arc to the telescope, as shown upon the alidade in

Fig. 317. This has a degree scale reading by vernier to about 3 minutes.

704.--_Various Devices for Fixing the paper on the Surface of the

Table_ have been made. Many prefer simply pinning it with drawing

pins on a quite plain pinewood surface. In this case the table is

better slightly sunk round the edges with a rabbet of the depth of the

thickness of the head of the pin, so that the alidade may rest firmly

even over the pin heads. The French plane tables have very generally

rollers at each end of the table, upon which a long slip of paper is

rolled, sufficient for twelve or more stations. The rollers for small

tables are made of brass tube about 5/8 inch in diameter. They commonly

move with a turn-key which is inserted in a square fitting in the end

of the roller. The rollers keep the paper tight by means of ratchet

wheels and spring pawls at their ends. This plan is very convenient

for topographical work, as for instance a river may be followed from

station to station right down its course and appear on a single slip,

its bearing being indicated by the compass north line. Fig. 320 shows

the manner in which the author has made this plane table.

[Illustration: Fig. 320.--_Plane table with rollers._]

[Illustration: Fig. 321.--_Gurley's plane table adjustment._]

705.--_Adjustment of the Plane Table._--There are a great many devices

for this. Mr. Pierce, in the admirable paper already mentioned, gives

illustrations of the different plans. Some of these have all the

complication of the adjustment of the stage of a theodolite, and one

has superadded to this a slide-rest motion. These things of course

are necessary if the field work is made to take the place of finished

office work. One general feature of plane table tripods is some means

of adjustment of the table to uneven ground, when the tripod-head

cannot be brought nearly level. Gurley's plane table adjustment is

perhaps the simplest of any of these devices, and appears to the author

to be as good as any other. Fig. 321, _D_ the table top; _A_ a ball

fitting turned inside and out, and attached firmly to the table top;

_C_ a socket fixed firmly in the head of the tripod; _B_ a bolt with

globular head fitting the interior of _A_, and carried through the

head to a winged nut which clamps it firmly. A spring is placed to act

against the winged nut, so that when this is slightly loosened the ball

fitting A may move between _B_ and _C_ with moderate firmness when the

table is being set to an angle.

[Illustration: Fig. 322.--_Stanley's high-class plane table._]

To meet all conditions the revisor has designed a high-class plane

table shown at Fig. 322. This has a ball and socket rough levelling

arrangement and parallel screws for fine levelling, circular motion to

table with clamp and tangent, spring rollers for taking any length of

paper and instantly clamping it, alidade with extra powerful telescope

with vertical circle divided on silver reading by two verniers to

minutes with clamp and tangent motion, cross levels, diagonal scale,

and adjustment for setting telescope to revolve in vertical plane,

circular compass with cross levels and plumbing bar. The board is

generally made 30″ × 24″. The telescope is stadia reading and is made

with long sensitive bubble mounted upon it or upon the verniers if

preferred as shown at Fig. 323.

[Illustration: Fig. 323.--_Stanley's high-class alidade with bubble on

verniers._]

706.--_Method of Using the Plane Table._--The table is first set and

levelled up at a commanding position to observe the extent of country

it is intended to plot from observation from a single station. Let

Fig. 324 1 be the first station for plotting the enclosure _abcdef_.

Draw lines by the alidade pointing to these angles represented by the

letters from a point near 1. Set up a picket or stadium at the station

2 where it is intended that the plane table shall next be set up, and

draw the line 1 2 distinctly on the paper. Measure the line 1 2 either

by its subtense on the stadium or by direct chain measurement, and

plot this from station 1 on the paper according to the scale to be

worked to in making the plan. On removing the table set up a picket or

distinct land mark vertical with the position of station 1 occupied on

the paper. Move the table to station 2 at the measured distance and set

the direction of the board by means of the alidade so that the line

2 1 cuts the picket left at station 1. Now draw lines from station 2

to all the points _abcde_, cutting the former lines as represented by

dotted lines in the figure, and the intersections of these lines will

give the true positions of _abcde_, according to the scale selected for

the base 1 2, and these may be tied up to represent the boundaries, as

shown on the plane table 2. It will be readily seen that the line 1 2

represents a bearing in azimuth; so that if the edge of the table be

set, say truly N. to S., in both positions the line on the paper 1 2

will agree in both these positions of the table; but the check by the

alidade of this line is valuable to save risk of error.

[Illustration: Fig. 324.--_Diagram of plane table work._]

707.--Where an extent of land is to be surveyed by the plane table,

longitudinal bands of a mile or so in width are taken. Where the roller

plane table with continuous paper, Fig. 320, is used, the forward

points of observation are lined in and the backward ones simply tied

up, being certain by observations written in pencil upon the paper that

identical objects are tied up from the positions of both stations.

Where an object cannot be seen from both stations its position may be

indicated by the stadium from a single bearing, or it may possibly be

tied up from a further advanced station.

708.--=Edgeworth's Stadiometer.=[52]--The general construction of

this instrument is given in the inventor's specification of patent,

from which the engraving, Fig. 325 is taken. The vernier plate of

an ordinary theodolite is extended to a plate of about 10 inches

in diameter. This is adjusted to level by means of parallel plate

screws. The plate or plane table is divided on its edge ¼°. The part

representing the limb of a theodolite is carried out from its axis

by two arms only: upon these the standards _RR_ of the telescope are

mounted. These standards leave a striding space near the plate, into

which any scale S of equal parts with a zero centre is introduced,

which is intended to be used for the plotting, the striding space being

so arranged that the fiducial edge of the scale shall pass exactly over

the axis of the instrument. The standards unite in the same casting to

form the horizontal axis bearing of the telescope. This axis permits

the telescope to move in azimuth. The telescope carries a vertical

arc divided to degrees, also a scale of centesimal differences of

hypotenuse and base, with the ordinary clamp and tangent adjustment of

a theodolite. It is also fitted with a level above it which is used in

setting up the instrument. Stadia webs are placed in the diaphragm and

are made adjustable to subtend upon the stadium a percentage of arc

agreeing with the unit to which the land is measured. The inventor does

not appear to have known the optical error of the system proposed for

measuring distance, art. 558. Neither does this appear to have been

recognised by others writing upon the instrument, who have generally

followed the late J. F. Heather's description.[53]

[Illustration: Fig. 325.--_Edgeworth's stadiometer._]

709.--_To Use Edgeworth's Stadiometer._--After it is set up, a circular

disc of paper of about an inch less diameter than the table is

held down upon it by four spring clips. The telescope is directed

consecutively from object to object, the positions of which it is

desired to take. It is clamped by the screw below the plate during the

observation. The stadium is placed against the object and the distance

taken by the subtense of the webs in the diaphragm, which may be exact

if a constant be added after proper adjustment, art. 558. If the

stadium be above or below the horizontal plane it is inclined by means

of a sight-hole through it, as originally proposed by Green, so that

the subtense is equal under all conditions. The horizontal distance

is taken by the difference of hypotenuse and base, as shown on the

vertical arc, so that the record of a complete observation appears for

calculation as--

_stadium reading_ + _constant_ - _altitude correction_.

This distance is at once set off from the centre of the instrument by

the scale on a line drawn upon the disc of paper, and observations are

written against the line. In making a number of observations from one

station two or more discs of paper may be employed to save confusion

of lines and interference of descriptions. These papers are separately

used in plotting as protractors by pricking holes through the stations

defined in the field from the centre of the disc which represents the

station of observation.

710.--=The Sandhurst Protractor=, Fig. 326, is a military protractor

adapted especially for topographical delineation, which is commonly

used with the plane table. It is different from many instruments of

its kind in having only useful matter upon it. It is made of boxwood,

upon which the protractor is cut, and has also one scale of 6 inches

to a mile in yards, at the lower edge, the tens of which are carried

across to make parallels of 90° in the manner of an ordinary military

protractor. Over the back of the protractor is a scale which gives

a standard for shading slopes of land upon topographical maps, Fig.

327, from 2° to 35°, also lines for contour shades. A small plummet,

the cord of which is passed through a hole in the centre, from which

the degrees are protracted, is supplied with the instrument. When the

protractor is held up, degrees downwards, the cord of the plummet will

pass over the degrees and indicate the angle at which it is held. By

looking over the edge the angle of inclination of the land may be taken

directly, as with a clinometer, or by looking along the edge by a

second person reading the plummet the angles of altitude may be taken

more exactly.

[Illustration: Fig. 326.--_Sandhurst Sketching clinometer protractor._]

[Illustration: Fig. 327.--_Example of scale of shades for slopes._]

711.--=Military Sketching Board.=--This sketching board, Fig. 328,

the invention of Mr. Graham F. Hodgson, will be found a very great

improvement upon the old pattern boards. It is designed to meet the

requirements of military and other officers not conversant with the

higher branches of surveying, and it will also be found of great use

to surveyors, explorers, and other travellers.

It consists of a mahogany board revolving on a circular metal plate

attached to a handle held in the operator's left hand.

Mounted on the metal plate is a compass, which is visible through a

glass plate flush with the face of the board and the tracing paper or

cloth on which the map is made; the paper being stretched tightly over

the board by means of rollers.

A magnetic north line is drawn on the paper over the centre of the

compass. The operator invariably has his board in its relatively

correct position by always keeping the magnetic north line immediately

over the top of and aligned with the compass needle when his sights are

taken.

[Illustration: Fig. 328. Top view. Under view.]

The sights are taken by means of an alidade moving along a slotted bar,

which itself slides along a bar fixed at the side of the board.

The object aimed at is sighted in a mirror attached to the end of the

alidade and aligned with a point at its other end, and the alidade is

clamped into position by a thumb-screw on the slotted bar.

The advantages of this instrument are many and will only be fully

realised when it is in practical use. No backsights are necessary.

Sights can be taken with one hand. The operator is always in a

comfortable position and the object aimed at is always immediately in

front of him. The alidade remains in position by means of the clamp

screw along the ray drawn till the object sighted at is reached and the

distance known, which is merely marked off by means of the scale on the

alidade. It is invaluable when sketching rivers from launches or canoes

when backsights are often impossible. It is light and portable, being

easily carried slung over the shoulder in a canvas case.

712.--The method of using is simple and ensures a great degree of

accuracy with a minimum amount of time and trouble:--

(1) The tracing cloth is first fixed by means of the rollers over

the board. (2) A magnetic north line is then ruled across the paper

and passing immediately over the centre of the compass visible under

the paper. (3) The operator then, holding the board by the handle

underneath, proceeds to make his map and first brings the magnetic

north line immediately over the top of and aligned with the compass

needle. (4) He then from some point, marked as his starting point

on the paper, proceeds to take sights to any objects he may wish to

delineate on his map. These sights are taken by means of the alidade

fixed above the board. The sighting rule is pushed along a slotted

bar, which itself slides along a bar at the edge of a board until the

edge of the alidade is against the starting point and is sighted on

the object aimed at. The object is sighted in a small mirror fixed

at the end of the alidade and aligned with the point at its other

end. The alidade is then clamped into position with the thumb-screw

on the slotted bar and the operator draws his ray corresponding with

the direction of the object aimed at. The distance is then marked off

on the scale on the alidade. (5) When the operator moves to the next

station no back-sight is necessary. He at once puts the board into its

relative correct position by merely revolving it until the magnetic

north line is again lying over the top of and aligned with the compass

needle and he then proceeds to take all necessary sights at that

station.

[Illustration: Fig. 329.--_Cavalry sketching case._]

713.--=Cavalry Sketching Case.=--This forms a very convenient exploring

sketching board, permitting sketches to be made on horseback while en

route. The pattern shown in Fig. 329 is that of Captain W. Vernier. It

consists of a small board 9¼ inches by 7½ inches, at two sides

of which there are small rollers to hold paper 7 inches wide and from

3 feet to 6 feet in length, according to its thickness. Two stout

indiarubber bands, which hold a small straight-edge to scale in any

position on the paper with sufficient firmness to be able to draw a

line against it, are passed over the board. A small compass on one side

of the board indicates direction. After one sketch is made, a new part

of the paper is rolled forward.

714.--=Camera Lucida--Optical Compass.=--In new countries where

landmarks are not clear a sketch of the general aspect of the country

will make the points of triangulation more clear. Where the plane table

is not used these sketches may be made with accuracy as to positions

by the use of the photograph camera, the camera lucida, or points of

observation may be taken in correct bearing by the optical compasses.

These latter instruments are described in the author's _Treatise on

Drawing Instruments_, seventh edition.

FOOTNOTES:

[51] _Proc. Inst. Civil Engineers_, vol. xciii. part iii. paper No.

2308. See also _Military Surveying in the Field_, by Major The Hon. M.

G. Talbot, Prof.; _Papers Royal Engineers_, vol. xiv. p. 25.

[52] Patent No. 1202, D. R. Edgeworth, April 1866.

[53] Heather's _Surveying Instruments_, 1870, p. 85.

CHAPTER XVI.

INSTRUMENTS FOR MEASURING LAND AND CIVIL WORKS

DIRECTLY--CHAINS--VARIOUS TELLERS--STANDARD CHAINS--ARROWS--DROP

ARROW--VICE FOR ADJUSTING CHAIN--CAINK'S RULE FOR INCLINES--STEEL

BANDS--WIRE LAND MEASURES--COMPENSATION SYSTEMS--LINEN TAPES--OFFSET

RODS--PINE STANDARD RODS--RODS WITH IRON CORE--BEAM COMPASS RODS--

COINCIDENT MEASUREMENTS--COMPENSATED RODS--BASE LINE APPARATUS--COAST

SURVEY LINES--PERAMBULATOR--PEDOMETER--PASSOMETER--SOUNDING CHAINS--

SOUNDING LINES--TELEMETERS--HAND RODS--RULES.

715.--_The Instruments Generally Employed for Measuring Land_ are

chains, steel bands, and tapes. Where roads are roughly measured,

pedometers are commonly used. Where very exact measurements are

required, rods have been used. Rough approximate measurements are

obtained by stepping, with the use of the passometer to count the steps.

716.--=Land Chains.=--Although these are made in many qualities

the forms vary very little. They are too well known to need much

description. In the British Isles and some of our colonies the chain

of 100 links, equal to 66 feet, the invention of Edmund Gunter about

1620, is generally used, 10 square chains (100,000 square links) giving

the statute acre, presenting a decimal system of measurement much in

advance of any other at the present time. The best land chains are

made of steel, which is afterwards hardened and tempered to spring

temper, in the process of which the surface is burnt off with asphalt

varnish in order to produce a covering to resist the rusting effects of

moisture. Steel chains are made _light_ and _strong_. The light chain,

of No. 12 Birmingham wire gauge, weighs under 5 lbs. The strong chain,

of No. 8 B.W.G., weighs about 12 lbs. A light chain of 50 links, of

weight under 3 lbs., is sometimes used with the complete chain of 100

links for taking offsets.

[Illustration: Fig. 330.--_Land chain and arrows._]

All the best chains, whether of steel or iron, are made with long links

formed by turning up the ends of a length of wire. Three small oval

links are placed between each pair of long links. These three interval

links are found to cause the chain to kink less than when only two are

used. Each oval link is sawn through at the meeting line, which is

brought up on one flat side of the oval in bending it from the wire.

The saw-cut forms the point of adjustment. The small link is afterwards

re-sawn and closed to shorten it, or forced open to lengthen it. There

are generally four swivels in the length of the chain, two of which

are at the handles: these prevent the chain from becoming twisted in

turning the handles over in use. A swivel is shown Fig. 331 at S. Iron

chains are sometimes galvanized to prevent rust. This process, however,

makes the chain much more brittle, and cannot be recommended. It may be

noted that all link chains lengthen with use.

717.--=Tellers= are small pieces of brass suspended to the chain

by a spare link placed at every ten links. They divide the chain

decimally from either end equally. Proceeding from one end of the chain

the tellers read 10, 20, 30, 40, 50, and the other end they read by

subtraction from the complete chain: 100 - 10 = 90, 100 - 20 = 80,

100 - 30 = 70, and 100 - 40 = 60. Fig. 331 shows detached pieces of

chain with value of the tellers figured under. _S_ inserted swivel.

The 50 teller shows the link attachment. _A_ shows the position at

which the arrow or other mark is placed to commence or finish the chain

measurement, the handle being included in the first link. These tellers

are liable to catch and get dragged off in chaining. When this chain

is used abroad, or far from home, it is well to have an extra set of

tellers to repair losses.

[Illustration: Fig. 331.--_Gunter's land chains._]

718.--_Inserted Tellers._--This form of teller is preferred by many,

Fig. 332. It is much less liable to get dragged off, but it is not

considered quite so distinct, and it is a little liable to get clogged

with grass and weeds.

[Illustration: Fig. 332.--_Inserted tellers._]

719.--The author's design for inserted tellers is shown Fig. 333.

These are perhaps quite as distinct as the last. The holes in wet

weather fill up with mud and the surfaces keep bright, so that they

remain very readable. There is much less drag, and the chain therefore

wears longer.

[Illustration: Fig. 333.--_Stanley's inserted tellers._]

720.--=Feet Chains= are usually made 100 feet, more rarely 50 feet.

They are generally made in foot lengths, but sometimes for flexibility

are preferred in 6-inch lengths. They are commonly made of No. 8 B.W.G.

steel or iron. The weight of 50 feet is 6 lbs.; 100 feet, 11 lbs. If

made of light steel, No. 12 B.W.G., the 100 feet weighs 6 lbs.

721.--=Mining Chains= used in mineral districts are made generally 10

fathoms, or 60 feet, 6-inch links counted off by tellers in fathoms.

They are made entirely of brass. The weight is about the same in

brass as steel--No. 8 B.W.G., 9 lbs. Occasionally they are made extra

strong, No. 7 B.W.G.; weight 12 lbs. In coal mines Gunter's chains are

generally used.

722.--=Metre Chains= are made 20 or 25 metres long. They are marked

with tellers at every two metres with a plain ring at the metre.

The tellers are generally of the inserted kind, Fig. 332. In taking

measurements the sign of the teller is doubled: thus the ordinary 1 or

10 is counted 2 metres; the 2, 4, and so on. 20-metre chains in light

steel, No. 12 B.W.G., weigh 4½ lbs.; strong, in No. 8 B.W.G., 9 lbs.

25-metre, light, 6 lbs.; strong, 11 lbs.

A land chain is generally secured for carrying by a leather strap with

a buckle. Occasionally it is carried in a sailcloth bag with a strap

over the shoulder.

723.--=Standard Chains.=--These are of the same form as the ordinary

steel chain, but all the links are hard soldered after being adjusted

link by link. They are not intended to be used for regular chaining,

except it be for laying down rough base lines. Their special employment

is to test chains, or to set out with two pegs on a straight piece of

ground a standard length or station where the common chains in use may

be tested daily. A standard chain is commonly enclosed in a box with a

lock to prevent its accidental use for an ordinary chain.

724.--=Arrows.=--These are sometimes called _pins_. Ten form a set.

They are shown with the chain in Fig. 330, and are commonly made

of the same wire as the chain--No. 8 B.W.G. They are much better

made one gauge stouter (equal to about 1/7 inch), and preferably of

hardened steel than of iron. The common length is 15 inches. Where

heath, stubble, or woodlands prevail 18-inch are better for use, and

in some exceptional cases even 2-feet are very convenient. Surveyors

going to new countries are recommended to take the longer arrows as

well as those supplied with the chain. It is common either to tie a

short length of scarlet webbing upon each ring of the arrow or to sew

a piece of red flannel or bunting upon it to find it easily in long

grass. Arrows are sometimes carried in a quiver with a strap over the

shoulder, Fig. 334, which leaves the hands of the fore chainman free to

remove obstructions where they occur.

725.--=Drop Arrow=, Fig. 335. Where ground is very hilly it is common

to roughly level the chain by holding the lower position shoulder

high, either by guess work or by using any kind of rough hand level

or clinometer to ascertain this. The arrow is then dropped, and the

point, held at first lightly in the ground, is pressed hard down or

another arrow supplanted for it. The chain in this case is used in odd

multiples of links as they occur, of which record is taken separately

at each station. In going downhill a drop arrow answers very well. In

going uphill a plummet to the last arrow is better. Some use the drop

arrow as a plummet, carrying for this purpose in the pocket a piece

of fine whipcord, with a bent hook tied to one end, to be used when

required.

[Illustration: Fig. 334.--_Quiver with arrows._]

[Illustration: Fig. 335.--_Drop arrow._]

726.--_Examination and Adjustment of Chains._--Respectable makers

send out chains tested to within half of one of the small links of

standard, that is, within a quarter of an inch; but in use this error

may increase either by the bending of the long links of the chain, when

it becomes shorter, or in the more general case of friction from wear

and from strain, by which it becomes longer. In London, standards are

fixed upon the pavement in Trafalgar Square and at the Guildhall. These

standards are also fixed at many municipal town halls. Surveyors very

commonly lay down a standard on the pavement, or by pegs on a level

gravel path. Where a peg is used it should be driven home nearly to the

surface. It should if possible be made of a piece of heart of oak 12

inches long and about 2½ inches square. The standard length, which

may be set off by a standard chain or new steel tape, should be from a

saw-cut across the centre of one peg to a similar cut on the other. It

is well also to have the centre space (50 links) indicated by a smaller

peg.

727.--_The Chain to be Adjusted_ should be first examined and its long

links set straight by means of a hammer on a flat, hard stone or anvil,

after which the error will be, if it has been much used, that it is

too long. It should be then laid in direct line on the standard just

described, and stretched lightly with a pull of about 7 lbs., and then

left to rest. Assuming it too long, the centre of the chain should be

observed to ascertain which half is of the greater length, then short

links should be taken out at distributed distances, if more than one be

required, by twisting the link open in a vice, and opening and closing

another link to restore the chain.

[Illustration: Fig. 336.--_Stanley's vice for adjusting and repairing

land chains._]

728.--=Chain Vice.=--The links of steel chains can seldom be twisted

open without breaking, and broken links cannot be restored by steel

links. Iron links answer, but they are very stiff to twist open.

Generally it will be found best for professional men to repair the

chain with spare _brass links_. These wear very well. Where a smith is

near with his vice and a light hammer the links are readily opened. It

often occurs in open districts and abroad that no smith's shop is to be

found. To meet these cases the author has constructed a special vice,

as shown Fig. 336. This vice is let into a piece of hard wood--an old

oak post answers admirably. In stone districts it is perhaps better to

let it into a stone and fix it by pouring hot lead round it. The part

_B_ is used for an anvil for straightening the links. The vice _V_

holds the link edgewise very firmly by bringing up the slide _J_ by

means of the screw _S_. The link may then be knocked open by the pane

end of a light hammer. The link is closed again in the same manner. If

the vice be left out of doors the screw should be well greased and the

whole covered with a leaden cover. The weight of the vice is about 6

lbs. It is made of cast iron with chilled face, or the jaws are faced

with steel.

729.--_Opening and Closing the Chain for Use._--The chain is most

readily unfolded by taking the two handles in the hand and walking

away from it as it lies on the ground. It is most convenient to place

it about 45°, and half a chain length from the first station, each

chainman taking a handle and moving to his position. The only danger in

undoing a chain is from two chainmen taking one handle each and walking

in opposite directions, in which case, if there happens to form a kink,

the opposite movement of the two men will probably stretch or break the

chain. In closing the chain it is taken by the middle links and folded

up two links at a time till the handles are reached. If the links be

placed consecutively in position round the axis formed by the first

links, it may be folded up very compactly in a twisted form ready for

the strap, by which it is carried, to be passed round it.

730.--_Chaining_ is performed by two chainmen, termed the _leader and

follower_. The follower, having pressed a stake into the ground for a

starting point, then places the centre of the outside of the handle of

the chain against it. The leader takes ten arrows in his right hand

and one handle of the chain in his left, and walks directly towards a

point which is to be the termination of the measurement, stopping at

nearly the length of the chain, examining the chain to see that it is

straight. He then places an arrow lightly outside the centre of his

handle. The follower looks over this arrow to the distant station to

see whether it is in direct line. If it be not so, he waves his right

or left hand once, twice, or thrice for 1, 2, or 3 inches for movement

to right or left. The follower picks up the arrows consecutively as

left by the leader, and when he has the ten, 10 chains have been

measured, which is then recorded in the field-book, or earlier than

the ten if a shorter distance or object completes the measurement. It

is most important to observe that if an arrow be taken for the first

station, _the follower having ten counts nine only for the first ten_.

To prevent accident it is therefore safer to start from a stake or

other landmark, _not one of the arrows_. Some surveyors advise eleven

arrows. If eleven be used, one should be distinctly marked from the

rest so as never to be counted. This may be done by omitting the red

webbing tie, or using a green tie for the odd arrow. The French always

make the drop arrow the eleventh arrow, which is never counted in

direct chaining.

[Illustration: Fig. 337.--_Caink's rule for correcting inclines._]

731.--=Caink's Rule= for correcting inclines in chaining is the

invention of Mr. Thos. Caink, C.E., of Malvern, Fig. 337. It is made

four-fold, each fold being one link. The link is divided decimally

along the inside of the rule. On the outer edge of the rule there is

a scale marked degrees, a part of which is subdivided where the scale

is open to read closer, that is, to 20 or 30 minutes. These degree

divisions, which read up to 16° on one side of the rule, indicate the

space from the end of the rule to be allowed in addition for the same

degrees of inclination of the land up to 4 links of measurement. On the

opposite side of the rule the inclination scale is carried from 16°

to 22° 10′. For these higher numbers the length of the rule is first

set off, and then plus such part of the rule as is indicated by the

position marked upon it of the required number of degrees.

732.--_To Use Caink's Rule._--The follower has a clinometer of one of

the kinds shown, Figs. 260 or 264. He notes at starting the position

upon the face or body of the leader that corresponds with the height of

his own eye. He takes the inclination of the land to this point of the

leader's body while he is standing upright at one end of the chain and

the leader standing at the other, noting the number of degrees shown by

the clinometer. He then places the rule in the direction of the chain,

with the number of degrees indicated, in front of the arrow, and moves

the handle of the chain to this position. For the sake of verification,

if he has a second arrow he may place it in the new position, which

gives the true allowance. In either case the leader moves the chain

forward by the amount required and places his arrow ready to continue

the work. By this method it is seen that there is no after calculation

or separate record necessary for undulating land, but the true

horizontal position is given correctly at each chain measured. The same

form of rule is made for feet and metres.

733.--In mountainous countries the eight links of the rule is

insufficient allowance for common inclinations. Such countries are

measured much more accurately by some system of subtense measurement,

for which see Chapter XII.; but where a small piece of sudden steep

inclination occurs half a chain may be taken, and the number of degrees

indicated upon the rule be doubled, so that the full rule, instead of

taking 22° only, will take 44°.

734.--=Steel Bands= for measuring, termed _steel band chains_, are made

in various forms in this country, and sold by nearly all opticians.

They are much lighter than chains of equal strength, and are made of

standard length. They are also lighter to use, being smooth and without

any projection. On the other hand the reading is less distinct than

with the chain, and they need more careful usage in chaining. They also

require oiling before being put by. From the thinness of the metal they

are altogether more delicate and less durable than the chains for hard

wear; but it is thought by many to be a compensation that they are

always of true length.

[Illustration: Figs. 338, 339, 340.--_Steel bands and tapes._]

[Illustration: Fig. 341.--_One link of steel band._]

735.--The bands commonly used for land measuring are made 3/8, ½,

5/8, and ¾ inch wide, of Nos. 26 and 24 B.W.G. in thickness,

respectively. The chain is divided into links by a small stud riveted

through the centre of two small washers, a large stud being placed at

the fives and an oval plate held by two rivets at the tens, which are

numerically indicated in plain engraved figures, as shown in detail,

Fig. 341 _b_, or perforated with holes indicating the number of tens.

These band chains are made in links, feet, metres, or to any foreign

measure to order, and of any length corresponding with land chains.

Weights, approximately--100 feet: ¾ inch, 7 lbs.; 5/8 inch, 4¾

lbs.; ½ inch, 4 lbs. 100 links: ¾ inch, 4¾ lbs.; 5/8 inch, 2¾ lbs.;

½ inch, 2¼ lbs. 20 metres: ¾ inch, 5 lbs.; 5/8 inch, 4 lbs.

736.--Steel band measures are also made with divisions throughout,

etched upon them with acid in such a manner that the divisions and

figures stand in relief up to the original surface, whereas the new

surface, which is etched back to form the ground, appears dull. The

brightness of the figures and divisions on the dull ground makes them

easily read. These bands are divided into links, feet and inches,

metres and decimeters, or closer quantities either on one or both sides

of the band as required. With the etched band there is perhaps a little

risk of weak places from over-etching, although these bands are most

carefully made, but perhaps this is not greater than in the inserted

stud band, where weak places are necessarily caused by the loss of

width at the points where the holes are made for the studs, wherein

moisture hides after use in damp weather.

737.--The steel bands have handles the same as a land chain. They

are wound upon a steel cross, Fig. 340. They are commonly placed in

a wind-up case similar to that of an ordinary measuring tape, but in

steel, provision being made that one of the pair of handles may be

secured about the position of the axis of the tape for winding it up.

In Fig. 338 the axis is made very large, so that the handle may be

pressed in from an opening in one side of it. The newest idea is to

cut a slit in one side of the plate up to the centre, as shown, Fig.

339. In this case the handle and band are put in from the side, so that

the axis is no larger than is necessary to take the handle. A strap is

placed on the side of the case for holding it. This is shown cut off to

admit sight of the handle.

738.--The French make the handle generally T-shaped and hollow in

the cross part, which renders it very light and perhaps less cramping

to hold. The arrows are very commonly held by loops to the cross on

which the band is wound. This general arrangement is very portable and

convenient to carry; it is shown Fig. 342.

739.--=Wire Land Measures.=--Where long open stretches of new country

are to be measured, it is common to employ a steel wire chain, of 5

chains or of 500 feet in length, fitted with a pair of strong cross

handles only.

[Illustration: Fig. 342.--_French land measure._]

[Illustration: Figs. 343, 344.--_Marchant's 500-feet band._]

740.--The author has made many chains of 500 links; in Fig. 344 a part

of one is shown full size. This _band_, as we may term it, is wound

upon a reel in an iron case, Fig. 343. A spring brake is placed at the

position _A_, which holds the reel and prevents the band from springing

out into loose hoops when it is run out. The 50 and 100 links are

indicated by short lengths of brass tube placed over the band--single

at the 50 links, but numerically indicated by number of bands as 2, 3,

and 4 chains. In Fig. 344 a 50 and a 300 links are shown; weight, 3½

lbs. This flat, narrow, steel band chain was unknown until introduced

to the notice of the profession in the first edition, 1890. It is now

in very general use, and lengths may be had from stock of 2, 3, 4, or

5 chains, or 200, 300, or 400 feet wound upon a steel cross.

[Illustration: Fig. 345.--_Richmond's tension handle._]

741.--=Richmond's Tension Handle.=--Various devices have been employed

for giving equal tension to chains and bands to ensure equality of

measurements. Salter's spring balance has been very commonly used

attached to one handle of the chain to give a uniform pull, say of

15 lbs. This appears to answer very well. Mr. Richmond, surveyor, of

Sydney, has devised a very simple plan for tension of light bands,

which, being lighter and attached, is much more convenient than

Salter's balance. This is shown Fig. 345. The band passes through a

fitting in the centre of the handle, and a spiral spring is fixed to

this and the band at a short distance along it. By pulling the handle

a given tension can be applied, which is shown by the mark it reaches

towards the end of the band. This is adjusted to standard length, and

a small notch is placed in the centre of the end, from which a plummet

may be suspended if necessary.[54]

The engraving is of a slightly modified form by the author, in which a

thin tubular cap covers the free end of the band to save this exposed

part from accidents.

[Illustration: Fig. 346.--_Copper case thermometer for suspending to a

band chain._]

742.--=Chain and Band Thermometer.=--Where very great accuracy of chain

or band measurement is aimed at, temperature is taken to allow for

expansion of the metal. A thin plain glass thermometer of the

_clinical_ form is the most sensitive of any. This is carried in a

wooden pull-off case lined with indiarubber. When it is used it is

placed upon the ground by the side of the chain. The delicacy of the

clinical form of thermometer is often objected to by the practical

surveyor, hence there are several other forms with boxwood and ivory

scales. These are not very satisfactory, as the boxwood and ivory

retain the heat of the body, from being carried in the pocket, for a

long time after exposure. The author has enclosed the clinical form of

thermometer in a copper case with open face, Fig. 346. The copper being

a good conductor of heat, this is very sensitive to the temperature

of the air. Two turn-down hooks are placed at the ends of the tube to

suspend it on the band. The thermometer stem has two indiarubber caps,

so that it will bear dropping on grass. It is contained in the same

form of pull-off case as the clinical.

[Illustration: Fig. 347.--_Littlejohn's temperature handle._]

743.--The coefficient of expansion for steel between 32° and 212° Fahr.

is about ·000012, which is less than ·01 inch per degree per chain.

Temperature corrections can therefore be recognised only upon very

exact work, appreciable only when long bands of the Marchant type,

lately described, of from 5 chains to 10 chains in length are used.

744.--Mr. Littlejohn has patented an adjustable handle for temperature.

This is divided for allowance for the 100-feet or other band for

every degree Fahr. or centigrade. Fig. 347. The handle is set to the

temperature as it changes during the day. It offers, perhaps, the

highest refinement in ordinary land measurements.

[Illustration: Fig. 348.--_Stanley's repairing sleeve._]

745.--=Repairing Sleeve for Steel Bands.=--The reviser has patented a

sleeve which will be found useful, as by its use a broken band can be

immediately and permanently repaired in the field without the use of

tools. They are made to fit all sized bands, but it is necessary that

the correct sized sleeve should be used. One of these sleeves is shown

attached to a band at Fig. 348.

In order to effect a repair it is merely necessary to clean the broken

ends of the band, and insert them into the sleeve, then hold a lighted

match under it until the soldering material is melted, when the repair

is completed.

The central hole in the sleeve is to enable the user to see when the

broken ends are in contact, and the other two are to indicate when the

soldering material is melted, which takes place when it either bubbles

up in or runs away from these holes.

[Illustration: Fig. 349.--_Linen Tape._]

[Illustration: Fig. 350.--_Small steel pocket tape._]

746.--=Linen Tapes.=--This most useful implement, Fig. 349, is one

of the most unsatisfactory measures the trader and user has to do

with. It consists, as is well known, of a tape oiled, painted, and

varnished, which is rolled up in a leather case when out of use. When

the weather is moist it shrinks, and when dry it expands. If it be too

heavily painted it becomes brittle and rotten; if it be lightly painted

it remains more flexible, but is more affected by moisture. A good

tape bears very well a stretching force of 7 lbs. to 14 lbs., but if

strained over this it is permanently stretched. There is no plan known

to the author by which these defects can be remedied. Numerous attempts

have been made--often valueless or worse--some, although popular, mere

claptraps, such as the insertion of wire. The best tapes for strength

and permanency are made entirely of green, hand-made, unbleached flax.

The tape is said to come from Holland to this country. These are at

first oiled with a drying oil (boiled linseed oil), and when seasoned

for a month or so, painted once or twice with white lead colour--not

too thickly. The printing is more permanent if done in oil; but the

tape is somewhat more flexible if the figures are stencilled in Indian

ink and the whole afterwards thinly varnished over with copal varnish.

The great secret for preserving the tape is to use it very carefully

and only in fine weather. In wet weather for taking offsets a light

steel 50-link chain is quite as convenient as the tape, and safer.

Tapes are divided into links, feet and inches, metres, and all measures

as required. A decimal yard is commonly placed on tapes for measuring

earth work. For use with the chain a 66-feet tape is usually employed,

but many think a 33-feet better, using the chain for dimensions above

this. For measuring buildings, 50-feet or 100-feet tapes subdivided to

inches are employed.

747.--=Steel Tapes.=--Thin steel tapes, 3/8, ½, and 5/8 inch wide

are in very extensive use. They are more accurate and more costly than

linen tapes, but less flexible and less durable. Where dimensions

are important they should always be used for short measurements. In

all cases it is advisable for a surveyor to keep a steel tape for

examination of the lengths of linen tapes in use. They are made to all

the measurements of linen tapes.

748.--=Pocket Steel Tapes= 6 feet to 12 feet, Fig. 350, are used more

generally by mechanical engineers. These tapes, which are very light,

are held open by a catch, and closed by a spring.

[Illustration: Fig. 351.--_Jointed offset rod, top and centre._]

749.--=Offset Rods= are generally made 10 links long, either in one

piece or jointed in the centre with a bayonet joint. They are about

1-1/8 inches in diameter, diminishing towards the top to 7/8 inch, and

made either of yellow pine or ash. A hook is commonly put at the top,

Fig. 351, which takes the handle of a chain to draw it through a hedge

or other obstruction. The author's plan of making this is shown at _H_.

The lower end of the offset is shod with a steel or wrought iron socket

point, so that it may be set up in the ground and used if required as

a picket. Bands are painted alternately black and white at every link.

Square or flat rods are occasionally used for the same purpose, but

they are not generally so convenient.

_The offset is Used_ in the manner of an ordinary rule to take

rectangular short measurements from the chain as it lies upon the

ground, commonly in order to obtain the contour of irregular outlines.

750.--=Measurement by Rods= has become less general than formerly,

from the greater accuracy of Konstat or Invar steel tapes, by which

practically correct base lines may be laid down. For geodetic works

requiring the greatest accuracy the bases have been laid with rods of

various forms. These rods will be briefly described. It is only in the

construction of iron bridges, roofs, etc., that rods are at present

generally employed in the work of the civil engineer.

751.--=Pine Standard Rods=, made of straight-grained pinewood seasoned

five or six years and then well soaked in linseed oil, make good

standard rods. The ordinary length in use is 10 feet by 1¾ inches

square. If the rod be used for butt measurement the ends are tipped

with gun-metal in which a turned steel stud is hard-soldered. The stud

is afterwards ground to true face in a lathe, and left of standard

length at 60° Fahrenheit (15·5 centigrade), Fig. 352. A disc of brass

1 inch diameter is inlaid at every foot for 5 feet from one end of the

rod, with a line at the true foot. These rods, after the work upon them

is finished, are lightly French polished to keep them clean and to

prevent the effects of moisture. The effect of temperature upon deal

was found by Roy to be about the same as upon glass--·0000085, average

of total length per degree centigrade, which is about three-fourths

that of iron.

[Illustration: Fig. 352.--_One end of a pinewood butt rod._]

[Illustration: Fig. 353.--_S--Block square._]

752.--Where butt rods are used for continuous measurement, it is

necessary that they be brought very carefully together. In base line

measurement three or four are used, but for metal work or masonry two

10-feet only are generally employed. It is necessary that the rods

should lie upon a straight surface or be supported in a straight line.

In bringing them together, a piece of indiarubber 1/8 inch or so in

thickness temporarily placed at one end will prevent any palpable

disturbance of the percussion if the fixed rod be well weighted. One

5-feet more fully divided butt rod is very commonly supplied with a

pair of 10-feet rods for supplemental measurement.

753.--=Angle-piece.=--A solid angle-piece with two planes at right

angles is very convenient for use with butt rods to give means of

scribing the true length down to a surface, Fig. 353 S.

754.--=Butt Rods with Iron Core.=--Where rods are to be used for

preparing iron work it is better to have an iron core through the

rod, that may expand and contract with the metal on which they are

used. The rods that the author has designed for this purpose are made

out of a length of seasoned pine 2¼ inches square, sawn down and

turned cut sides outwards to prevent warping. A 10-feet length of iron

steam tube about ½ inch diameter is painted several times and then

bound round with paper soaked in paraffin. This is placed in a pair of

meeting grooves, as shown in section Fig. 355. The two pine flitches

are cross-tongued together and glued up with the inserted tube between

them. The tube has a turned steel cap placed over each end, Fig. 354,

and this is ground in a lathe to true standard at the temperature

of 60° Fahr. A steel pin is placed through the centre of the rod to

indicate 5 feet. The finished size of the rods is 2 inches square.

The author has made these rods in sets, consisting of two 10-feet and

one 5-feet packed together with an angle-piece, Fig. 353 S, in a deal

case.

755.--_The 5-feet Rule_ is of steel, ¾ inch by ¼ inch, inlaid in a

piece of dry pine, altogether of only half the thickness of the rods,

so that it stands the correct height for central butt measurement,

Fig. 356. The rule is divided into feet and inches, with one foot to

eighths. A centigrade thermometer is placed in one of the rods to

indicate the prevailing temperature, and a small piece of scale showing

amount to be allowed in 10 feet per degree centigrade for temperature

above or below 15·5° centigrade is engraved upon the thermometer scale.

The coefficient for the expansion of wrought iron is given by Lord

Kelvin as ·000019 mean per degree centigrade.

[Illustration: Figs. 354, 355, 356.--_Butt rods with iron core._]

Where a long length is laid down for a base line or other purpose, it

is better to take the thermometer reading at each measurement and defer

correction to the completion of the work; the temperature errors may

then be added together as a total, and the space allowance may become a

measurable quantity. For example, say ten 10-feet lengths give by these

united centigrade degrees, plus and minus, shown at separate readings

+ 167°, and that the standard of the rods is true at 15·5°. Then 167 -

(10 × 15·5) = + 12° per foot total allowance, that is, 12° × 10 feet ×

·000019 = ·0228 feet or ·2736 inches to be added. In measuring iron of

course no correction has to be made.

756.--_Beam Compass Measurements_ are occasionally preferred for

iron work. In this case the beam is moved from centre punch mark to

mark along a surface by the beam producing a scratch for the forward

position in which to place the punch mark. Rods of pine are commonly

employed. Figs. 357, 358 will sufficiently illustrate the instruments.

[Illustration: Fig. 357.--_Beam compasses._]

[Illustration: Fig. 358.--_Standard scale._]

[Illustration: Fig. 359.--_Coincidence rods._]

757.--=The Method of Coincidence= in measurements by rods has often

been applied to measurement of base lines. The plan consists in

allowing one rod, or a lighter continuous part of it, to pass the

other rod, so that a line cut to standard on one rod may be read into

one on the other. The best plan to do this is to have a scale fixed

along the face of one rod near its end, as shown Fig. 359, and to have

an extension from the other end of the second rod to pass alongside

this scale, so that two lines may be brought into coincidence. The

rod _B_ has a fixed scale _b_ placed on top of it at one end. The rod

_A_ has a scale protruding from it. This scale may be jointed with a

good ground joint at _J_ for portability. The rods are laid lightly

together, and any final adjustment is given by light taps with a small

hammer or mallet upon one or the other side of the stud _P_ until exact

coincidence of the lines shown at _b_ is brought about. This tapping

operation appears a rather rough process, but practically it is very

exact.

[Illustration: Fig. 360.--_Bessel compensated rod._]

758.--=Compensated Rods.=--The plan used by Bessel for the measurement

of a base upon the shores of the Baltic in 1836 is looked upon as a

model of the most perfect work of its kind. The rods were composed of

two bars of iron having surfaces accurately planed, with a similar bar

of zinc placed between them. The bars were laid one on the other, but

not in contact, the surfaces being kept apart by glass plates, upon

which they could slide with little friction. The linear expansion of

zinc per degree centigrade is about ·0000292 (Fizeau); that of iron

much less than half this--about ·0000119 (Thomson). The bars are

attached to each other in such a way that the expansion of the zinc may

act in the opposite direction to the expansion of the iron. The form

followed for the construction is shown in Fig. 360 where _II′_ are the

iron bars, _Z_ zinc. The length of the zinc required for compensation

between the junctions is found in the equation--

(_S_ + _Z_)(0000119) - _Z_(0000292) = _0_,

_S_ being the total length of the standard rod in feet, and _Z_ the

length of zinc in feet required for compensation. This plan is that

adopted for the compensation of pendulums. For the verification of

a rod it may also be made to form the rod of a pendulum, by which

temperature expansion and contraction upon the system will be clearly

indicated by difference of time rate in the change of temperature

during night and day. This test becomes important where great precision

is aimed at, as the expansion in metals varies according to their

purity and state. The standard lines in rods made upon this model are

placed upon small inserted discs of platinum placed near the ends,

which are read by microscopes in coincidence upon a pair of rods.

759.--=Colby Compensated Rods=, the invention of Major-General T. F.

Colby, who was for twenty-seven years superintendent of the Ordnance

Survey, upon which these rods were used. Each rod is composed of one

rod of iron and one of brass, which are so arranged in pairs that the

difference of expansion of these metals shall act to diminish the

amount of entire expansion at the points measured, a quantity equal to

its increase by temperature, in a manner to be described.

_The Rods_ are each made 10 feet 1·5 inches long, 5 inches broad, and

1·5 inches deep. Fig. 362 _i_ is a side elevation of one rod, Fig. 363

_ib_ plan of iron and brass rods, Fig. 365 _ib_ perspective view. By

this it will be seen that the rods are placed edgewise. The distance

apart is 1·125 inches. They are supported in the middle upon rollers,

Fig. 362 _F_. They are firmly fixed together at their centres by

transverse steel cylinders, Fig. 363 _RR′_ 1·5 inches diameter, each

rod being left free to expand or contract from the neutral central

point independently of the other. The neutral point is formed of a

T-piece _E_, Fig. 363, fixed firmly on the bottom of the box _bx_. At

the extremity, and at right angles to each of these bars, is a flat

steel tongue, Figs. 364, 365 _A_, 6·2 inches long, 1·1 inches broad,

and 0·25 inch thick, which projects 3·25 inches from the side of the

iron bar _i_. The tongue _A_ is jointed by double conical pivots at

_f_ and _f′_, which form axes perpendicular to the surface of the

tongue, allowing it to be inclined to slightly different angles to the

direction of the bars according to the expansion or contraction the

system experiences by heat. The pivots are 0·5 inch diameter, and are

placed at 2·3 inches from the end of the tongue next the brass bar.

On the tongue at _P_, flush with its upper surface, a small stud of

platinum is inserted, upon which a small dot is struck to form the

point of standard measurement.

[Illustration: _Colby Compensated Measuring Rods._

Fig. 361.--_End of rod mounted with microscopes, trestles and ground

plate._]

The bars are placed in strong wooden boxes, to the bottoms of which

are fixed the plates that hold the brass rollers upon which the bars

are supported, Fig. 362 _F_, and the central stay _E_ mentioned before

prevents any displacement of the bars when the rods are held by the

rollers _RR′_. To protect the tongue _A_, which projects beyond the

boxes, there is a special covering nozzle having a hole and cover over

the dot. A level is placed on one of the bars, which is seen through a

window in the lid of the box. At the ends of the box plates are fixed

for supporting the tripod of the double compensated microscope, Fig.

361 _D_, employed to observe the standard points of one pair of rods

brought by adjustment to true position _MM′_. A pair of sight vanes

which shut down are placed on the ends of the box for setting the rods

approximately in line.

[Illustration: _Colby Compensated Measuring Rods._

Fig. 362.--_Side elevation of point of support of rod._

Fig. 363.--_Side elevation of centre, with section of box bx._

Fig. 364.--_Plan of rods and compensating arm._

Fig. 365.--_Perspective view of the same._]

_Two Rigid Tripod Stands_ Fig. 361 _S_ are used to each of the rods

placed under the rollers Fig. 362 _F_ upon which the bars are supported

in the box. The tripods carry a universal slide-rest by which the rod

may be adjusted to position both in horizontal and vertical planes

Fig. 361 _A_. Six rods were used for the Ordnance Survey at one time,

and were designated by the letters A B C D E F. The weight of each rod

complete with microscopes in its case is 136 lbs.

_Compensated Microscopes._--The compound microscopes, Fig. 361 _MM′_,

used with the Colby apparatus form a complete separate instrument,

consisting of two microscopes placed parallel to each other and

united together for reading the rods when they are brought with their

standard points the distance apart that separates the axes of the two

microscopes. In the intermediate space between the two microscopes,

and parallel with them, a telescope _T_ is fixed on the same piece

of apparatus, with adjustment for reading a point on the ground _G_

perpendicular to the measuring rod. The microscopes are held apart

by two bars of brass and iron 7 inches long, 0·5 inch broad, and

0·375 inch thick, which are placed at 2·5 inches apart and secured

with the telescope, which forms the fixed centre, by collars to the

bodies of the microscopes. The difference of expansion of the iron

and brass maintains the separation of the microscopes at their foci

at one distance with every change of temperature of the air. The

object-glasses are of 2 inches focus. The microscopes are brought to

adjustment and bearing by levelling on a tribrach whose base is fixed

firmly to one of the rod cases, and by lateral adjusting screws.

Special microscopes are used with each of the six rods of the Colby

apparatus, and are distinguished by the letters M N O P Q R S. The

weight of each compound microscope is 5 lbs. Very full particulars of

the Colby apparatus with engravings of all parts, are given in "The

Ordnance Survey Account of the Measurement of the Lough Foyle Base."

In measuring a base line a piece of nearly level land is selected, and

the rods are supported upon the trestles or tripod stands at about 3

feet from the ground. The heights of the upper surfaces of the tripods

are ranged by a theodolite or level for all intermediate points between

the two ends of the line. Generally twelve trestles are employed with

these rods, which are fixed firmly to the ground at every station by

legs well rammed in, Fig. 361 _HH′_. The cases containing the rods, or

the rods themselves, are made sufficiently strong to be supported upon

two points only without serious deflection.

The Colby system of measurement of base lines varied in detail has been

employed by nearly all the nations of Europe and in America.

760.--=Modern Base-line Apparatus.=--The introduction of "Konstat"

steel (highest grade Invar) tapes and wires has revolutionised the

method of measuring base lines. These tapes offer a means which is

far superior to anything obtained by measuring bars, because they

combine the advantages of great length and simplicity of working,

with more precision than the shorter laboratory standards, providing

that suitable apparatus is used in applying them to their work. Base

lines may now be rapidly measured with long "Konstat" steel tapes so

that much longer lines are laid down than was formerly the practice

when measured with bars, with the result that any errors that may be

introduced do not affect the ultimate expansion so much owing to the

greater length of the base.

The coefficient of expansion of "Konstat" steel is under ·0000005 per

degree Fahrenheit, so that provided accurate means of suspending the

tape and reading it and transferring the readings to a plate properly

let in the ground are used, we have a most exact and rapid method for

this important work.

The tapes are usually 100 feet or 30 metres long, but 300 feet or 100

metres are often used. The tapes are a few feet longer than these

measurements, so that the rings are well clear of the reading lines. A

silk cord is attached to these rings and passes over the end suspension

supports, one of which is shown at Fig. 366. These are made with two

steel bars rigidly mounted on two tripods; upon the bars a sliding

carriage is mounted carrying a pulley running on ball bearings with a

vertical motion for final adjustment of the tape for height. A weight

is attached to the other end of the silk cord to give the same tension

as that under which the tape was divided.

[Illustration: Fig. 366.--_One of the two end supports of the band,

showing tension weight, with cord running over the ball-bearing

pulley._]

To prevent catenery light intermediate stands, as shown at _B_ Fig.

367, are used at about every ten feet; these have a rising cross piece

with guides which are adjustable for height and sideways to support

the tape in perfect alignment. Having the tape properly suspended the

reading instruments, _C_ Fig. 367, are placed in position at either

end. These are mounted on rigid-framed stands and provided with

levelling screws, cross levels, transverse screw motions and movement

in azimuth, with clamp and tangent motions and aligning telescopes. A

powerful microscope is rigidly fixed over a little table over which the

tape passes and reads its division with great exactness, coincidence

with the division being made by the traversing screw. By the side of

the reading microscope, and in exact collimation with it, a plumbing

telescope is rigidly fixed, and this sights down to a transferring

apparatus, _D_ Fig. 367, which is over the plate let into the ground.

[Illustration: Fig. 367.--_Two reading and plumbing instruments, C,

C; transferring instrument, D; and one of the adjustable intermediate

supports, B._]

The transferring apparatus is a spring centre punch rigidly mounted

truly vertical on a supporting plate having transverse motions,

cross levels and levelling screws. The top of the centre punch has

a small platinum disc let in a recess, and upon this disc very fine

cross lines are marked. This apparatus is placed on the ground over

whatever has been let in to receive the mark, it is then levelled and

the cross lines upon the punch top brought by means of the transverse

motion screws exactly to coincide with the spider web of the plumbing

telescope, and in this position the centre punch is lightly struck

with a mallet which marks the plate let in the ground in the exact

position of the centre of the cross lines at its top, so that if now

the transferring apparatus be removed the cross webs of the plumbing

telescope would cut the dot marked in the plate by the centre punch.

This method is far more exact than any hanging plumb-bob, as even if

they are screened to prevent swinging very few hang with the point

perfectly true. In laying down a base line No. 1 reading and plumbing

instrument is set up and levelled over the starting end block,

which is usually of hard stone or granite set on a firm foundation,

with a copper plate let in its top about the centre, the line having

been previously set out with a theodolite, and the intermediate

stations being roughly measured with an ordinary steel tape. At each

intermediate station or length of Konstat tape used a teak post is

driven into the ground and a zinc plate screwed upon its top; the

other end block is similar to the starting one. The Konstat tape is

now mounted between the end suspension supports, one being outside the

starting end block and the other outside the first teak post which has

been put in for the first length. No. 2 reading and plumbing instrument

is set up over this post, and No. 1 and No. 2 are aligned upon each

other by their aligning telescopes, and the Konstat tape adjusted over

the little tables under the microscope of each; the intermediate stands

are then put in and adjusted for height to prevent catenery, and the

guide pieces are brought up to the tape on either side and clamped to

prevent side deflection by wind. The tape being properly suspended it

can be easily moved with the fingers lengthways, as it is suspended at

either end by silk cords over ball-bearing pulleys. It is brought in

position with its starting end division somewhere under the microscope

of No. 1 reading apparatus, and the microscope is then brought into

exact coincidence by the traversing screws. The transferring apparatus

is put on the granite block with the centre punch in the field of view

of the plumbing telescope and then levelled; the cross lines in the

top of the centre punch are then brought to exactly coincide with the

plumbing telescope webs by the transverse motion of the transferring

apparatus, the centre punch is struck and the mark thus made in the

copper plate has a line engraved through it. The transferring apparatus

is then removed to the position under No. 2 reading and plumbing

apparatus. No. 2 microscope is made to coincide with the end division

on the Konstat tape by its traversing screws, the centre punch of

transferring apparatus, brought by its traversing screws to coincide

with the webs of the plumbing telescope and struck, marks the first

section. No. 1 reading and plumbing apparatus is then transferred to

the next post, No. 2 remaining over the first section post, the first

end suspension stand is transferred outside No. 2 post and the tape

mounted between as before, the traversing motion of No. 2 reading

apparatus must not be touched, but the end division of the tape brought

to coincide with its microscope web by shifting lengthways. No. 1

microscope at the further end is adjusted to coincide by its traversing

screws and the transferring apparatus as before, and so on until the

entire length is measured, the last centre punch mark on the copper

plate let in the further end block or stone having a line engraved

through it.

A few 1-100th of an inch divisions, or 1-10ths of a millimetre, are

divided on either side of one end division of the Konstat tape so

that any allowance for expansion or contraction may be made under the

microscope at the time, but with Konstat tapes this is very small

indeed. With fairly level ground any slight differences of level can be

allowed for in setting up the stands, so that the tape remains level;

if the difference is too great for this the difference of hypo and base

must be calculated. Thermometers are used, generally one suspended on

the tape at each end.

761.--=Perambulator.=--A very ancient instrument, described by

Vitruvius as being among the effects of the Emperor Commodus; it was

used by hand, or attached to a carriage to measure distances. The

instrument is at present used as formerly for measuring roads. Upon

pavements and asphalt roads it measures accurately, where by reason of

traffic it is sometimes a difficult or very slow process to use the

chain. The plan of manufacture is varied considerably. The author makes

the felloe of the wheel in segments of well-seasoned mahogany in two

rings, Fig. 368. These are rivetted together from side to side in such

a manner that the grain of the wood is crossed as much as possible to

prevent lateral warping. The tyre, which is 6 feet in circumference,

is made of hard rolled brass 1 inch by ¼ inch thick. The spokes are

light steel tubes covered with brass tube, and screwed into a brass

hub. The axle of the wheel is placed in a steel fork which is formed

by screwing, by means of a winged nut, two bars of about 18 by 1½

by 3/8 inches upon a boss formed at the end of the steel stem of the

turned wood handle. Made in this manner the handle may be easily

detached and placed flatwise upon the wheel, so that the whole may be

packed in a square deal case of moderate dimensions for transport.

[Illustration: Fig. 368.--_Perambulator._]

[Illustration: Fig. 369.--_Details of registering box._]

[Illustration: Fig. 370.--_Section._]

_The Registering Part of the Instrument_, Figs. 369, 370. The axle is

protruded through the fork on the left-hand side, and thence through

the registering box supporting one of its ends. The other end of the

box is supported by a stud which fits into the side of the fork. The

axle in the part contained within the box is cut into a screw, Fig.

370 _S_, of about sixteen threads to the inch. The screw works in the

edges of a pair of discs _R_, placed one upon the other upon the same

axis; these are cut on their edges with teeth to form worm wheels in

which the screw upon the axis of the wheel works. The upper disc has

110 teeth. This therefore moves one revolution by 110 turns of the

wheel. It is divided into 110 divisions at its circumference, but is

figured 20 yards to 220 yards or 1 furlong, so that each division

represents 2 yards, corresponding with the circumference of the wheel,

Fig. 369 _O_. The divisions are read by a point attached to the side of

the box shown at the top of the figure. Single yards are shown by the

intermediate position of the pointer between the divisions, but single

feet may very well be estimated approximately. The lower disc is cut

with 111 teeth. The ratio 110 to 111 gives a differential displacement

of one tooth only after 110 revolutions of the wheel, or of 220 yards

traverse. The two discs take, therefore, by revolution over the surface

220 × 110 = 24,200 yards or 13·5 miles before they return to the same

relative position as at starting. This is, therefore, the space this

perambulator will traverse without resetting. To enable the lower

disc to be read the upper disc is cut away for half the interior

circumference of its circle. A part of the upper disc is formed into a

point, to read direct from the centre into divisions on the lower disc,

in furlongs up to 13½ miles.

_The Measuring Box_ is covered with glass for protection. The box can

be taken off by removal of the milled-headed screw at any time to set

it back to zero, but in practice it is often found more convenient to

spin the wheel round to zero or an even mile of the outer circle, and

record differences of reading, if this can be done in the distance

within the record of 13 miles of the lower disc. The screw and axis,

which are of hard steel, should be occasionally oiled with watch oil to

keep the perambulator in good working order.

762.--The reviser has designed a light form of perambulator on the

bicycle wheel principle. It is shown at Fig. 371, and is very light

and portable. The rim of the wheel is of gun-metal and is usually made

two yards in circumference. It is fitted with a counter which denotes

two yards to every revolution, and the distance is given in number of

yards only. The handle is detachable from the fork for packing, and

the whole is contained in a light pine case. The wheel is also made two

metres and ten links in circumference.

[Illustration: Fig. 371.]

763.--=Pedometer.=--Used for roughly ascertaining distances passed over

in walking. This ingenious instrument was the invention of William

Payne in 1831 (patent No. 6078). It is the size of an ordinary watch,

and has a similar face; but between the figures, which indicate miles 1

to 12, there are four divisions only, to indicate quarter miles. The

pedometer is slung up by a loop, Fig. 371, _H_ fixed upon the handle,

which in use is passed over the edge of the waistcoat pocket so as to

keep the instrument in an approximately vertical position.

[Illustration: Fig. 372.--_Construction of pedometer._]

[Illustration: Fig. 373.--_Face of passometer._]

764.--_The Registering Apparatus_ consists of a pendulum, Fig. 372,

_P_ placed horizontally by being supported by a delicate spring _L_ to

its highest position, where it rests against a stud. The action of the

pendulum is caused by its following the motion of the body in stepping,

until stopped by the foot reaching the ground, when the momentum

attained by the pendulum carries it from its upper position of rest

where it is sprung against the stop to its lower free position, where

it is stopped by a screwed adjustable stud _S_, shown under it. The

axis of the pendulum is free upon the axis of the ratchet wheel _R_.

When the pendulum falls, a fine spring, fixed to its upper surface,

drops its end into the teeth of the ratchet, moving over two or three

teeth, which are held against retrograde motion by the spring pawl _D_.

When the pendulum rises, the ratchet is moved forward the number of

teeth that the spring at first slipped over. The ratchet is connected

with a pair of geared wheels, _not shown_, the axis of the second of

which forms the axis of the hand. In this manner each oscillation of

the pendulum is communicated to the index hand. The ratchet is made

with extremely fine teeth, so that by adjustment of the screw stud _S_

a greater or less number of these teeth may be taken by one beat of

the pendulum, and thus the mileage rate may be adjusted approximately

to the step. This is done, however, very imperfectly, as the variation

of the average steps of men amounts to one or two inches, and the

difference from the number of teeth taken will scarcely indicate less

than three inches in the step.

765.--=Passometer.=--This instrument was originally invented by the

author as an improvement upon the pedometer (1868). The instrument,

Fig. 373, is not intended to indicate miles or any distance, it

simply counts the number of steps taken. The action is just the same

as the pedometer, but the ratchet teeth are larger and less liable to

miss a tooth, as it is made to take one tooth only at a single step.

The dial arrangement is entirely changed. The steps are numerically

indicated by a separate hand which reads into the graduations up to

50 steps upon a small dial. Each revolution of the small hand reads

through gearing one division of the central hand, which moves over the

complete circumference of the dial, reading up to 25,000 steps. This

is the extent of indication. It is necessary in continuation beyond

25,000 steps to take a record of progression per 25,000 where a greater

distance is required to be measured.

766.--The average step may be estimated perhaps within 1 or 2 per cent.

by training in walking several miles steadily, counting the steps,

always remembering that we take shorter steps uphill and when we are

tired. But the mean step of the individual under all the different

circumstances is the only rule that can be followed.

[Illustration: Fig. 374.--_Sounding chain._]

767.--=Sounding Chains= used for coast surveys are generally made of

iron, but sometimes of brass. They are usually made of 10 fathoms

entire length. The links are 1 inch, and the feet are indicated by

tellers. The form of teller designed by the author is shown in Fig.

374 for the 3. A leaden weight, similar to that shown Fig. 375, is

used upon the end of the chain--of 28 lbs., for ordinary coast work,

or heavier if there are strong currents. The chain is contained in a

strong wooden box.

[Illustration: Fig. 375.--_Sounding line and weight._]

A very elaborate apparatus with steel wire line has been made for

deep-sea sounding by Lord Kelvin and others; but this subject is beyond

the province of the present work.

768.--=Sounding Lines=, used for survey of shallow coasts and harbours,

are made of water-laid line of fine green hemp, about ¾ inch

circumference, Fig. 375. White tapes are inserted as tellers at every

foot, and red tapes at every fathom. 3 to 6 fathoms are the ordinary

lengths employed. If the water is shallow the fathoms are easily

counted, but if thought necessary knots may be tied to indicate the

number of fathoms on the red tellers. The weight is about 7 lbs. for

50 feet line, about 15 lbs. for 100 feet. The under side of the weight

is commonly recessed to take tallow when it is desired to bring up a

specimen of the bottom, if this is loose sand or mud.

769.--=Coast Survey Lines.=--For surveying distances, from point to

point of soundings along a coast, lines of fine copper wire rope

marked with tellers at 50 and 100 feet are commonly used. The line

is generally allowed to rest on the bottom of shallow water, and is

floated up by means of attached corks in deep water. It is usually

laid and picked up by means of a reel fixed at the stern of the

surveying boat. The lengths of line used vary from 1000 to 5000 feet.

770.--=Telemeters.=--These scarcely enter within the practical limits

of surveying instruments, but as several attempts have been made to

introduce their use it is necessary to mention them. The general

attempt has been to measure a great distance, 1000 feet or more, by

means of the angles subtended from the ends of a short base to a

distant point. This base in the telemeter of Piazzi Smyth is 60 inches;

Colonel Clarke, 72 inches; Otto Struve, 73·5 inches; and Adie, 36

inches. The angles are usually taken upon the principle of the sextant

by coincidence of image. Very much greater success has been attained

recently by Messrs. Barr and Stroud by means of their range-finder

of 54 inches base. The author, as far as his information reaches, is

assured that no instrument of the class is satisfactory for surveying

purposes. Further, the subject is one to which he has devoted some

study, and designed two telemeters.[55] One of these appeared to

him for a time satisfactory within certain limits. The base in this

instrument was 50 feet, formed of a fine pianoforte wire stretched

between two observing telescopes, the tension of the wire directing the

one telescope to a right angle, and the other telescope to an arc which

read either degrees and minutes or absolute distances in the eye-piece

to the direction in which the telescope was pointed. In first trials

this instrument was found fairly satisfactory; but subsequently in

windy weather the deflection of the wire rendered the action of a pair

of instruments quite unreliable.

There are some instruments, as Colonel Gautier's telemeter used in the

French army, which depend upon combined reflectors placed normally at

15° to 45°, as in the apomecometer, art. 693, but with a tangent screw

to give a small motion of displacement to one mirror which reads on

a scale of calculated distances to angle from a certain base measured

between two stations of observation. A very similar instrument,

invented by Labez, has one reflector only at 45°. These instruments may

be useful for measuring approximate distances for range in the army,

but can scarcely rank as surveying instruments, the box sextant, art.

664, being in every way a superior telemeter for the purpose when a

measured base can be fixed and well-known trigonometrical calculation

used.

771.--The simplest and best telemeter for surveying purposes is the

subtense telescope, and all good, up-to-date surveying instruments

have their telescopes so fitted, but for those who do not carry an

instrument with a telescope the reviser has designed a small subtense

telemeter, Fig. 376, which consists of a small telescope fitted with

subtense points, and mounted in a collar which has vertical and

horizontal motions and a centre socket to fit a Jacob's staff. The

stadia is set to read 1 in 100. The telescope has rack and pinion

focussing and may be revolved in its socket so that the stadia rod

may be read held either horizontally or vertically. It is packed in a

leather holster case, and a four-fold 10-feet spring-pointed stadia rod

is supplied with it divided into feet, tenths, and hundredths.

[Illustration: Fig. 376.]

772.--=Hand Rods=, although used more generally by building surveyors,

are extremely useful also to the civil engineer and land surveyor

for town work among buildings and in mines. They are made 5 feet in

length, less generally 10 feet. The 5-feet are made of single blades

of lancewood or of two jointed to fold. The 10-feet are always jointed

and made much stouter than the 5-feet. The 5-feet are generally sold in

pairs.

[Illustration: Fig. 377.--_Ordinary 5-feet jointed rods--plan and

section of joint._]

773.--=Ordinary 5-feet Rods= are divided to every 3 inches, with feet

only stamped with numbers, as shown Fig. 377. Where the rod is jointed

the best form of folding joint is shown in the figure in section and

plan. The spring S is sunk into the face of the rod at the joint on

one side, and springs into a groove (_housing_) in the other side so

as to lock the joint when it is either open or closed. The most useful

dimension for the rod is 1 inch by 1/6 inch. Rods are nearly always

made of lancewood, but they are preferred dyed black for neatness by

many surveyors. A pair of rods is usually carried in a cowhide case.

They are also often carried in the stem of a walking-stick hollowed out

for the purpose. The rod or rods in this case are made much lighter,

generally ½ inch by 1/8 inch for a pair of rods, or 7/16 inch by 7/32

inch for a single rod. The single rod is to be preferred in this case

for its extra strength.

774.--=Fully Divided Rods.=--The author has made rods for many years

divided to single inches. These measure from both ends--one end direct

as Fig. 378 and the other end reversed by turning the rod over as Fig.

379. By this plan the rod gives direct measurement in feet, inches,

and parts from either end, and the division is always placed outwards

against the work, so that measures may be taken from either end by

turning the rod over sideways, without turning it end for end.

[Illustration: Figs. 378, 379.--_Stanley's surveyors' rods._]

775.--Connecting Link for Rods, which weighs only 1 oz. and may be

carried loose in the pocket, is often found convenient for measuring

heights, as it permits the ends of a pair of rods to be brought

together, Fig. 380. By this means the arm will raise the rods about 7

feet, and with 10 feet, the height of the pair of 5-feet rods, this

will make 17 feet of measurement. When the 10-feet is set against a

wall, its height, if 20 or 30 feet, may be guessed very approximately

by standing at a distance from it.

[Illustration: Fig. 380.--_Connecting link for rods._]

[Illustration: Fig. 381.--_Slip jointed rod._]

776.--=Slip Jointed Rod.=--This form is less general, but it is a very

convenient form of rod. The jointing is effected by two loops which are

fixed to the centre end of one part of the rod in such a manner that

the other part may slide through the loops. When the rod is extended

to 5 feet there is a stop which prevents further extension, and a

spring to keep it at this exact position, Fig. 381. The outside of the

rod is divided into feet and inches. The inside is divided so that

any addition to the half rod, produced by extending it; may give the

measurement from end to end of the rod at this position, thus:--The

half rod being 2 feet 7 inches closed, if the loose side be drawn out

19 inches the rod from end to end will be 4 feet 2 inches, which will

be indicated by the division and figuring inside the rod. This is very

convenient for measuring openings such as doorways or passages.

777.--=Brace-piece.=--A 10-feet rod is sometimes made with a

brace-piece, which folds up inside the rod. This brace-piece is jointed

to fix both half rods to 90° when it is desirable to use the rod as a

square.

[Illustration: Fig. 382.--_Civil engineer's rule._]

778.--=Civil Engineer's Rule= is made fourfold in both boxwood and

ivory, Fig. 382. The most convenient size is 1 inch wide. Some of the

profession prefer them narrow for lightness--¾ inch; and some wide

for strength--1¼ inches. This rule is generally well made, with

German-silver joints and outside joint-plates. The divisions placed

on the rule outside are inches in eighths and tenths; the inside, the

ordinary architects' scales, 1/8, ¼, ½, 1, and four chain scales,

20, 30, 40, and 50. A 10 is got by halving the 20; 60, by doubling the

30. A protractor reading to 5° is divided on the head. With silver

joints and in fine ivory this rule is often made a presentation

instrument.

FOOTNOTES:

[54] _The Surveyor_, vol. ii. No. 5. Sydney, Nov. 1889.

[55] Patent No. 2142, May, 1880.

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.

[Illustration: Fig. 383.--_Ranging pole or picket._]

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.

[Illustration: Fig. 384.--_False picket._]

[Illustration: Fig. 385.--_Spur-shod picket._]

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.

[Illustration: Fig. 386.--_Socket for station pole._]

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.

[Illustration: Fig. 387.--_Stanley's heliotrope._]

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.

[Illustration: Fig. 388.--_Heliograph._]

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.

FOOTNOTES:

[56] _Manual of Surveying for India_, p. 478, 1875.

[57] Patent No. 3390, October 1874.

[58] For full description and plate, see Everest's _Measurement of the

Meridional Arc of India_, Introd. p. cxv.

CHAPTER XVIII.

MEASUREMENT OF ALTITUDES BY DIFFERENCES OF ATMOSPHERIC PRESSURE--

HISTORICAL NOTE--MERCURIAL BAROMETER--CONSTRUCTION--OPERATION--ANEROID

BAROMETER--CONSTRUCTION--VARIOUS IMPROVEMENTS--HYPSOMETER.

808.--_Historical Note._--The observation that the atmosphere decreases

in density with increase of height is due to Alhazen the Saracen,

about a.d. 1000. By this he explains that a ray of light entering

the atmosphere obliquely follows a curvilinear path, bending towards

the denser strata, that is concave towards the earth. He showed that

a body will receive difference of pressure in a rare and a dense

atmosphere, and calculated that the height of the atmosphere to its

final attenuation would be from his data nearly 58½ miles. The

practical instruments that have been devised for measuring altitudes,

by the differences of pressure due to the weight of superincumbent

atmosphere are the barometer, the aneroid, and the hypsometer. The

barometer was invented by Torricelli about the year 1640. Its principle

was demonstrated and first applied to altitude measurement by Pascal in

1647. The aneroid barometer was suggested by Conti in 1798, and said to

be devised as a practical instrument by Vidie in 1808. The hypsometer

or boiling-point thermometer, which depends for its boiling temperature

upon the pressure of the atmosphere above the liquid which surrounds

it, was suggested by Fahrenheit in 1724, experimented with by de Luc

in 1772, and brought to its present practical form by Regnault about

1840. At the present time the aneroid is almost exclusively used by

the civil engineer, as this instrument when made with great care is

sufficiently reliable, more portable, and not so delicate in use as

the others. So that it is only when very great precision is desired,

or when the one instrument is used as a check upon the other, that the

mercurial barometer, or the hypsometer, or both are now employed. At

the same time it must be understood that the aneroid barometer scale

is in a certain degree arbitrary, as the divisions at best are only

made up from a certain number of points taken from observations of the

mercurial barometer placed simultaneously with the aneroid under an

air pump, and therefore its errors comprise those of the particular

mercurial barometer with which it is compared, and those due to the

difficulties of the comparison, and of making subdivision afterwards in

the same relative proportion, by copying to the scale of the aneroid.

809.--=The Mercurial Barometer.=--The principle of the barometer is

generally understood. If a glass tube, closed at one end, 33 inches

long, say of ¼ inch or over in bore, be filled brimful of mercury and

the point of the forefinger be firmly pressed on the surface of the

mercury, the tube may be inverted without the admission of air. If the

covered end of the tube be now plunged into a basin of mercury and the

finger slowly withdrawn from under the tube beneath the surface of the

mercury, the latter will sink in the tube to about 30 inches above the

surface of that in the basin--that is, if the experiment be performed

at about the sea level. The empty space in what now becomes the top of

the tube is termed a _Torricellian vacuum_.

810.--In removing the pressure of the atmosphere from its surface in

the tube, which in the above experiment produces the barometer, the

pressure of the atmosphere then falls only upon the exposed surface

of the mercury in the _basin_, or what is technically termed the

_cistern_. This pressure is equal per area, according to hydrostatic

laws, to the upper surface area of any equal column of mercury that the

barometer may contain. Therefore the weight of the column of mercury

in the tube, if cylindrical, above the surface of that in the cistern,

is the same as that of a column of air of equal size reaching upwards

to the full height of the atmosphere. In fact the one exactly balances

the other, and it is by the difference of the weight or quantity of air

above the barometer _per area_ of bearing surface that it is possible

to ascertain the altitude of its position by means of the height of

mercury in the tube, after proper allowance is made for sudden changes

of conditions of the atmosphere itself from time to time, capillary

attraction of the tube, temperature, etc.

811.--The mean height of the barometrical column in Great Britain, at

sea level at the temperature of 32° Fahr., is about 29·95 ins. A cubic

inch of mercury at this temperature weighs 0·48967 lbs. Therefore

29·95 × 0·48967 = 14·66 lbs.

gives the mean pressure of the atmosphere on each square inch of

surface of the earth in this latitude. Nearer the tropics the pressure

is greater, near the poles less. It can be shown that as the heights

ascended by the barometer increase in arithmetical progression, the

pressure upon the mercury diminishes in geometrical progression.

812.--=Mountain Barometer.=--The barometer used for measuring

altitudes, to which the above term has been applied, is now made only

upon Fortin's plan, in which the bottom of the cistern wherein the

glass tube is plunged is made of fine, close-grained leather, the best

for the purpose being a stout kid. The pores of the leather must be

sufficiently fine not to admit of the escape of the mercury, and yet at

the same time sufficiently soft and pliable to transmit the exterior

pressure of the air. Fortin's construction permits the cistern to

be closed entirely secure from leakage of the mercury, in whatever

position the barometer may be placed. The closing is effected by means

of an adjusting screw, Fig. 390 _F_, which by its pressure decreases

the capacity of the cistern and forces the mercury up the tube, or

adjusts it to a given height, so that the scale of the barometer may

be read correctly from a given point _X_ placed within the cistern. To

prevent injury to the tube the adjusting screw is made of a length just

sufficient to force the mercury to fill it, so that when it is closed

home there is no jar or percussion of the mercury in carrying the

barometer. The details of the mountain barometer may be best followed

by the illustrations.

813.--_The Glass Tube_ is made of mild flint glass thoroughly annealed

and sufficiently stout to resist all the strain and percussion that

may occur with fair usage. One end of the tube is slowly sealed by the

blow-pipe, so that the closed end may be as strong as the other parts.

814.--_Mercury--Filling the Barometer Tube._--The mercury of commerce

is generally impure, and it contains occluded air. For standard

and mountain barometers the mercury should be distilled in an iron

apparatus, at just its boiling heat, leaving about one-sixth of the

mercury in the still. The tube, which should be perfectly clean, is

left about 12 inches too long for the barometer. It is charged with

clean mercury for about 36 inches in height. It is then boiled in

a special circular charcoal stove, in the centre of which there is

a vertical iron tube of 2 inches diameter. The barometer tube is

introduced from the bottom of the stove, to heat about 4 inches of

the top of the mercury only. The tube remains in this position till

the mercury boils. It is then elevated for another 4 inches and again

brought to boiling point, and so on until the end of the tube is

reached. Under this process the air and some impurities rise to the

surface of the mercury, and the tube is considered to be properly

boiled. The end of the tube is then cut off to its proper length and

inserted in the cistern, in which there is left sufficient clean

mercury to complete the barometer.

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. · 第 25 / 26 章

[49] See British patents--Winter, 1760, No. 752; Ould, 1791, No. 1842;