815.--The lower part of the barometer tu_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.

attached to a thin boxwood socket of about an inch in depth by means of

hot thin glue. The socket piece is afterwards bound over with sewing

silk, which is again covered with glue, and is finally varnished so as

to form an elastic, secure fitting upon the glass. The socket-piece is

secured to a wide boxwood collar, Fig. 390, _D_. Upon the under side of

the collar an ivory gauge peg _X_ is inserted, which forms the index

point for reading the surface of the mercury in the cistern upon the

Fortin principle.

816.--_The Cistern._--The glass sighting tube, Fig. 390 _H_, of the

cistern, through which the mercury and gauge point _X_ are visible,

is made about 1½ inches long and from 1 inch to 1½ inches

internal diameter, the glass being 1/8 inch to 1/5 inch in thickness,

ground square at its ends. The upper end of the glass fits upon the

boxwood collar _D_, with the interval of an indiarubber band to make

the fitting air-tight. The lower end of the glass tube fits upon the

boxwood collar _I_, with an interval of a turned leather collar. The

boxwood collar prolonged forms the lower part of the cistern. This

has a second boxwood collar screwed upon it, to which the leather bag

_E_ is attached by silk and glue. A stout leather capping plug is

glued upon the lower end of the bag, upon which the boxwood cap of the

adjusting screw _F_ presses for adjustment of the mercury, or to close

the tube.

[Illustration: Fig. 389.--_Mountain barometer erected for use._]

[Illustration: Fig. 390.--_Section through the cistern._]

[Illustration: Fig. 391.--_Vernier reading, showing gauge point S._]

[Illustration: Fig. 392.--_Sling case for carrying._]

817.--_The Cistern Casing_, which is of brass, consists of upper and

lower collar pieces, Fig. 390 _AA′_ and _BB′_, and their attachments.

The upper collar is fixed to the casing tube of the barometer. In the

inside of this collar a leather washer is placed, which comes above the

boxwood collar on the glass tube _D_ and makes soft contact between

these parts. The lower collar has been partly described with the

cistern. This has a brass tube _E_ screwed upon it, covering the bag

and lower part of the plug of the cistern. The lower closed end of the

covering tube is formed into a nut for the adjusting screw _F_ placed

in the axis of the tube. There are four bolts or screws _GG′_ which

bring the two collars of the cistern casing towards each other, support

the lower part of this casing, and produce a pressure between the

boxwood collar on the barometer tube and the top of the glass sighting

tube with the intervening rubber collar, so that the mercury at this

point is secured.

818.--_The Stem, or Barometer Casing Tube_, is made of brass, about ¾

inch diameter. This has a slot, of about ¼ inch in width, down two

concentrically opposite sides, from near the top of the tube downwards

for about 20 inches. The tube is graduated along one open edge next

the slot in inches and tenths, these being again subdivided to

twentieths, and figured to read from 13 inches to 32 inches of mercury,

as shown in detail for the upper part in Fig. 391. The same space is

divided into centimetres and millimetres if metrical measure be used.

Within the outer tube an inner tube of about 12 inches in length fits

telescopically to move with a soft smooth motion. This inner tube

carries one vernier at top and one at bottom, Fig. 389, _rr′_. The top

vernier, shown Fig. 391, is placed above a slot in this tube which

corresponds with the outer tube, so that the level of the mercury can

be seen below the top vernier-piece at _S_. The verniers are divided

into 50, so that, reading into the 20, they give reading 50 × 20, or

1000 to the inch. The inner tube carries a rack about 11 inches long,

which moves by a pinion fixed upon a _cock-piece_, Fig. 389 _m_, on

the outer tube in the same manner as before described for telescope

racking, art. 96. Two _stay-pieces_ placed over the outer tube hold

the slots firmly at an equal opening. A ring is placed at the head of

the barometer to suspend it in a room, to be used, if required, as an

ordinary meteorological barometer, as shown at the top of Fig. 391.

819.--_Mounting of the Barometer._--The barometer is mounted upon a

tripod formed of three light tubes with steel points, as shown Fig.

389. These screw into a collar which is packed in the cap of the

leather case. The collar has two opposite screws that screw into a

second collar, which is also held by two opposite points at right

angles to the first. The points of the screws form axes in the manner

of a Hook's joint, permitting the barometer to take a vertical position

by the superior gravity of its cistern and lower parts.

820.--_The Thermometer_, shown at Fig. 389 _t_, has its bulb brought

as nearly as possible into contact with the glass tube enclosed in the

casing tube. It is commonly divided with both centigrade and Fahrenheit

scales. Correct observation of the thermometer is necessary to be made

with every observation of the barometer, as the specific gravity of the

mercury, and consequently the height of the column, depend partly upon

this for its correct determination.

821.--_The Packing Case_, Fig. 392, is made of solid leather lined

with thick felt to fit the barometer. The legs are placed in packings

outside the case. In packing for carriage the screw of the cistern is

turned nearly home, leaving only sufficient space for any probable

expansion of the mercury from increase of temperature. The barometer

should always be carried in an inverted position, as this precludes the

possibility of air getting into it, and even tends to exclude, by the

jarring motion of carrying, any air that may have accidentally become

occluded. A strap is attached to the case for holding it over the

shoulder.

822.--_Reading the Barometer._--It will be observed that the mercury

against the sides of the tube presents an upward curved appearance, due

to the resistance of the glass to perfect contact, and the cohesion

of the mercury in what is termed capillary action. This _beading_,

as it is termed, varies according to whether the mercury is rising

or falling. It is always necessary before taking an observation to

raise the mercury, by turning the screw _F_, until its surface just

touches the peg _X_, to make observations uniform. The reading is

taken by slowly lowering the index-piece by means of the milled screw

until light is just excluded between the fore and back index surfaces,

as shown Fig. 391 at _S_, at the highest point of the surface of the

mercury. The inches, tenths, and half-tenths (·05) are read on the

scale, and the thousandths on the vernier. Thus, suppose the scale

reads 26·45 and the vernier 25 = 25 thousandths, the reading will be--

26·45

·025

------

26·475

For altitude the upper and lower stations are taken, and the difference

subtracted for difference of barometrical scale.

823.--_Difference in Altitude in feet taken from Barometrical

Inches._--Complete barometrical tables for this comparison will be

found in Molesworth's and other pocket-books in use by all engineers.

It is therefore unnecessary to occupy our space with them. A very

approximate rule may be given, which was proposed by Mr. R. Strachan in

the _Meteorological Magazine_, as follows:--

"Read the barometer to the nearest hundredth of an inch; subtract the

upper reading from the lower, leaving out the decimal point; and then

multiply the difference by 9, which gives the elevation in feet. Thus:--

Lower station 29·25 inches

Upper " 28·02 "

------

123

------

Elevation 1107 feet."

824.--_Capillarity._--For meteorological observations a quantity

must be added to the reading equal to the resistance of the tube in

capillary action to the rise of the mercury. This is greater in an

unboiled tube than in one in which the mercury is boiled. For altitude

measurements with a single barometer, or by two barometers with equal

tubes, it may be neglected, as it will be equal in all parts of the

tube. Where two barometers of different bores are used, the following

table gives the correction:--

_Correction of Capillarity to be Added to the Reading._

Diameter of Tube in Inches ·6 ·55 ·5 ·45 ·4 ·35 ·3 ·25 ·2

Unboiled Tube, Inches ·004 ·005 ·007 ·01 ·014 ·02 ·025 ·04 ·059

Boiled " " ·002 ·003 ·004 ·005 ·007 ·01 ·013 ·02 ·029

825.--_Temperature Correction._--As the mercury increases in

temperature it becomes specifically lighter, therefore rises higher in

the tube under equal atmospheric pressure. The temperature is indicated

by the thermometer, shown at Fig. 389 _t_. The expansion of mercury

for 1° Fahr. is 0·000101; but the brass tube also expands 0·0000104,

and it is the difference between the two expansions that we require,

the mercury expanding about 7·15 more than the brass. If we subtract

from the reading ·00014 of the observed altitude for every degree of

Fahrenheit above 32°, the correction will be practically very near.

Thus for a single reading--thermometer, 52° Fahr.; barometer, 30 inches

-(52 - 32) × 30 × ·00014 = ·084,

making the true reading 30 - ·084 = 29·916 inches at 32° Fahr.

Tables for correction without any calculation will be found in

Molesworth's and other pocket-books.

826.--_Gravity Correction._--The force of gravity decreases as we

ascend to a higher level in proportion to the square of the distance

from the centre of the earth. It follows that the force of gravity as

we ascend at the equator diminishes at a less rapid rate than at the

poles. Its amount is always small--on an average it may be taken at

about 0·001 inch of mercury per 400 feet of ascent.

_Time._--Humboldt discovered that the barometer varied within the

tropics at different hours of the day. This has also been found to

be general to some extent in all countries, depending upon many

conditions. It is only important for consideration of altitude

measurements, that it is advisable if possible to take the upper

and lower stations simultaneously by a pair of barometers for exact

determination of altitude.

827.--=Aneroid Barometer.=--The first introduction of this instrument

into England was by Pierre Armand, le Comte de Fontainmareau.[59]

This instrument consisted of a vacuum chamber as its prime mover. The

chamber was made a flat cylindrical box, with its upper surface of thin

metal, with corrugations covering its surface in concentric rings.

The chamber was filled with a number of spiral springs which resisted

the pressure of air, to prevent the collapsing of the corrugated

surface when the chamber was exhausted, and so placed the surface in

equilibrium with the pressure it received from the atmosphere. The

movements under various pressures were multiplied by gear work and

levers so as to make a small movement of the corrugated surface evident

in the extent of motion of an index hand reading upon a dial.

[Illustration: Fig. 393.--_Stanley's civil engineer's aneroid._]

The aneroid practically in its present form was devised by Lucien Vidie

from 1848 to 1862.[60] In this instrument the vacuum chamber, which is

a thin, flat, circular box, is corrugated equally on both sides, so as

to obtain double area of active surface under atmospheric pressure to

that of the older form. The chamber has its surfaces drawn apart by an

exterior spring, the point of communication or tension being placed at

the centre of its corrugated sides only.

[Illustration: Fig. 394.--_Perspective view of the interior of an

aneroid._]

828.--The construction of this aneroid is shown in Fig. 394, which

is of a 4½-inch instrument, aneroids made for surveying being of

two sizes, 3 inches and 4½ inches. _A_ is a solid plate of metal

1/8 inch in thickness, termed the base plate; _B_ the vacuum chamber,

circularly corrugated on both sides, made of thin, hard-rolled German

silver containing a large percentage of nickel.

829.--An axis is projected from the lower side of the chamber, of about

1/5 inch diameter. This is tapped with a screw and screwed firmly

down into the base plate with a counternut. On the upper side of the

vacuum chamber the axis is projected upwards to receive the tension

of a strong, very flexible spring _D_ above it, to be described. A

bridge-piece _EE_ of steel of strong section strides over the vacuum

chamber. This piece has a stout arm-piece projecting from it towards

_A_, which is secured to the base plate by a screw that is left open

to a hole indicated near _A_ through the outer case of the instrument,

by means of which the bridge-piece can be rocked so as to produce more

or less tension of the spring _D_ upon the vacuum chamber for final

adjustment. The bridge-piece has two points of rigid support in right

line, which form a primary--adjusted when the instrument is made--of

the spring _contra_ to the pull of the vacuum chamber. The _main spring

D_ is made of fine thin steel, carefully tempered, as broad as the

chamber. This spring is constructed so that by its elasticity it may

have sensitive movement under the pull of 10 lbs. to 15 lbs. per inch

of active surface of the vacuum chamber. It is upon the perfection of

this spring as much as upon the construction of the vacuum chamber

that the sensitiveness of the instrument depends. The upper axis of

the vacuum chamber is secured by a cross cotter pin _C_ which gives an

exact point of resistance and yet secures flexibility of the spring

at the junction. This cotter pin is placed in the centre of the three

points of support of the bridge-piece _EE_. A _lever arm G_ is fixed

to the main spring _D_ upon a stout plate of metal which is in direct

connection with the point of tension of the vacuum chamber. It is the

small movement of this lever arm (about ·01 inch at the chamber) that

gives motion to the indicating apparatus. The lever moves a cranked arm

on the axis _HK_, which communicates through the axis to a second

cranked arm placed at right angles to the first _I_. This pulls a

chain _Q_ attached to the arm _J_. The chain is wound round a small

drum fixed upon the axis which carries the hand near _R_. The drum

keeps the hand in one direction _contra_ to the pull of the chain by

a hair spring _R_ which is just sufficient to overcome the friction

of the axis of the hand _F_. The hand and drum and their fixings are

carried by the plate _M_, which is a light piece of brass projected

from a stiff standard fixed from the base plate _K_. The compound lever

apparatus described moves the point of the hand about five hundred

times the amount of movement over the first fulcrum of the lever at the

chamber.

830.--_Compensation for Temperature._--This is a somewhat difficult

matter, which is generally brought about by several modifications

of parts. Some ordinary aneroids will move upwards about 1/10 inch

of mercury by a rise of temperature of 8° centigrade only. This is

caused principally by the increase of temperature softening the spring

to render it less rigid, and the softening of the vacuum chamber to

render it more flexible or sensitive to atmospheric pressure. Some

little difference is also caused by the unequal relative expansion of

the lever, arms, spring, and chain, these parts being of steel and

brass. Compensation can be made in the lever arm _G_ by making this

curved and of two unequally expansive metals, as zinc and steel, so

that the curvature increases with increase of temperature and the

lever shortens. Compensation can also be partially made by making the

base plate in two metals--iron and brass--so as to press the standards

fixed through the two metals nearer or further apart with temperature

changes. But the whole subject is too technical to be entered upon in

our limited space, as it depends so much upon the construction of the

instrument, which is modified in various ways by different makers in

order to effect this correction.

831.--_Dial and Hand._--From the delicacy of the structure of the

aneroid it becomes evident that no two instruments can be made to

exactly the same rate of movement; therefore each instrument has to

be separately graduated when it is intended to measure altitudes with

it exactly. However close or open the scale may be, it becomes closer

as greater altitudes are ascended, the density of the atmosphere as a

gaseous fluid decreasing in geometrical progression as the altitude

increases in arithmetical progression. From this we can understand

that a vernier to the index hand can only read approximately, although

it will act fairly well at a certain point of the scale. The best and

possibly only correct method of dividing the scale is to put at first a

false scale to the instrument, and to read this scale by the index hand

with a microscope under an air-pump, compared at every half-inch of

height of the column of the mercury by the gauge attached to the pump.

When this is carefully done, a zero point is taken of the position of

the index hand at the atmospheric pressure at the time, as indicated

on the false scale. The proper scale, as it appears upon the dial, is

divided from the position of the readings of the false scale, the two

scales being superimposed upon a special dividing machine. The dial is

afterwards figured and finished.

832.--The ordinary method of reading the aneroid is to let the index

point read over the divisions. The author devised a plan, which he has

used for many years, of fixing a small plate of aluminium upon the

point of the hand, level with the scale, which is raised on a step to

read it upon its inside edge, to a fine line on the aluminium. By this

means error of parallax in reading is entirely avoided. The author also

places an adjustable magnifier to move over the index for reading.

This last improvement is now followed by other makers. A pointer also

revolves with the outer rim to show the last reading before ascent or

descent.

Instruments made with care in the points just indicated must

necessarily become expensive. Where the aneroid is to be used as

a weather glass, or even as a travelling companion to judge of

approximate heights in climbing mountains, such care is not needed,

and the instrument may be produced very cheaply of useful quality. On

the other hand, where precision is required, a delicately made aneroid

will indicate a movement of 3 feet or less in raising or depressing,

when holding the instrument horizontally in the hand and giving a light

tap on the glass with the finger-nail before reading, so as to put all

motive parts in equilibrium.

833.--_The Altitude Scale_ is generally placed near the periphery of

the dial; it is the all-important part to the surveyor. This scale is

usually set out from a mean of atmospheric pressure at sea level, taken

from Sir George B. Airy's tables, which give the extreme pressure of 31

inches barometric pressure for zero at sea level. With this pressure

altitudes are taken at intervals according to the indices tested under

the air-pump, and the intermediate divisions are graduated to scale.

These index points are shown in the table below for a few points:--

_Table of Altitude with Barometrical Scale._

Height Barometer in Height Barometer in

in Feet. Inches. in Feet. Inches.

0 31 6000 24·875

250 30·717 7000 23·979

500 30·436 8000 23·125

750 30·159 9000 22·282

1000 29·883 10,000 21·479

1500 29·340 11,000 20·706

2000 28·807 12,000 19·959

2500 28·283 13,000 19·236

3000 27·769 14,000 18·535

4000 26·769 15,000 17·853

5000 25·804

It may be generally observed that the more open the scale the less

altitude can be obtained by a single revolution of the hand; therefore

the more points can be taken per 1000 feet. Thus, with an altitude

barometer reading to 3000 feet, readings can be pointed in construction

at every 250 feet; with one of 6000 feet, at every 500 feet; and over

this at every 1000 feet.

834.--_Movable Altitude Scale._--In this the altitude scale revolves so

as to be able to set it at zero for ascending from any point. As the

barometrical scale diminishes, it is necessarily inaccurate, and cannot

therefore be used upon a surveying aneroid; but the plan is pleasant

for approximate measurements for amusement in making ascents. It is

only mentioned here for the reason that the inaccuracy of the movable

scale is not always recognised.

835.--_Adjustment of the Aneroid._--There is a screw at the back of

every aneroid somewhere under the point _A_, Fig. 394, by means of

which an aneroid may be brought to the reading of a mercurial barometer

at the position the mercury may be read. Where a good instrument has

been set by the maker to a standard barometer, it is not wise to

alter it frequently if it keeps in good working order for altitude

measurements without being again set by a standard. On the other hand,

however well the aneroid may have been made it works gradually to a

slight change, caused by the smooth wearing of parts in action. It is

well to have an aneroid, after one or two years' wear, cleaned and

adjusted by the maker. It will then, if a good instrument, work well

for many years.

836.--_Directions for Measuring Altitudes._--Turn the outer rim of the

instrument until the index carried thereby reads to the same point

as the index hand. Raise the magnifier until the reading comes into

sharp focus. Hold the instrument as nearly horizontal as possible,

and tap the case lightly with the thumb-nail two or three times, so

as to overcome any slight friction of its mechanism. This places the

action of the works in equilibrium. Write down the observation as it

now reads in the pocket-book, taking thousands from the right hand

(large figures), hundreds from the right hand (small figures), tens

from the lines to the left of this, and units from observation of the

position of the index line in the space between the last and the next

line. Say this observation reads 2465. Whether we ascend or descend,

the instrument acts similarly. We will now presume we ascend to the

height we require to ascertain, and take a second reading, 1945; the

difference between these numbers, 2465 - 1945 = 520 feet, is the

number of feet ascent. It is necessary, where exact measurement is

required, to take the reverse reading, as the atmospheric pressure

may have changed. We now descend, taking the last observation, 1945,

and find the reading at the first position 2463 instead of 2465,

that is 2 difference, which proves that the atmospheric pressure has

decreased. If we take half this difference = 1 and correct the first

deduction, 520 - 1 = 519 will give us the correct measurement, subject

only in this instance to the irregular possible fall of atmospheric

pressure, which will not in many instances, if the times of observation

have been nearly equal, be a quantity worthy of consideration. It is

not necessary to make any correction for the height of the observer

in positions above ground, as the instrument must be placed at a

uniform distance from the eye to obtain the reading. In mines it will

frequently be necessary to measure the heights from the ground at which

the observation is made.

837.--_Various Improvements in the Aneroid._--It is uncertain whether

any great internal improvements have been made in this instrument,

except by Vidie, at various times. Many attempts have been made to

increase the length of scale to obtain more open reading. These

attempts have all been in the direction of increasing the difference

of space between the fulcra of the levers or by additional gearwork,

producing thereby a greater multiplication of the small unit of

displacement of the axis of the vacuum chamber beyond the normal ×

500, which is already great. The multiplication has been taken up

to × 2000 or more. This increases the difficulty of manufacture and

certainty of permanent action. Many of these plans were tried by Vidie

and abandoned. A plan of Vidie's[61] of giving the hand three or four

revolutions, and to register this upon a spiral scale upon the dial,

also by counting on a second dial the number of revolutions, has been

repeated with slight variation by E. T. Loseby in 1860[62] and by Major

Watkin later. Vidie's plan of drawing back the hand to read the spiral

has been modified also by Major Watkin in a manner which may be a

little less frictional.[63]

[Illustration: Fig. 395.--_Watkin's extended scale surveying aneroid._]

838.--=Watkin's Extended Scale Aneroid.=--This instrument is shown

at Fig. 395, and has a very extended reading, consisting of _three

complete_ circles, in place of the usual single scale, with a hand or

pointer sufficiently long to extend across them all. In order to show

clearly which circle of scales should be read there is an indicator

attached to the movement of the instrument which causes a series of

figures (I., II., III., corresponding with the three circles) to be

exhibited through an aperture in the dial. For instance, when the

instrument is in its normal state the hand will point to the first or

outer circle, and the figure I. will appear and remain in the aperture

until the barometer falls to 27·8, where the break takes place in the

circle, as will be seen in the illustration. The hand then takes up

the reading on the second circle (where the break appears at 27·8) and

figure II. replaces figure I. in the aperture, remaining there until

the barometer falls to 25, when the reading is transferred to the third

circle, and figure III. appears in the aperture.

[Illustration: Fig. 396.--_Face._]

[Illustration: Fig. 397.--_Back._]

839.--=Watkin's New Patent Mountain Aneroid Barometer.=--This

instrument, of which both a front and back view is shown above at Figs.

396 and 397, is the invention of Colonel H. S. Watkin. The special

feature is that it can be put in or out of action as required, and

when out of action is impervious to the influence of variations in

atmospheric pressure. This relieves the strain on the mechanism of

the aneroid, as it is only put into action when a reading is required.

The lower portion of the vacuum-box instead of being a fixture (as

is the case with ordinary instruments) is allowed to rise, which is

effected by attaching to the lower portion of the vacuum box a screw

arrangement actuated by a fly nut on the outside of the case. Under

ordinary conditions this screw is released, and the vacuum-box put out

of strain. When a reading is required, the fly nut is screwed up as far

as it will go, thus bringing the instrument into the normal condition

in which it was graduated.

It has an aluminium case for lightness, is made in two sizes (3 inch

and 4½ inch), and has a sling leather case.

These plans are again on their trial. It is the author's opinion on

the subject, knowing the delicacy and skill shown in Vidie's work,

that little improvement is likely to be obtained by magnification of

the small motion of the vacuum chamber by mechanical means, which

must necessarily be by a process both delicate and highly frictional.

Attempts, he thinks, may otherwise be successfully made in the

magnification of the small motion of the hand in a frictionless manner

by optical means to obtain clearer definition.

840.--An improvement was made in the aneroid in one direction by the

late Thomas Cooke[64] by replacing the chain by a thin gold band upon,

and leading from, the drum. This obviated the small difference of rate

of displacement due to separate jointed links as they leave the tangent

of the drum. It is said, however, to cause a little springiness at

this point, where it should be very dead, which somewhat minimises the

improvement; so that it has not been very generally adopted.

841.--=Bourdon's Aneroid=, invented by C. Bourdon in 1849.[65] The

motor of this instrument consists of a flat, oval tube bent into a

circular form. This tube opens to greater and lesser curvature by

difference of external pressure upon it. The small motion given at one

free end of the tube is multiplied up by gearwork. This instrument

is found to act most delicately as a steam gauge; but experience has

shown that it is not so sensitive or durable for indicating atmospheric

pressure as the vacuum-chamber aneroid last described.

842.--=Hypsometer=, _or Boiling-point Thermometer_.--That water or

any other liquid boils at a certain temperature, according to the

amount of atmospheric pressure surrounding it, is easily observed by

placing a cup of boiling hot water under the receiver of an air-pump.

At first the surface will remain still, but as the pressure of the air

is pumped off it may be made to boil time after time until it arrives

at a low temperature. The temperature at which the water boils as the

air is rarified may be easily followed by observation of a thermometer

immersed in the cup of water; and at the same time, if a barometer be

placed in connection with the receiver it will indicate the pressure,

from which the scale of differences may be practically made. For the

civil engineer this instrument, accompanied by the aneroid, is in every

way superior to the mountain barometer, which must necessarily have a

three-feet tube, as the hypsometer is much lighter, more portable, and

less liable to injury, and perhaps, from the uncertainty of keeping a

pure vacuum in the barometer, safer as a means of observation.

[Illustration: Fig. 398.--_Hypsometer, or boiling point thermometer._]

[Illustration: Fig. 399.--_Case for hypsometer._]

843.--The modern form of instrument is shown in Fig. 398. The boiler

shown immediately over the lamp is filled about half full of rain water

by lifting off its covering tube _C_. The covering tube has a smaller

tube, about 3 inches long and ½ inch diameter leading upwards from

it, through which the thermometer bulb is passed into the boiler. This

tube is covered by the _jacket J_, formed of four telescopic tubes

that are extended, as shown in the figure, for use, but which close

up quite compactly when the instrument is put in its case. The upper

drawer of the jacket tube is about ¾ inch diameter, so that the tube

enclosing it passes over the leading tube when the apparatus is closed.

The lamp, which is filled with pure spirit, draws out from the bottom

of the outer casing _O_. It carries a wick holder with screw cap, and

this again has a covering cap to secure the spirit perfectly when the

instrument is carried about. The inner casing _A_ is perforated with

holes to admit air at the level of the body of the lamp. When the lamp

is lighted and complete for use it is placed vertically in its outer

case _O_, which is jointed in two parts and perforated by large holes

surrounding it top and bottom: the bottom holes are covered with wire

gauze. By this arrangement the flame is not seriously disturbed by

wind or rain.

844.--_The Thermometer_, upon which the action of the instrument

depends, has a stout stem about 6 inches long and ¼ inch diameter,

with a very fine, flat, oval bore about ·01 inch wide and not much

over ·005 inch in thickness. The stem is divided very openly for about

25° below 100° centigrade, each degree being subdivided into 10, below

212° if Fahrenheit scale be used, with each degree divided into 5. The

divisions are filled in with lamp-black, and the stem is backed with

white enamel to give clear reading. The thermometer _T_ when in use is

surrounded by a vulcanized indiarubber collar _I_ which slips over its

stem to adjust it to position in the boiler tube as shown.

In placing the thermometer in its jacket, it is important to hold it

erect to be sure it passes into the leading tube from the boiler,

as there is generally just room for it to catch by the side of this

tube, where if it were pressed down it would break the bulb. When

the thermometer is out of use the rubber collar is removed, and the

thermometer is placed in a tubular metal case which is lined with

indiarubber tubing, so that no jar can injure it.

The whole apparatus when closed is carried in a solid leather case,

which contains divisions for the separate parts of the apparatus, and a

strap for passing over the shoulder for carrying it. Fig. 399 shows the

general form of case.

845.--_Use of the Hypsometer._--Saussure calculated, from data of his

ascents of Swiss mountains, that the temperature of boiling water

decreased 1° centigrade for every 978·5 feet of ascent, where the

mean temperature of the atmosphere was estimated at 0° centigrade, or

freezing point. If the temperature of the surrounding atmosphere be

taken as 5·5° centigrade, the ascent per degree of that scale is 1000

feet. This becomes, therefore, the most convenient data to calculate

from, allowing 3·9 feet per 1000 per degree centigrade for temperature

above or below 5·5° centigrade at any two stations of observation,

of which the difference of level is required. Thus:--If at the first

station the temperature of air be 15·6 centigrade, the boiling point

95·5° centigrade; second station temperature of air 14·1° centigrade,

boiling point 94·2° centigrade, the barometrical pressure of the lower

station being taken as a constant, or referred to the aneroid for

correction; then 15·6° - 5·5° = (9·1) (3·9) = 29·2 + dif. 95·5 - 94·2 =

(1·3) (1000) = 1329·2 - dif. external temperature (15·6 - 14·1) (3·9°)

= 1323·4 difference of level in feet.

Sometimes the thermometer is divided to Fahrenheit degrees, subdivided

into 5 to read by interspace and line to ·1° F. This may be changed to

centigrade for use of the above formula by taking 32° F. lower than the

reading and multiplying by 5/9. Thus--

60° Fahr. = 5/9 (60 - 32) = 15·55° centigrade.

The calculation proposed by Lefroy is, however, simpler for Fahrenheit

scale. To allow for diminution of boiling temperature, with height

from 212°, with barometer at 30 inches, take 511 feet of altitude for

the first degree and add 2 feet for each succeeding degree. Thus,

taking height of first station = h corrected for 212° Fahr., 30 inches

barometer, remembering decrease of barometrical pressure acts the same

as increase of height. Then--

211° boil point _h_ + 511 feet.

210° " _h_ + 511 + 513 = _h_ + 1024 feet.

209° " _h_ + 511 + 513 + 515 = _h_ + 1539 feet.

FOOTNOTES:

[59] British patent, No. 10157, April, 1844.

[60] British patents--No. 13332, November, 1850; and No. 682, March,

1862.

[61] Patent, No. 13332, May, 1850.

[62] Patent No. 3454, December, 1862.

[63] Patent No. 3425, March, 1886.

[64] Patent No. 2714, October, 1865.

[65] Patent No. 12889, December, 1849.

CHAPTER XIX.

MISCELLANEOUS SURVEYORS' AND ENGINEERS' INSTRUMENTS, APPLIANCES,

AND ACCESSORIES--CROSS STAFF--MECHANICS' LEVELS AND CLINOMETERS--

BONING RODS--FOOTNER'S RAILWAY GAUGE--GIRTH STRAP FOR TIMBER

MEASUREMENT--GIRTH TAPES--TIMBER MARKER--SLASHING KNIFE--BILL-HOOK--

RECONNOITRING GLASS--TELESCOPE--SUN SPECTACLES--WHISTLES--PIONEER

TOOLS--SKETCH BLOCK BOOK--CAMERA--GEOLOGICAL TOOLS--WEALEMEFNA--

OPISOMETER--BOUCHER'S CALCULATOR--SLIDE RULES--FULLER'S CALCULATOR--

ENGINEERS' POCKET BOOKS--CHRONOMETER--OUTFITS.

846.--=Cross Staff.=--Those of Tycho Brahé and of Gunter were very

elaborate affairs, consisting of a pair of notched cross-bars sliding

on a divided rod which gave directions to form any angle in a quadrant

from the eye by sliding the bars further from or nearer to it. The

surveying cross staff, after better instruments were invented to take

angles, became a cross at right angles, sawn upon a disc of wood and

supported upon a staff which was pressed into the ground. This was used

by looking along the saw cuts to take offsets to the chain, and for

setting out buildings. The fixed cross-head was much improved by making

it a cross of metal with turned-up ends, down the centre of which

vertical saw cuts were made at right angles, Fig. 400. This, in the

author's opinion, is still the best form.

847.--Cylindrical heads superseded the open cross-head. The modern

instrument in use is the French form, Fig. 401, which is made of

octagon brass tube. This is cut with alternate sight slit and opposite

window, with vertical hair on each of four rectangular sides of the

octagon. On the other four sides there are plain slits subtending

45° to those first mentioned. The octagon tube is mounted upon a

socket-piece which fits upon a conical pointed staff. The defect

of this cross-head is the closeness of the slits, due to the small

diameter of the tube, which renders the direction given for sighting

uncertain.

[Illustration: Fig. 400.--_Open cross-head._]

[Illustration: Fig. 401.--_French form._]

[Illustration: Fig. 402.--_Adjustable cross staff head._]

848.--=Adjustable Cross Staff Head.=--The cross staff head is sometimes

made cylindrical, in two parts, Fig. 402. The upper part is centred

upon the lower so that the upper series of sights move to any angle in

relation to the lower. In this construction a wheel is cut about the

axis of the upper part, which works into a pinion in the lower part,

so that the upper part may be revolved horizontally by it. The meeting

planes of the two cylinders are divided, the lower into degrees and the

upper with a vernier. The vernier is almost an unnecessary refinement,

as the sighting distance from slit to hair is only about three inches,

and no very great exactness can be obtained in the sighting. This

instrument has commonly a magnetic compass upon the upper surface.

It is about as expensive as the semi-circumferenter, shown Fig. 232,

p. 347, and very inferior to that instrument owing to the extreme

closeness of the sights. Its use is obvious.

Many of the following articles, briefly described, may be beyond the

direct province of this work; but the utility of these implements for

completing the equipment of a surveyor or engineer for special work it

is hoped will be sufficient apology for their introduction. The subject

can scarcely be treated except in a desultory manner.

849.--=Mechanics' Levels.=--In crowded Eastern cities, in levelling

through close passages, in many cases the surveyor has to resort to

mechanical levelling to carry his levels through. Mechanics' levels are

too well known to need much description. The ordinary good kinds are

made from 6 inches to 18 inches long, generally of rosewood, as this

wood is very hard and stands well. They have a brass plate at the top,

and tips of the same metal at the base. The illustration, Fig. 403, is

of a 12-inch level. The level tube, which is of blown glass, is fixed

in plaster of Paris, and the upper plate screwed down over it.

[Illustration: Figs. 403, 404.--_Mechanics' Levels._]

850.--=The Author's Hand Level= is shown Fig. 404--12 inch. This is

made of a casting either of iron or brass. The level tube is ground to

curvature and is somewhat superior to the ordinary run of this class of

work. The level tube is fitted with ball socket at one end and stiff

spring fitting at the other, which is adjustable, so that the tube may

be easily replaced if broken.

These levels are commonly fixed upon a stout fir straight-edge of about

5 feet to 10 feet in length by the lugs at the ends. The level is taken

by blockings upon the ground. Corrections of error, both in level and

straight-edge, may be made for any considerable distance by reversing

the forward and backward position of the level with its straight-edge

alternately.

851.--=Square Level--Circular Level.=--Fig. 405 represents a very

useful class of level for setting up some instrument stands, plane

tables, etc., in which a pair of level tubes are placed at right

angles to each other. It is generally made very small--1½ inches

square only. A circular level, the upper surface of which is formed

of a worked concave glass, was lately very popular, and is still used

to a small extent. As the spirit cannot be hermetically sealed in, it

evaporates, and this level soon fails. Mr. J. J. Hicks has taken out a

patent for a hermetically sealed circular level, described p. 96, which

appears to answer very well.

[Illustration: Fig. 405.--_Square level._]

[Illustration: Fig. 406.--_Surface level and clinometer._]

852.--=Incline Level.=--For laying railway rails and drainage works

the bubble is frequently made adjustable by the tube in which it is

contained being hinged at one end and fitted in slides to rise with a

screw at the other end, as shown Fig. 406. A scale of percentage of

inclination _S_ is commonly divided upon the adjustable end. The tube

is raised or lowered by the key _A_, which is removed after setting

and cannot be tampered with.

[Illustration: Fig. 407.--_Stanley's sight for mechanics' levels._]

[Illustration: Fig. 408.--_Section._]

853.--=Sighted Levels.=--A mechanic's level is commonly made with a

hole longitudinally through it of about ½ inch diameter, closed at

one end, except a small hole of 1/30 inch or so, and a cross upon a

piece of glass at the other end. This plan permits a sight to be taken

through it which gives an approximate level. Occasionally the same form

of sight as that described is hinged on the top surface at each end of

the level. The author has found a better plan of sighting to be given

by a pair of sights placed on a centre upon the ends of the level to

turn up when required for use, as shown Fig. 408, _P S_ one of the

pair of points. This, when turned up, shoulders on the stop-piece _A

B_. The stop-piece is made of sufficient thickness to admit the point

in the hole near _B_ for protection when it is folded away out of use.

The section of the level, as shown by the end view _D_, is the same

as that of the level, Fig. 404. Very fair accuracy may be obtained by

making these sights appear coincident upon a distant staff or rod.

[Illustration: Fig. 409.--_Boning-rod._]

[Illustration: Fig. 410.--_Boning-rod with standard._]

854.--=Boning-Rods=, Fig. 409. These are very commonly employed with

mechanics' levels. They are made somewhat like a stout T-square of 3

feet to 4 feet in length, about 3 inches in width, and ¾ inch in

thickness both of the stem and head. They are at first placed at a

distance apart, 9 or 10 feet, and a straight-edge of this length is

laid from one to the other, upon which the mounted level is afterwards

placed, the boning-rod being tapped down in the ground till the bubble

is in the centre of its run. A third boning-rod is then placed at

the same distance as the first pair, and the straight-edge with the

level upon it is reversed end for end. This, if the work be fairly

down, leaves the two outer boning-rods level, however imperfect the

straight-edge and level may be, if the run of the bubble be taken

correctly. By removing the central boning-rod from the outer pair

of rods, levels may be continued by sighting over them, or _boning

forward_ as it is termed. On the Continent boning-rods are commonly

fixed by driving a separate standard into the ground, which has a pair

of brass slings by its side to hold the rod, Fig. 410. This is a

much neater plan than that in common use of blocking the rod up with

stones. Boning-rods are also sometimes used conveniently with a proper

surveying level, from the tops of water-pipes, etc.

[Illustration: Fig. 411.--_Footner's railway gauge and clinometer._]

855.--=Railway Gauge=, combining level and clinometer. This high-class

gauge, Fig. 411, is the invention of Mr. H. Footner, C.E., late of the

London and North-Western Railway. It is formed of a bar of Spanish

mahogany neatly shaped. The end fittings are of steel. The gauging

part is formed of two turn-up steel flap-pieces with back stops. A

spirit level is sunk in the end fitting, shown in the figure towards

the left hand. The clinometer is formed by a gun-metal pin of ½ inch

in diameter; 9 inches long. This slides perpendicularly in a spring

fitting sufficiently stiff to support the gauge, and is made to fall

on the centre of the rail. The pin is divided into inches and eighths.

When it is out of use it slides up the end of the gauge and leaves the

whole instrument smooth and portable to carry open or go into a leather

case. Its use is implied.

856.--=Timber Girth Strap.=--The direction for removal and estimate of

the value of timber often falls into the hands of the surveyor. The

height of standing timber may be taken by a long rod, or a pair united

by a link, art. 775, or by the apomecometer, art. 693. The girth is

most conveniently taken by a leather girth strap, of which there are

various patterns: but that illustrated below, Fig. 412, is perhaps the

most popular form. This strap is made of two straps of bullock's hide 1

inch wide, thinned down to about 1/8 inch in thickness; the two pieces

are stitched together to make it 12 feet to 14 feet long. The strap is

divided by lines into inches, but figured in units at every 4 inches

= single inches of quarter-girth. The figures and lines are stamped.

A brass weight, shown at one end of the strap, is thrown by the strap

with a swing round the standing tree, and encompasses it in a second of

time. The weight is caught by the hand and the strap brought up to it

to read the quarter-girth. The _quarter-girth_ gives roughly the equal

sides of a square; as, for instance if a quarter-girth reads 10, the

size of the tree is 10 × 10 = 100 inches, or 8·4 cubic foot-inches per

foot run.

[Illustration: Fig. 412.--_Leather girth strap with throwing reel._]

Some surveyors prefer a hook instead of a weight, as being more

convenient to measure close timber. This is shown Fig. 413. The hook is

stuck into the bark and the tree is girthed by walking round until the

hook is met.

[Illustration: Fig. 413.--_Leather girth strap with clutch hook._]

857.--=Girth Tapes=, similar to measuring tapes, Fig. 349, p. 506, are

occasionally used, but these are more convenient for felled timber.

Tapes for the purpose are made from ¾ inch to 1 inch wide, and 6, 12,

and 24 feet long. They have the ordinary feet and inches on one side

and quarter-girths on the other.

It is customary to allow 1 or 1½ inches, and sometimes more for

bark, according to the species of tree and the custom of the country.

858.--=Marking off Timber.=--For this a special tool with a gouge

point, Fig. 414, and strong buck-horn handle, termed a _timber-marker_,

is used for standing timber intended to be felled. The contents of the

tree are sometimes marked with the marker upon it if for sale, good

bark allowance being made in cases of difficulty of extraction from the

forest, etc. A plain knife is usually put with the marker, which is

useful as a food knife.

[Illustration: Fig. 414.--_Timber marker, nearly full size._]

859.--The author makes a very small, neat surveyor's knife, with

marker, for the waistcoat pocket, Fig. 415, which combines--_M_ tree

marker (small); _S_ screw-driver for small screws of instruments; _P_

tommy-pin for turning capstan heads; _F_ file for sharpening lead of

pencil, when this is used for the field-book; and _E_ _R_ two penknife

blades. The knife is similar to the author's architect's knife, which

is well known. The tree marker is not strong enough for constant work.

[Illustration: Fig. 415.--_Surveyor's pocket knife._]

860.--=Slashing Knife--Bill-Hook--Axe.=--In new countries where sight

way has to be obtained for the survey through forests and jungles, one

or more of the tools illustrated next is most valuable as a part of

the surveyor's equipment. The slashing knife, Fig. 416, which is over

a yard long, wielded by a strong man will remove light brushwood very

quickly. Where the wood is close and of larger growth the bill-hook,

Fig. 417, is better; and with thickset timber the axe becomes

necessary. The well-known Canadian axe is found to be the best.

[Illustration: Fig. 416.--_Slashing knife._]

[Illustration: Fig. 417.--_Bill-hook._]

861.--_Hedging Gloves--Iron Hooks for Climbing Trees._--For clearing

land to avoid spines hedging gloves are generally used; these are made

of soft horse-hide, and although pliable resist thorns to a great

extent. Clutch hooks are also very convenient to climb trees, to look

forward for the easiest direction for sight way.

862.--=Rods for Measuring Standing Timber.=--These are generally made

25 feet long, jointed in 5 feet lengths, similar to a fishing rod, but

much stiffer. The rod is set by the side of a tree to be measured and

observed from a distance where the first breech cuts its length.

[Illustration: Fig. 418.--_Reconnoitring glass, India pattern._]

863.--=Reconnoitring Glass.=--At present it is customary to use a

binocular field-glass in preference to a telescope. The telescope

gives greater penetration from its higher power; the field-glass is

preferred for its wider field of view. The field-glass the author has

supplied to the Indian Government has neutral-tint glasses centred on

the eye-pieces to take off the glare when looking towards the sun,

Fig. 418. These have also hinge joints between the pair of bodies,

which permit adjustment of distance of centres to the distance of the

eyes. The object-glass should be 1¾ inches, not over this. Where a

telescope is used, the 30-inch--the original, not the present--India

military telescope is to be recommended, Fig. 419. This is portable,

has a sling case and a good 2-inch object glass. For lightness,

aluminium bodies are preferred by many for both field-glasses and

telescopes; at present the price of this metal is very low, so that it

is probable it may become in a short time general for the purpose.

[Illustration: Fig. 419.--_Army telescope._]

864.--=Prism Binoculars.=--These will be found a great improvement on

the old form of field-glasses, as owing to the optical arrangement a

high power is obtained combined with a larger field of view and good

illumination. Fig. 420 shows the most modern form with all refinements;

hinged body, central focussing and separate focussing to suit each

eye. It has a very compact and strong body, and the size magnifying 8

diameters or about 64 times weighs only 13 ozs.

[Illustration: Fig. 420.--_Stanley's prism binocular._]

865.--=Dome Spectacles--Bogles.=--Spectacles of neutral tint are most

comfortable for general wear in sunny or snowy countries. The dome or

globular form is generally preferred. Where there is hot dust gauze

sides are to be preferred. There is a very cheap form with gauze sides,

which holds on the head by an elastic band, termed _bogles_. These are

rather hot to the face, and the band after a time becomes sticky. The

spectacle form is much better. The glasses are made in various shades

to choice: some very dark or even black, the latter being made for

viewing and tending arc lights.

866.--=Whistles= made very powerful are much used in exploring abroad

to bring the party together, and for signalling generally by sound,

using the Morse signals, art. 803.

867.--=Pioneers' Tools.=--A small set of these is often very useful

to the surveyor in new forest countries. The common set consists of a

claw-hammer, wood-chisel, stone-chisel, pincers, screw-driver, gimlet,

and brad-awl. The leather case is 8 by 4 by 2½ inches; it weighs

1¾ lbs. with strap. This may be supplemented by a small American

saw, cutting both edges, about 20 inches long, and the axe previously

described, with a few pounds of wire nails. The tools serve for marking

trees or rocks, erecting signals, temporary covers, etc.

868.--=Sketch Block Book--Pocket Book.=--In reconnoitring no better

information can be given of a track than forward sketches from

commanding station to station. Sketch books about 7 inches by 5 inches

are generally found sufficient. The drawing-paper should be thin, and

the pocket large enough to contain all the separate sheets as they

are taken off by the penknife after completion from the block. The

sketches may be made with pencil, or a fine fountain pen; or if the

surveyor be a colourist a light box of moist colours and a water bottle

will often leave pleasing sketches as reminiscences. Pocket-books with

section lines to 1/8 inch or 1/10 inch scale are sometimes used to give

approximate plans to scale of buildings, etc., where required, as well

as the ordinary field-book record.

869.--=Camera.=--Recently the camera has been much used for

reconnoitring. These are now made very light and portable to take ¼

plate or 3 × 3 inch films, either on rollers or in separate films.

870.--=Cement Testers= are made in various manners, generally to test

the cohesion of the cement as a homogeneous hard body. Mr. Mann's

cement tester, Fig. 421, goes on another principle--it tests the

adhesion of the cement to stone, which appears to the author to be its

most important function; it is always hard enough.

[Illustration: Fig. 421.--_Mann's cement tester._]

871.--=Watson's Improved Vicat Needle= is a most refined and accurate

instrument for determining the time taken by cement in setting. The

cement is placed in the circular container shown in the illustration,

and the weighted needle is lowered into it by means of the handle

at the top. The depth of penetration is shown in millimetres on the

divided arc.

[Illustration: Fig. 422.--_Watson's vicat needle._]

872.--=Geological Tools.=--_Acid-bottle--Blow-pipe--Touch-stone._--

Where countries are prospected for railways it often becomes important

to examine the rocks, both to detect the softer rocks for cutting and

to find limestone suitable for mortar. A geological hammer, weight

about 2 lbs. to 3 lbs., is the ordinary tool. This, with a chisel

and sailcloth bag with strap, is all the necessary appliance. In

searching for limestone a small bottle of sulphuric acid sewn up in

a leather case is useful. A dipper is blown on the stopper of the

bottle, and a single drop of acid will detect limestone by the bubble

of froth it produces. Where minerals are to be examined, a small

blow-pipe apparatus is necessary. This should be accompanied by a book

of instructions. Where the surveyor has not been trained to use the

blow-pipe, one with constant blast should be employed. For examination

for precious metals a touch-stone and two-acid bottle--sulphuric and

nitric--for silver and gold, are useful. The metal is merely rubbed on

the stone and the acid applied. If the metal is base the acid removes

it from the surface of the stone. If precious it removes other matter

and leaves it visible.

873.--=Wealemefna--Opisometer.=--The wealemefna is a very neat form

of space runner invented by Mr. E. R. Morris, which is found a very

convenient instrument for measuring distances on maps in prospecting.

It is very small and light, and may be, if desired, attached to the

watch-chain. It gives distances run over in inches and eighths, to be

afterwards calculated to the scale of the map, Fig. 423. The opisometer

for the same purpose, Fig. 424, is formed of a spur wheel at the end of

an ivory handle running upon a screw. This instrument gives measurement

by reversing its run upon the scale of the map.

[Illustration: Fig. 423.--_Wealemefna._]

[Illustration: Fig. 424.--_Opisometer._]

874.--Boucher Calculator, the invention of M. Alex. E. M. Boucher,

engineer, of Paris.[66] This is one of the most convenient pocket

calculators that a civil engineer can desire, being only of the size

of an ordinary watch. The instrument was formerly made in France for

this country in a very slovenly manner. It is now made in London by the

author, of sound work and accurate centring, Fig. 425. It has face back

and front. The front one, which is shown in the illustration, carries

logarithmic scales of sines, numbers and square roots, and is made to

revolve by turning the milled head placed under the handle, as the

winder of a keyless watch. The back dial, which is fixed and does not

revolve, has upon it a scale of equal parts giving the decimal parts

of logarithms, and a logarithmic scale of cube roots. There are three

index hands, one fixed on the side of the case over the front dial,

as shown in Fig. 425, and one on each end of the central axis made

to revolve simultaneously over the back and front dials by means of

the milled head at the side of the case. Any operation involving

multiplication, division, proportion, powers or roots can be performed

approximately with great rapidity by the aid of this calculator, and it

is practically as simple to use as an ordinary slide rule, as will be

seen from the following explanation of its use:--

[Illustration: Fig. 425.--_Boucher's calculator._]

[Illustration: Fig. 426.--_Stanley-Boucher calculator._]

Multiplication, using the second circle of divisions from the outside

of the front dial:--Bring the first factor under the fixed index, set

the movable index to 1, then bring the second factor under the movable

index, and the product will be found under the fixed index.

Division is performed on the same scale as follows:--Bring the dividend

under the fixed index, set the movable index to the divisor, then bring

1 to the movable index, and the quotient will be found under the fixed

index.

For proportion the second circle is also used:--Set the first factor

under the fixed index and set the movable index to the second one, then

the proportionate equivalent of any number brought under the former

will be found at the movable index.

Square roots, using the same scale:--Bring the number under either of

the indices, and the square root will be found upon one of the two

inner circles of the same dial.

Cube root:--In this case it is necessary to first bring the 1 on the

front dial under the fixed index, then set the movable index to the

number, and the cube root will be found on one of the inner circles of

the back or fixed dial.

To use the trigonometrical dial:--Bring the needle of this dial over

the angle of which the sine or tangent is required, and read upon the

other dial (indicated by the needle) the natural trigonometrical line

upon the inner circle, or its logarithm upon the outer circle.

The book of instructions supplied with the instruments, written by

Professor George Fuller, C.E., for the author, gives all directions for

working and also gauge points from which calculations are made as with

the slide rule.

875.--In reduction of factors of a calculation collectively Boucher's

calculator may take more than one turn or less than unity. The author

has added a central index to record the number of turns. This is said

to be of great value for the perfection of the instrument, Fig. 426.

876.--=Slide Rules=, of which there are great varieties, are of too

complex a nature to discuss, except very briefly, in our limited

space, particularly as general descriptions have been often given. The

ordinary logarithmical scales of Gunter (1619), known as _Gunter's

lines_, are placed upon most slide rules. The arithmetical lines are

lettered _A_, _B_, _C_, _D_, and _E_. _A_ and _B_ are alike: these are

technically termed _double radius log. lines_. They are used for all

processes of multiplication and division. _C_ and _D_ are also alike

and are termed _single radius log. lines_. They are used together for

ordinary multiplication and division, and in conjunction with A and

B scales for squares and square roots. The _E_ line, not originally

a Gunter's line, but found early in the century on several rules, is

termed a _triple radius log. line_. The numbers of the divisions on

this line are the cubes of the numbers of the corresponding divisions

of the _D_ line, with which it generally works. All these lines work

reciprocally together, performing the most complex calculations by

simply setting them to numbers or gauge points of which given solutions

are required, as for instance, the first four lines in combination

give answers to such questions as:--To divide by a number two numbers

multiplied together, one of which is squared; to divide the product

of two numbers by the square of a third number, etc., each of which

calculations is performed at a single setting. By inversion of the

slide _A_ to _C_ the reciprocal of a given number is found, also the

mean proportional between two numbers, the fourth term is inverse

proportion, etc. Trigonometrical calculations are performed by the

lines of sines, tangents, etc. Instructions are to be found in the

books supplied with the rules, and as a part of many works. Among the

most complete books may be mentioned "The Slide Rule," by R. G. Blaine,

M.E., and "The Slide Rule," by Chas. N. Pickworth. These both contain

very full information on the subject.

877.--_The Slide Rules_ in most general use are A. Nestler's and A.

W. Faber's. Both these well-known firms make a very complete series,

applicable to a great variety of technical calculations.

878.--The reviser has recently completed from the designs of the author

an entirely automatic dividing engine for these rules, which is the

only one in existence.

A great number of slide rules are made for special purposes only: some

of these are very useful to the civil engineer.

879.--_Hudson's Slide Rules_ give strength of shafts, beams, and

girders; pump duty; and computation of horse-power in engines.

880.--_Honeysett's Hydraulic Slide Rule_ gives discharge of water from

channels and pipes of different forms and inclinations.

881.--_Tacheometrical Slide Rules_ with scale of sine^2 and sine ×

cos. for calculating the horizontal equivalents and vertical heights

from tacheometrical observations. These are made either for use with

instruments divided sexagesimally to 360° or centesimally to 400.

882.--_Sheppard's Slide Rule_ has duodecimal lines, double reading, for

squaring and cubing timber.

883.--_Young's Slide Rule_ is designed for squaring and valuing timber

simultaneously, which operations it performs in a very expeditious

manner.

884.--Essex's Slide Rule is the best for calculating the rates of

velocity and discharge from sewers, water mains, channels, and culverts

of different forms, as it works with all formulæ.

885.--=The Slide Rule of Prof. Geo. Fuller, C.E.=, Fig. 427, presents

perhaps the highest present refinement of this class of rules, capable

of greatly facilitating the numerous arithmetical calculations of the

civil engineer. Its range is greater than most calculating machines,

and besides the operations of multiplication and division, squaring and

cubing, results requiring the reciprocals, powers, roots, or logarithms

of numbers can be quickly and easily worked out by its use.

[Illustration: Fig. 427.--_Professor Fuller's calculating slide scale._]

The rule consists of an outer cylinder that can be moved up or down,

and turned round upon the cylindrical axis which is held by the handle.

Upon the outer cylinder a single spiral, logarithmical scale is

continued from end to end, the total length of which makes the scale

500 inches long. This is graduated into 7250 divisions. One index is

fixed to the handle. A second index is attached to the inner tube

blocked out by a flange to read upon any part of the scale; so that

altogether there are three tubes which work together telescopically,

by means of which the indices may be set to any position on the

graduated cylinder. Stops are placed so that the indices may be brought

to zero. By these means, the indices being set to any of the gauge

points, the logarithmical scale, moving by itself, will maintain

the same proportion for any numbers. In this rule a single log.

radius is repeated by coincidence of indices, so that its scale of

divisions, 41 feet 8 inches long, if compared with an ordinary double

radius slide rule, becomes equal to a slide rule of 83 feet 4 inches

long. The ordinary 12-inch slide rule has about 80 divisions to each

radius, so that it is easily seen how much more exact quantities may

be brought out with a rule of 7250 divisions. It is a most valuable

rule for calculations for the tacheometer. Copious tables of gauge

points for civil engineers are printed upon the central tube, which is

supplemented by a book of instructions. The value of this rule has been

much extended by scales to facilitate subtense calculations, by Mr. W.

N. Bakewell, C.E., in the "Fuller-Bakewell" slide rule.

[Illustration: Fig. 428.--_Improved Fuller's slide rule._]

An additional improvement, as shown at Fig. 428, has now been effected

in these instruments by adapting the case to support the rule when in

use, thus overcoming the objection of being always obliged to hold it

in the hand.

The use of Professor Fuller's rule is, however, confined to

arithmetical computations. The numerical solution of formulæ comprising

trigonometrical functions can only be performed by extracting, with

considerable loss of time, the values of these functions from a

book of tables. To do so requires a certain effort of mind with its

consequent risk of mistakes. This limitation has restricted its use

in a considerable body of calculations, such, for example, as in

the computation of the co-ordinates of surveys from the lengths and

bearings of their lines, a method of plotting which is very largely

used by land surveyors at present; in astronomical computations; in

civil and mechanical engineering, etc.; the use of logarithms being

preferred on the score of speed, although the degree of accuracy

attained with Professor Fuller's rule is amply sufficient in the large

majority of cases.

[Illustration: Fig. 429.--_Barnard's co-ordinate spiral slide rule._]

886.--=The Co-ordinate Spiral Slide Rule= has been designed to meet

these requirements by Mr. H. O. Barnard, A.C.H., F.R.A.S., etc.,

Superintendent of Trigonometrical Surveys, Ceylon, Fig. 429. Like

Professor Fuller's rule, upon which it is an improvement, it enables

the user to perform with speed and accuracy arithmetical computations

involving multiplication, division, proportion, continuous fractions,

powers, roots, and logarithms; but in addition, the natural and

logarithmic values of trigonometrical functions of any angle can

be determined by inspection with the same accuracy as in numerical

computation, while the products, quotients, etc., of these functions,

by lengths or numbers, integral or fractional, are obtained with equal

ease, rapidity and precision. The scope of its operations will be

gathered from the examples which are given to illustrate its use in the

instructions supplied with the rule.

[Illustration: Fig. 430.--_Thacher's slide rule._]

Although the co-ordinate spiral rule, as all varieties of slide rules,

is based primarily upon the theory of logarithms, a knowledge of that

theory is by no means essential to its practical use.

887.--=Thacher's Slide Rule.=--Fig. 430. This contains a shorter scale

than Professor Fuller's, and the system is not quite so simple. Full

printed instructions are given in the book supplied by the inventor,

Mr. Edwin Thacher, of Pittsburg, U.S.A., or of the author, who is his

agent for this country. The original divisions of this rule were made

by the author. The scale is manufactured in the United States. There

appears to be found some difficulty in its construction to keep the

scales to true length and get them to exact position.

888.--=Pocket Sets of Chain Scales.=--These are made 3, 4, 5, and 6

inch. Three of 6 inch form the ordinary set. The chain scales, if three

only, are 10, 20, 30, 40, 50, and 60; if six they generally contain

the same scales with feet equal to the links. An extra scale with

the ordnance or other scale of the country is found also useful for

measuring from maps or plans. Some civil engineers prefer the pocket

scales made wide with quite square ends, to be used as offsets or for

sketching. These scales are generally made in ivory and placed in a

light morocco or Russia leather case. The numbers of divisions of the

scales should be stamped on the ends to prevent the wrong scale being

drawn from the case.

[Illustration: Fig. 431.--_Biram's anemometer._]

[Illustration: Fig. 432.--_Lowne's anemometer._]

889.--=Anemometers= are used by mining engineers for testing the

ventilation of mines. The original and best known form is that of

Biram, Fig. 431. This instrument is held in any current of air, and the

velocity of the current is registered by the motion of oblique fans,

by means of ordinary decimal gearwork on five dials giving feet and

multiples by 10. Lowne's anemometer, with the author's improvements,

Fig. 432, is of similar principles of construction, but it is arranged

in portable form to go in a pocket case. Another well-known form of

anemometer is built upon the same principle, but of cubical form. It

is customary to take the velocity of the current for one minute by a

watch, there being a detent provided in most instruments to start and

stop the motion of the hands upon the dials.

890.--=Books of Tables and Formulæ.=--Few British Surveyors are

without Molesworth's pocket-book. This contains all the useful

tables and notes of reference valuable to the civil engineer in his

ordinary work--weight, 5 ozs. Many pocket-books have been written on

the same plan. Hurst's pocket-book contains all matters of reference

for the town surveyor among buildings. Trautwine's _Civil Engineer's

Pocket-book_ (American) is the most complete, but it is of double the

weight of the Molesworth. Spon's _Engineers' Tables for the Waistcoat

Pocket_--weight, little over ½ oz.--is a very useful little book. Of

_Traverse Tables_ both Gurden's and Boileau's are comprehensive and

reliable. There are several pocket-books of _Curve Tables_, those of

Cutler & Edge, Beazeley, and Kennedy & Hackwood being perhaps in the

most general use.

891.--=Technical Books--Ordnance Maps= are published on special

districts and subjects which are often relative to the country or the

special conditions of work abroad and at home for minerals, etc. It is

very useful to possess such of these as may be required, and the note

is only made here as a reminder.

892.--=Sling Case for Drawings.=--The most convenient method of

carrying maps or drawings for public works in execution is to have a

solid leather case similar to a telescope case. This is best if made

with the cap or lid of the same length as the body: it can then be

drawn out any distance according to the length of the rolled drawing.

If thought more convenient, and the map or drawings are heavy, a strap

may be added to pass over the shoulder, Fig. 433.

893.--=Chronometer.=--This may be any form of watch with compensated

escapement. At present the prices run high for this class of work; but

from the simplicity and moderate certainty of compensation it does

not appear that this should be necessary for the production of a fair

working instrument useful for the surveyor in new countries to check

his longitude. Where a good chronometer is used it is better to keep it

to Greenwich time without alteration. If there is a gaining or losing

rate this will most probably remain constant in equal times, so that

corrections may be made _pro ratâ_ for all observations until a check

can be taken with certainty when arriving at a town which possesses

an observatory. The quality of a chronometer is fully ascertained by

having a certificate from one of our observatories, that of Kew being

the most popular.

[Illustration: Fig. 433.--_Sling case for drawings._]

894.--=Chronograph.=--For the observation of stars in transit for the

purpose of taking longitude, a dead-stop watch or chronograph is most

useful. This can now be had in combination with an otherwise fair going

watch at a very moderate price.

895.--=Outfit of a Surveyor for Work in a New Country.=--The ordinary

items of strong, dust-coloured woollen clothing, good boots, saddle,

firearms, etc., do not come within the province of this work. The

instruments he will require will depend partly upon the nature of the

country and the kind of work to be done. If for prospecting only, light

instruments are commonly selected--the sextant, or box sextant with

glass artificial horizon, good pocket chronometer, _telescope_, aneroid

barometer, prismatic compass, and clinometer. If a general survey is

to be made, the first instrument of importance is the theodolite, the

4 or 5-inch being the most usual. With this, pickets, land chain and

arrows, a steel tape for testing, and a linen tape. If for survey in

mineral districts, a good mining-dial is required, with all accessories

of chains, etc. If for railway work, a 5-inch theodolite, a good

level, staves, pickets, clinometer, and prismatic compass. In all

cases, field-books, drawing instruments, supply of paper, drawing

boards, squares, parallel rule, pencils, Indian ink, colours, stencil

plates, and other articles for office use, of which the established

optician or trader will give full information from his experience, or

general reference may be taken from any complete catalogue of such

instruments.

FOOTNOTE:

[66] Patent, No. 4310, November, 1876.

INDEX

Abney's clinometer, 411

Achromatism explained, 35

Adjustable axis, of plane table, 479

of theodolite, 237

Adjustable tripod, 329

Alidades for plane tables, 473

Alloys used for surveying instruments, 7

Altazimuth theodolite, 295

Altitudes, measurements of, 550

Aluminium alloys, 8

Anallatic telescope, 364

Anemometers, 596

Aneroid barometers, Vidie's, 558

Bourdon's, 568

Apomecometer, 469

Arrows, for chain, 494

Artificial horizons, 443

Atmospheric pressure, measurements of, 550

Axes, workmanship in, 11

Bakewell's tangential index, 384

Ball and socket adjustments: Hoffmann's, Pastorelli's, 330

Bands, steel, for measuring land, 449

Barker's clinometer, 415

Barometer, aneroid, 558

mountain, 550

mercurial, 549

Base line apparatus, 517

Beam compass measurements, 510

Bellamy's road tracer, 420

Bill-hook, 582

Binoculars, prism, 584

Black, optical, 14

Boiling-point thermometer, hypsometer, 569

Boning rods, 578

Books, levelling, 173

Boucher's calculator, 587

improved, 590

Box sextant, 451

with continuous arc, 461

with supplementary arc, 458

Bronzing instruments, 14

Brunton mine transit, 353

Bubble trier, 88

Burel's reflecting level, 144

Burnier's clinometer, 416

Caink's rule for correcting inclines, 498

Calculators: Barnard's, 594

Boucher's, 587

Fuller's, 592

Thacher's, 595

Camera, 243, 585

Cases, for carrying maps, 597

leather, for instruments, 23

Cavalry sketching case, 488

Cement tester, Mann's, 585

Centesimal division, 185

Chain scales, pocket sets, 596

Chain vice, 496

Chaining, 497

Chains, land, various, 490

Chains, sounding, 526

Chronometer and chronograph, 598

Circumferentor, 307

Clamp and tangent motions, various, 202

Classification of instruments, 5

Clinometer compasses, 418

hanging, 344

Clinometers, various, 411

Coast survey lines, 527

Coincidence rods, 511

Collimation, 55

Collimator, 121

Compass, surveying with, 80

Compasses: magnetic, bar, 59

Barker's, 84

Burnier's, 80

hanging, 344

Hutchinson's, 79

luminous, 84

mariners', 73

prismatic, 75

pocket, 82

ring, 72

trough, 74, 83

Compensated rods, 512

Connecting link, to extend hand rods, 531

Convex and concave lenses, 32

Cooke's level, 138

Co-ordinate slide rule, 594

Cross-staff heads, 573

Curvature, correction for, 170

Cushing's level, 136

De Lisle's reflecting clinometer, 413

Declination of needles, 67

Deville's theodolite, 263

Diagonal eye-piece, 45

Dials, mining, 309

Diaphragm of telescope, 50, 114, 135

Dip compass, 354

Dip of needles, 66

Dispersion of light, 35

Dividing engine, 176

Division of the circle, 175

Double optical square, 467

Drop arrow, 494

Dumpy level, 110

improved, 123

Dynameter, 43

Edgeworth's stadiometer, 482

Engineer's level, 133

Engraving, note on, 16

Everest's theodolite, 271

tripod, 273

Excise ink bottle, 174

Eye-pieces: Ramsden, 41

erecting, 44

diagonal, 45

reflecting, 46

Field-books, 289, 374, 380

Field-glasses, 582

Finishing of surveying instruments, 14

Formation of images in a telescope, 33

French forms of miners' dials, 336

Fuller's rule, 592

Geological tools, 587

George's artificial horizon, 446

Girth straps, tapes, etc., 579

Glass diaphragm, 53

Glass, working, 16

refraction of, 25

Gradient scale, 213

Gradienter Screw, 386

Gradiometer, 404

Gradioplane, 409

Graduation, 179

Green, William, Subtense instruments, 355

Hadley's quadrant, 423

Hanging mining compass, 344

Hedley's dial, 322

improved, 326, 329, 333

Heliograph, 540

Heliostat and heliotrope, 537

Henderson's miners' dial, 315

Hick's patent level, 96

Historical sketch of surveying instruments, 1

Hoffmann's ball and socket head, 330

Horizontal scale of tangents, 213

Hypotenuse and base, 212

Hypsometer, 569

Illumination of axis of telescope, 234

Inclinometer: Lister's, 389

Lacquering work, 15

Lamp, magnesium, 545

for levelling, 169

mining, 348

theodolite, 235

Land chains, 490

vice for adjusting, 496

Lanterns, oil, 545

Lean's miners' dial, 315

Leather cases, 23

Lenses, 33

achromatic, 36

Level: for levelling staff, 163

mechanics', 575

with inclines, 576

Levels: surveyors', 97

Cooke's, 138

Cushing's, 136

dumpy, 110

same improved, 123

engineers', 133

reflecting, 144

pocket, 142

simple construction of, 141

supplementary parts to, 139

water, 146

Y-form, 98

same improved, 107

Level tubes, 86

circular, 96, 576

curvature of, 87

divisions upon, 90

readers for, 95

Scott's, 93

sensitiveness of, 89

Strange's, 92

with air cell, 93

Level tube trier, 88

Levelling: books, 172

staves, telescopic, 148

semicircular, 150

mining, 158

papering of, 159

preservation of, 161

various patterns of, 151

holder for, 163

pads for, 161

pegs for, 171

practice of, 163

Light for night observations, 169, 545

Light, refraction of, 27

Line, sounding, 527

Lubrication of joints, etc., 20

Luminous compass, 84

Magnesium lamp, 546

Magnetic compasses, 59

correction of, 64

declination of, 67

inclination of, 66

trough, 74

variation of, 68

various forms, 82

Magnetic needles, 59

lifting, 65

mounting, 64

Magnetic needles, various, 59

Magnetism, 59

preservation of, 71

Magnifying power of telescope, 43

Measuring rods, 530

Mechanics' levels, 575

Mercurial barometer, 549

Metals employed in surveying instruments, 7

Micrometer microscopes: various, 192

Stanley's, 197

Micrometer theodolites, 259

Military sketching board, 485

cavalry, 488

Miners' circumferentor, 307

Miners' compasses: French, 336

hanging, 344

Stanley's prismatic, 343

Miners' dials: various, 309

hanging, 344

Hedley's, 322

Henderson's, 315

Lean's, 315

improvements in Hedley's dial, 326, 329, 333

Mining, survey lamp, 348

targets, 349

theodolite, 342

Morse signaling, 544

Mountain barometer, 550

theodolite, 258

Nautical sextant, 429

Needles, magnetic, 59

Night signalling stations, 545

Octant or quadrant, 422

Offset rods, 507

Omnimeter, 374

Opisometer, 587

Optical black, 14

Optical principles of telescope, 25

Optical square, 465

Outfits for surveyors abroad, 598

Packing of instruments, 21

Parallax, in eye-piece, 55

Parallel plates, to level, 99

to theodolite, 219

Passometer, 525

Pastorelli's ball and socket head, 330

Pedometer, 524

Perambulator, 521

Permanent stations, 535

Pickets or ranging poles, 533

Pine measuring rods, 508

Pioneer tools, 585

Photographic camera, 243, 585

Plain theodolite, 215, 267

Plane tables, 472

Platinum-iridium points, 53, 129, 135

Plummets, 232

Pocket-books, 534, 596

Pocket levels, 96

sighted, 142, 525

Pocket magnetic compasses, 83

Point diaphragm, 53, 129, 135

Polishing work, 14

Preservation of instruments, 20

Prismatic clinometers, 414

Prismatic compasses, 75

Hutchinson's, 79

stands, 78

Prismatic mining survey compass, 343

Prisms as reflectors, 29

Protectors for the eyes, 584

Protractors, sketching, 82, 374, 485

Quadrant, 422

Qualities of work, 7

of a telescope, 56

Quick-setting surveyor's level, 132

Quick-setting theodolites, 257

Quiver for arrows, 494

Railway gauge, 579

theodolite, 258

Ramsden eye-piece, 41

Ranging poles, 533

Ray shade, 129

Reader for level tube, 95

Reading microscopes, 188

Reconnoitring glass and telescope, 582

Reflecting cap to telescope, 334

Reflecting circle, 424

Reflecting clinometers, 413

Reflecting levels, 144

Reflection of glass, 25

Reflector in eye-piece, 46

Refraction of light, 25

Repairing sleeves for steel bands, 505

Revolving compass to dial, 318

Richmond's tension handle, for steel band, 503

Road tracer, 420

Rods: coincidence, 511

compensated, 512

hand, 530

standard, 508

Rule, civil engineer's, 532

Rule for correcting inclined measurement, 498

Rule form clinometer, 418

Semi-circumferentor, 346

Sextants, 425

box, 451

with supplementary arc, 461

continuous arc, 458

sounding, 449

surveying (open), 464

Sight director to stadium, 364

Sighted pocket level, 142

Silvering sextant glasses, 437

Sketch books, etc., 585

Sketching board, military, 485

Slashing knife, 582

Slide rules, various, 590

Sliding stage to theodolite, 249

Socket for station pole, 535

Solar attachment to theodolite, 239, 259

Soldering, 13

Sounding chain, 526 lines, 527

Sounding sextant, 449

Spectacles for protecting the eyes, 584

Spherical aberration, 33

Spur shod picket, 535

Stadia points, 131

webs, 114

Stadium for tacheometer, 155, 373

Stadiometer, 482

Standard rods, 508

Stands of instruments, 19

Stations for observation, 533

Staves, levelling, 149

Steel bands and tapes, 499, 507

Striding level, 237

Style of work, 16

Subtense, instruments, 355

diaphragm, 114, 363

Supplementary arc to box sextant, 461

Sun glass to telescope, etc., 47

Tacheometers: general description, 370

Stanley's, etc., 371

Tacheometers: stadium for, 155, 373

field book, 374

Tangent motions, 202

Tapes, linen, etc., 505

Targets, mining, 349

Telemeters, 528

Telescope: general description of, 24

Kepler's, Galileo's, 40

body of, 47

Telescope: optical arrangements, 47

optical principles, 25

qualities, 56

reconnoitring, 582

Telescopic pocket level, 143

Tension handles for steel bands, 503

Theodolites, 214

adjustment, 276

adjustable axis, 237, 248

Deville's, 263

Everest's, 271

micrometer, 259

mountain, 258

plain, 265, 267

railway, 258

simple construction, 275

solar attachment to, 239

Souterrain 348

Stanley's, 247

transit, 215, 231, 247

14-inch, 295

36-inch Colonel Strange's, 298

Universal, 265

Thermometer for steel band, 503

boiling point, 569

Timber girth strap, 579

marking knife, 581

rods, 582

Tools used in manufacture of instruments, 11

Triangle for levelling staff, 162

Tribrach adjustments, 126

Everest's, 272

with mechanical stage, 251

Tripods, 114, 217, 322

framed, 250

miners', 313

Tripods, jointed, for mining instruments, 313

Trough compass, 74, 83, 236

Vernier scale readings, 180

Vicat needle, 586

Vice for adjusting land chain, 496

Water levels, 146

Waterproof covers, 23

Watkin's aneroid, 567

Watkin's clinometer, 417

Wealemefna, 587

Webs, collecting and mounting, 51

Whistles, 585

Y-levels, 98

improved construction, 107

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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 26 / 26 章

815.--The lower part of the barometer tube, after it is filled, is