Fig. 155, W. In the transit theodolite it is fixed firmly by screws
to the vernier plate and is made to form a steadying piece to the
_A-frames C′ C″_ which support the upper part of the instrument. For
this purpose the compass-box is made as a solid casting in brass,
which is much stiffened by the raised step which forms the divided
circle. Four solid lugs in the same casting project from the rim of
the compass, and form stiffening pieces between the lower parts
of the A-frames; these are secured to the lugs by four screws, one
of which is shown, Fig. 153, at _a_. The lug screws hold the whole
superstructure together quite independently of the vernier plate, to
which it is afterwards firmly fixed. The compass needle is lifted by
means of a milled head, just inside one of the standards, _not shown_.
For a general description of the compass-box see art. 138. The vernier
plate carries two or more verniers. The verniers are read by a pair of
microscopes, Fig. 155 _MM′_ placed one on each end of a radial arm _N_
having its axis of motion upon a large collar of the vertical axis. By
this plan, when one microscope is set to read by the coincidence of
lines upon one of the verniers, the other microscope on the other arm
or arms will be set also in like position over the other vernier or
verniers.
[Illustration: Fig. 154.--_Vertical circle with clipping arm of transit
theodolite._]
The verniers are adjusted ready for reading when the telescope is
accurately directed upon any object of which it is desired to ascertain
the angular position in relation to magnetic north, or a definite
object. The vernier plate also carries a spirit level at _O_, Fig. 153,
which is adjustable by a pair of capstan-headed screws.
[Illustration: Fig. 155.--_Cross section of the upper part of a transit
theodolite._]
383.--_The Standards or A-Frames_, shown _C′ C″_ Fig. 155, are solid
castings in brass of about 7 inches in height. They are set up upon
the vernier plate, to which they are attached by four stout screws,
as also by cross screws to the compass as stated. This renders
the superstructure of the transit as firm as may be in a built-up
construction. Upon the front of one of the standards a spirit level,
Figs. 153, 155, _I_, is placed adjustable by two capstan screws. This
level, and one shown Fig. 153 at _0_ on the vernier plate are used
entirely in setting up the instrument; and being placed at right angles
to each other, are a means of making the vernier plate quite level.
Upon the inside of each of the standards, at about 2 inches from the
vernier plate, a _clip-piece_, Figs. 154, 155, _P_ is secured by two
screws. This takes the clipping screws, Fig. 154 _HH′_ to be described.
At the top of the standards two V's are formed, upon which the transit
axis rests. One of these is cut out of the solid casting. The other
as shown in half section Fig. 155 _c_ is formed as a parallel sliding
piece with the V at the top placed in a vertical slot formed in the
standard. This sliding piece has a screwed stem continued from its
lower surface that passes through a vertical hole at the top of the
A-frame, which is formed here as a cross-piece. Upon the screw two
capstan nuts are placed, one on each side of the cross-piece, Fig. 155
_xx′_; these permit the adjustment of this in height so as to get the
transit axis _perfectly horizontal_ when the vertical axis is perfectly
perpendicular to the horizon. The sliding piece is covered by plates
back and front to render it firm in its position. The transit axis in
practice is adjusted with a striding level which will be described
presently.
With the author's theodolites from 6 inches downwards the old-fashioned
adjustment to one upright for levelling the horizontal axis has been
dispensed with for many years, and is only fitted if specially ordered,
as it has been found to be a frequent source of error. Long experience
has proved beyond doubt that the fewer adjustments there are, and the
more parts that can be fashioned from the solid metal correctly, the
longer will the instrument keep in adjustment. Should there ever be any
wear on either of the V's a few strokes with a piece of very fine emery
paper upon the opposite one will put it right in half the time that it
could be corrected with the old-fashioned adjustable V, and no amount
of vibration can alter it as with the adjusting screws.
An axis _cover cap bb′_ is placed on the top of each standard. The cap
is screwed down at one end with a cut screw and collar. The screw is
used for adjustment to gentle pressure on the axis. The second screw
is a milled head _EE′_. Under this screw the cap is slotted out to one
side, and turns on the cut screw as an axis to open the cap without
removing its milled-head screw, so that the telescope can be lifted out
to turn its face to the opposite side of the instrument. In the under
side of the centre of the cap a cell is bored out, into which a small
cork is fitted, which produces, when the cap is clamped down, a soft
elastic pressure on the axis.
384.--_The Transit Axis_ which supports the telescope rests at its ends
upon two trunnions, Figs. 154, 155 _AA′_, technically called _pivots_,
in the V's of the standards already described. The pivots are turned
as true as possible, and afterwards ground to exactly equal size in
a collar, so that they may be reversed end for end in their bearings
without changing the linear direction of the transit axis, except by
the little difference of pressure that one end of the axis imposes by
the weight of the vertical circle and its attachments being eccentric.
In larger instruments this difference of weight is counterbalanced,
as shown in dotted lines at _p_, Fig. 155. The centre of the transit
axis is formed into a _collar e_ of about 1¼ inches in width, which
exactly fits the outer tube of the telescope, and to which it is fixed
with soft solder. The collar is directly connected with and supports a
_flange f_. Upon this flange the vertical circle _FF_ is fixed by three
or four screws.
385.--In front of the vertical circle a flanged collar-piece carries
the _vertical vernier frame VV′_, Fig. 154, centred upon it. The
vernier frame is attached by three screws to the _clipping arm_ to
be described, and in front of this the vertical microscope arms are
centred. These carry two readers _U_, Fig. 155, exactly similar to
those which read upon the horizontal circle, and they are similarly
centred, so that by setting one, the other is set at exactly 180° from
it. In front of the centre of the microscope arms on the transit axis,
an _axis collar-piece j_ is attached by three screws cut directly
into the axis. This collar and one at the other end of the axis _A′_,
turned out of the solid, are nicely fitted to the opening between the
standards to prevent lateral displacement of the axis.
386.--_The Clips._--The clipping arm, which is centred on the transit
axis and attached to the verniers, is shown Fig. 154 _BB′B′_. It is
fitted to move freely on its axis at _A_, so as to permit unrestrained
motion of the telescope. A milled-head clamping screw with clamp,
Fig. 155, _K_, and the same partly cut away to show the slot in which
it works, are shown at K′ Fig. 154. This is used to fix the verniers
stationary on the circle, except for the adjustment by the tangent
screw _G′_, which has its collar attached to the clipping arm, and
its ball nut attached to the clamp at _D_ when using the telescope
for levelling. This clamp and tangent sets the vernier to zero on the
circle. It is also used in setting the telescope before angles of
altitude or depression can be measured. The clipping screws _HH′_ are
used to bring the principal bubble _B_, Fig. 153, on the top of the
telescope to the centre of its run after the verniers have been brought
to zero by means of the clamp and tangent screws. The clipping screws
hold the clips, Fig. 155, _P_ or _P′_ to the one standard or the other.
The whole of the vertical adjustment is exactly equivalent to that
already described for the horizontal motion, except that it is placed
in the vertical plane.
387.--_The Vertical Circle_, Figs. 154, 155, _F_ is carried by four
arms from a central boss attached firmly by screws to the transit
axis. It is grooved at the edge to take the _clamp-piece_. The silver
is inlaid in this circle in the manner shown Fig. 117. The vernier is
read upon the circle on the plan shown Fig. 127. The circle is divided
generally to half degrees or 20′, and is figured 0 to the horizontal
with 90° upwards and downwards. The zero lines are made directly
coincident with the optical axis of the telescope when it is level. The
vernier reads to half minutes or 20″, in either direction, the rising
arc above the level datum being considered as plus, the falling arc as
minus.
388.--On the outer edge of the circle or at the back a scale of
difference of hypotenuse and base reads to a line on a fiducial
edge upon a part of the clip _BB′_, Fig. 154, at _N_. This scale is
calculated for decimal quantities, and gives the percentage number of
links, feet, or metres to be deducted from the chain measurements upon
the ground line to give the horizontal distance corresponding to the
angle of inclination at which the telescope is set for observation.
389.--_The Telescope_, Fig. 153, _DD′_ has been described art. 94. Its
general construction is also shown in partial sections in the figure.
Its body tube passes through the transit axis in which it is soldered.
390.--_The Principal Level Tube_ is generally mounted on the telescope
upon two stiff screws which rise from plates attached to the telescope
body by pairs of screws. Each level screw has a pair of capstan nuts.
The level is mounted in a brass tube with stop-pieces at the ends, each
of which carries a _tenon_ with a hole in its centre through which the
level screw passes to be clamped top and bottom by the capstan nuts.
These nuts give adjustment to the level, so that the centre of its
inner upper surface may be placed parallel with the optical axis of the
telescope.
[Illustration: Fig. 156.--_Stanley's new model of 4-screw transit
theodolite._]
391.--Until 1898 the author was unwilling to attempt to remodel the
old form of transit theodolite, believing the 4-screw adjustment would
soon become a method of the past, but as a small demand continued
from the Colonies and United States for this form of instrument he
felt bound to make it of more solid construction to bring it somewhat
up to date. The illustration shown, Fig. 156, is of an instrument,
following in construction the transit theodolite already described in
many details, the marked exception being that the standards are in one
casting with the compass-box and axis, these being entirely shaped out
in the solid metal. The upper parallel plate is of special design,
being far stronger, yet lighter, and gives a much longer bearing to
the levelling screws. The lower parallel plate is also shaped with
three feet so that the instrument may be set up without its stand when
required. It has also modern spring tangent adjustments with covered
screws. The limb is covered, and the readers are jointed across the
axis to turn up without separation. It has a floating aluminium compass
read by a microscope, so that the instrument, except in the four-screw
arrangement for setting up, embraces many modern improvements
formerly applied only to special high-class theodolites. The improved
construction permits greater rigidity with fifteen per cent. less
weight.
[Illustration: Fig. 157.--_The plummet._]
[Illustration: Fig. 158.--_Gurley's plummet._]
[Illustration: Fig. 159.--_Loop._]
[Illustration: Fig. 160.--_Ring plummet, Shortt's Patent._]
Of later years, however, the demand for four-screw levelling
instruments has been maintained, especially from Canada, owing to
the influence of the American school of teaching, and in consequence
all the author's improved theodolites are fitted with either three-
or four-screw levelling, whichever is desired. It is a strange fact,
however, that with all the American makers, although they list all
their ordinary instruments with four-screw levelling, their refined
ones, which they term "precision" instruments, will be found with
three-screw levelling.
392.--=Detached parts of a Theodolite.=--_The Plummet_ supplied with
the theodolite is made to hang from a hook under the centre of the
axis of the instrument, the cord, which is of soft silk, being looped
or knotted to hold in the hook. The lower end of the plummet is brought
to a point which, when in use, falls directly under the vertical centre
of the instrument upon the surface of the ground. In Fig. 157 the screw
and plummet are shown detached. The cord _C_ is attached to the plummet
by passing it through a hole in the milled-head screw _S_ at the top
of the plummet, and by making a knot _K_ in the cord. Fig. 160 shows
an ingenious ring plummet recently invented and patented by Mr. W. H.
Shortt, A.M. I.C.E. The chief object was the production of a plumb
bob whose plumbing point should be situated at, or very close to, the
centre of oscillation in order that the position of the point might be
unaffected by oscillation of the bob itself, apart from any swing which
it might have about the point of suspension of the string. A further
object was to shape the bob so that a person holding the string, or
standing close to it when attached to an instrument and looking down
at the bob, should be able to see readily the exact position of the
plumbing point.
These objects have been attained by making the bob in the form of a
ring, so that the centre of oscillation which lies in the centre of the
ring can itself be used as the plumbing point, since it can be readily
seen and indicated by the extremities of pointers projecting towards
the centre from the inside of the ring.
A great advantage of this bob is that when plumbing on to a flat
surface it does not fall over when lowered, but may be allowed to
actually lie on the surface while the position of the point is being
marked. Also it can best be steadied by lowering into contact with the
ground and raising again.
The plumbing pointers are largely protected from injury when the bob
is in use, and when not in use the suspension string can be wound
diametrically across the bob, in recesses provided for the purpose,
thus completely protecting the points.
393.--_The Loop._--It is somewhat difficult in the ordinary way to
adjust the plummet to the station mark on the ground or on a peg. The
cord is sometimes placed in an ivory runner fixed to the top of the
cord, Fig. 159. This gives friction on the cord and permits extension
and contraction of the loop for adjustment. Where the plummet has to be
suspended from the instrument as well as from a hook inside the stand,
which is sometimes convenient, it is better to have the runner cut out
on one side. This permits easy change and it is just as firm.
394.--Messrs. Gurley Bros. of Troy, N.Y., have a good plan for
shortening the plummet line. This is effected by making a reel in the
plummet, which is wound by a milled head at the top of it, Fig. 158.
395.--_Screw-drivers, Tommy Pins, etc._--A screw-driver and a tommy
pin, the last to turn the capstan heads, are placed in the case with
the theodolite. Two screw-drivers with proper handles are better,
as there are small and large screws. A camel-hair brush to dust the
instrument, a piece of wash-leather, a little vaseline, and a small
bottle of good watch oil are also very useful. These little refinements
are generally kept out to keep down the price of the instrument.
396.--=Additional Parts, and Variations in Theodolites.=--_Illuminated
Axis._--4, 5 and 6-inch transits sometimes, and larger instruments
always, have the transit axis bored on one side through to the interior
of the telescope, as shown on Fig. 155. Through the hole a small pencil
of light is sent by a lamp _l_ with a plano-convex lens front, to a
lens placed in the end of the axis. This, by a slight adjustment of
the lamp on its stand, focusses the light upon a small mirror placed
within the telescope, which reflects its rays to the diaphragm. The
lamp gives a faint light only sufficient to distinguish the webs for
night and underground observations. The mirror is about 1/10 inch in
diameter, and is generally mounted upon a milled head screw tapped
into the trunnion band of the telescope _m_. The point of the screw
is extended as a thin stem into the axis of the telescope, where the
mirror is held by it. This arrangement permits the mirror, which is
generally made of silver, but is much better of platino-iridium, to
be removed for cleaning. The lamp is mounted upon a wooden stand _w_
carried upon a slide _n_ or upon two brass pins direct to the A-frame.
The wood is employed in this case to cut off conduction of heat to the
near standard from the lamp as much as possible to prevent disturbance
of the axis from expansion by heating. The stand may be removed when
the lamp is not required and placed in the case. In large theodolites a
pair of lamps are used, that the transverse axis may not be heated more
on one side than on the other.
397.--_The Lamp_, which is found so convenient for bringing a star or
distant light to read with the webs, becomes difficult to use when
the object is very faint, as the light thrown into the telescope by
the lamp takes off the effect of blackness of the night sky or that
of total darkness. This becomes important in taking observations of
small stars, as for instance, the circumpolar stars of the southern
hemisphere. In some theodolites, made first for the Sydney Government,
the author placed a very small lamp to throw light upon the face of the
webs only, making these appear as light lines on a black ground. The
reflecting eye-piece, Fig. 20, will be found to answer very well, and
this is a simple, inexpensive contrivance. Any amount of illumination
desired may be thrown on the front of the diaphragm, according to the
distance at which the light is held from the eye-piece: generally a
very faint light only is required.
398.--The author has illuminated the webs front and back by means of
a very small (one-quarter candle power) incandescent lamp, which is
charged by a portable battery, or a secondary battery where a dynamo
is at hand for charging it, and for countries where these cannot be
renewed or where the extremes of temperature are too great for their
use, he has devised a small hand dynamo for generating the current and
a rheostat for controlling the power of the lamp, so that resistance
may be employed to reduce the light to the faintest possible glimmer.
The electric lamp is far superior to the old oil lamp and safe to
use in gaseous mines; it is far cleaner, does not give out a tithe
of the heat, and may be removed from its socket and used in the hand
for reading the verniers in a bad light. All the author's modern
instruments that are required with illuminated axis are now fitted with
electric lamps.
[Illustration: Fig. 161.--_Trough needle for transit theodolite._]
399.--_A Trough or Long Compass, used in place of Circular Compass._--A
long compass, Fig. 32, p. 74, is often applied to a theodolite, either
upon the top of the telescope, or more generally and conveniently
for reading under the limb. In this last case the trough needle is a
separate piece, which is only attached to the limb of the theodolite
by means of _loop slides_ or _bayonet fittings_ under the limb, when
required to take a bearing. The engraving Fig. 161 shows the long
compass with bayonet fittings. There are four slots, two of which are
shown _SS′_, which fit in under the heads of round-headed, shouldered
screws. The author has somewhat modified this pattern recently by
making it slide into grooves.
The trough needle is generally made 5 or 6 inches long, and reads into
a short scale of about 10° at each end. The divisions are best placed
upon sliding fittings, so that they may be adjusted by four screws from
the outside of the box--screws shown _AA′_. This enables the needle
to be adjusted to its own axis, and also to the 0° reading of the
horizontal limb of the theodolite. A slide lift to the needle is shown
at _L_. When the same form of compass is used upon large instruments a
reader is placed at each end of the needle.
[Illustration: Figs. 162, 163, 164.--_Striding level._]
400.--_Striding Level._--For the adjustment of the transverse axis of
a theodolite a very sensitive spirit level is used. This is mounted
upon a _bed_, which may be formed of brass tubing, from the two ends
of which adjustable legs descend, the ends of which are _forked_, the
hollows of the forks forming V bearing surfaces. The V's rest upon
the pivot of the axis. By reversing the striding level on the pivots
the transverse axis of the telescope, or transit axis, can be readily
adjusted truly perpendicular to the vertical axis. In the construction
of the striding level, shown in detail in Fig. 162, the two striding
standards _SS_ are carried down from the ends of the casing tube B of
the spirit level. These are adjustable: one, Fig. 164, by raising or
lowering the end of the level tube by the capstan screws _CC′_, and the
other, Fig. 163, by a lateral adjustment of the capstan screws _PP′_
that act upon the stud _S_, which is fixed upon an arm centred upon the
axis of the tube. This connection is shown by dotted lines. By these
two motions the standards are brought to perfect parallelism with each
other for their bearing surfaces and adjustment of the crown of the
bubble tube.
[Illustration: Fig. 165.--_Wallis' shifting centre for theodolites._]
401.--_Adjustment of the Axis for Setting it up over_ _a
Point._--Every surveyor experiences an amount of difficulty in getting
the plummet to fall from the axis of the instrument exactly over a
point upon the ground, or a mark upon a rock, or still more so upon a
point in street paving in a town, which is necessary for exact work.
It is easily set near the point, that is, within half an inch or so,
by pressing or shifting the legs; but the difficulty increases as the
exact point is approached, so that the setting has generally to be
left at a certain state of approximation. There are a great number of
schemes in use for moving the axis by adjustment of the instrument the
small quantity required, without disturbing the legs of the tripod
when they are firmly set down nearly correct to position. One of these
would no doubt be generally applied to the theodolite, except for
the reason that every means yet devised adds to its weight, and also
to the expense of the instrument. A moderately simple plan, which is
especially adapted to the parallel plate adjustment, is to make the
lower flange of the theodolite, upon which it stands when set down
off its tripod, somewhat larger and thinner. This flange, instead of
being screwed directly down upon the tripod head, is placed between
two ring plates, which are clamped together when the theodolite is set
in position. The large hole in the centre of the ring permits movement
of the lower plate of about 1 inch. Fig. 165 is an arrangement of
this kind by Mr. J. Wallis. This is made entirely independent of the
theodolite, and may be used or not as required. _I_ is a screw that
corresponds with the head of the tripod which takes the theodolite;
_T_ similar female screw to take the tripod head when the shifting
centre is used; _CC′_ a box formed by screwing two tray-pieces firmly
together; _S_ clamping flange; _HH′_ clamp screwed into the top of box
_C_. This has two handles by which the screw is moved to clamp when
the instrument is in position. The weight of this additional part is
about 3 lbs. The arrangement is particularly adapted to parallel plate
adjustments.
402.--In an American plan of a transit by Messrs. Heller & Brightly,
the flange is lifted by the parallel plate screws, which tighten it at
the same time.[18] Messrs. Troughton & Simms have a plan of shifting
the axis by means of a pair of eccentric plates, which carry the
instrument in two directions nearly at right angles to each other. By
this arrangement an amount of leverage is secured which produces an
easier motion than that of shifting the weight of the instrument on the
plans mentioned above. The author's schemes will be described as a part
of his new theodolites a few pages on.
403.--_Stadia Webs or Lines_ used for taking subtense angles by the
telescope for measuring distances, which are frequently applied to
theodolites, will be fully described, Chapter XII., in treating of
subtense instruments generally.
404.--_Solar Attachment to a Theodolite._--This appliance is an
adaptation to the theodolite of the solar compass of W. A. Burt, of
Michigan, which was made to replace the magnetic compass in determining
a true meridian, or north and south line, by observation of the sun
only. It was brought into general use in the surveys of the United
States public lands. The solar compass consists mainly of three arcs of
circles by which the latitude of a place, the declination of the sun,
and the hour of the day can be set off. In the solar attachment to the
theodolite the latitude arc is found unnecessary, as this is formed by
the vertical arc of the theodolite; therefore the hour and declination
arcs need only be described.
[Illustration: Fig. 166.--_Burt's solar attachment to a theodolite._]
405.--_The Hour Circle_, Fig. 166, _H_ is fixed upon the centre of
the telescope upon a socket axis _S_, which is placed perpendicularly
to the optical axis and to the transverse axes or pivots of the
theodolite. This circle is divided to read five minutes of time, and
is figured I to XII twice, or I to XXIV, the index being a fine line
carried down on a plate from the lower arm of the _declination arc_,
which is fixed to the socket _S_. The hour circle, when set to any
reading, may be clamped to this position by means of the milled head
placed over the socket _M_.
406.--_The Declination Arc_ is of 5 inches radius, divided to read on
the same plane with a vernier _V_ to single minutes of arc. The vernier
arm is fixed by a clamp at _C_, which carries tangent adjustment _T_.
At the back of the vernier arm two spur-pieces are carried out directly
from it, _L_ and _I_. These are blocks of metal about 1½ by 1¼ by
¼ inches, which carry each a lens of a focus _L_ to _I_, and a silver
plate to be presently described, upon which the sun's image is received
in one direction or the other.
[Illustration: Fig. 167.--_Image plate of solar attachment._]
407.--_The Image Plate_, Fig. 167, is marked with two sets of lines
intersecting each other at right angles. The lines _bb_ are termed
_hour lines_, the lines _cc_ _equatorial lines_; these lines having
reference respectively to the hour of the day and the position of the
sun in relation to the equator. The intervals between the lines _bb_
and _cc_ are just sufficient to include the circular image of the sun
formed by the solar lens on the opposite end of the vernier arm. The
axes of the solar lenses and corresponding image plates are placed
parallel with each other, and with the direction of the vernier arm.
Below the lower line c three other lines are cut at 5 minutes apart.
These are useful for making allowance for refraction. The following
description for the use of the instrument is partly extracted from
Messrs. Gurley's manual.
408.--When the instrument is made perfectly horizontal, the equatorial
lines and the opposite lenses being accurately adjusted to each other
by a previous operation, the sun's position in the heavens with
reference to the horizon will be defined with precision. Suppose the
observation to be made at the time of one of the equinoxes; the arm _R_
set at zero on the declination arc _V_; and the polar axis is placed
exactly parallel to the axis of the earth. Then the motion of the arm
_R_, if revolved on the polar axis around the hour circle _H_, will
exactly correspond with the motion of the sun in the heavens on the
given day and at the place of observation; so that if the sun's image
be brought between the lines _cc_ on the image plate in the morning it
will continue in the same position, passing neither above nor below the
lines as the arm is made to revolve in following the motion of the sun
about the earth.
409.--In the morning as the sun rises from the horizon, the arm _R_
will be in a position nearly at right angles to that shown in the
illustration, the lens being turned towards the sun and the silver
plate, on which his image is thrown, directly opposite. As the sun
ascends, the arm must be moved around, until when he has reached the
meridian, the graduated side of the declination arc will indicate XII
on the hour circle; and the arm _R_, the declination arc _V_, and the
latitude arc, that is the vertical arc of the theodolite, will be in
the same plane.
As the sun declines from the meridian the arm R must be moved in the
same direction, until at sunset its position will be the exact reverse
of that it occupied in the morning.
410.--_Allowance for Declination._--Let us now suppose the observation
made when the sun has passed the equinoctial point, and when his
position is affected by declination. Then, by referring to the
_Nautical Almanac_ and setting off on the arc his declination for the
given day and hour, we are still able to determine his position with
the same certainty as if he remained on the equator.
When the sun's declination is south, that is, from the 22nd of
September to the 20th of March in each year, the arc _R_ is turned
towards the plates of the instrument in the opposite position to that
shown in the engraving, using the solar lens at _I_, with the silver
plate opposite at _L_.
The remainder of the year the arc is turned from the plates, and the
lens at _L_ and plates at _I_ are employed in the position shown in the
figure.
411.--When the solar compass is accurately adjusted and its plates made
perfectly horizontal, the latitudes of the place and the declination
of the sun for the given day and hour being also set off on their
respective arcs, _the image of the sun cannot be brought between the
equatorial lines until the polar axis is placed in the plane of the
meridian of the place, or in a position parallel to the axis of the
earth_. The slightest deviation from this position will cause the image
to pass above or below the lines and thus discover the error.
412.--We thus, from the position of the sun in the solar system, obtain
a certain direction absolutely unchangeable from which to run our lines
and measure the horizontal angles required.
The transit theodolite will, without the solar compass, perform the
same functions; but by means of this instrument the calculation for
position is much more simple.
413.--=Photographic Apparatus in Connection with the Theodolite.=--The
application of photographic apparatus as an accessory to surveying
instruments has been tried tentatively for many years. A practical
introduction to the subject was first given by M. Laussedat in a paper
published in the _Comptes Rendus de l'Academie des Sciences_, 1859. The
subject has since been well studied by many writers, and is written
up extensively by Dr. E. Deville, LL.D., Surveyor-General of Canada,
in a work entitled _Photographic Surveying_, published in Ottawa, to
which we must refer the reader for full discussion of the subject. In
England, Mr. J. Bridges Lee has invented a very suitable camera in
which a negative glass photograph of 4½ × 3½ inches is taken,
with an axis line from the shadow of a hair permanently photographed
coincident with the axis to the telescope as it appears to view. At
the same time degrees and subdivisions are taken on the photograph to
right and left of the axial line. The edge of the magnetic circle is
also photographed upon the plate, indicating clearly the bearing of
the station taken by the axis line. The whole of these operations are
performed at once in a perfect manner.
414.--Mr. J. Bridges Lee's photo-theodolite was made in excellent
workmanship by Messrs. Troughton & Simms. The inventor has published
a paper on the subject, to be had of the Society of Engineers,
Westminster.
[Illustration: Fig. 168.--_Light camera upon the telescope of a
theodolite._]
At the present time a camera is very commonly taken by a civil engineer
for prospecting in new countries,--a convenient form of this will be
discussed at nearly the end of this work--but it is not generally held
that photography will ever offer a means of expeditious surveying,
except possibly in very mountainous countries where the necessary
stations for observation become difficult of approach and of clear
definition. The objections to the more general adoption of photography
are, otherwise, that the processes are in degree tedious, and require
special skill in manipulation, and that the apparatus is heavy and
expensive with sensitive glass plates for use with it.
415.--There are many cases, no doubt, where a photograph would be
valuable for the exact definition of a station. To meet this case
the author has made a small light camera, shown Fig. 168, giving
photographs 2 × 2 inches only, with axis line from shadow of a
point. The camera to be placed when required upon the telescope of a
theodolite for special cases. He has lately used his patent slide for
this camera that carries films which will be further described at the
end of this work. The films are unbreakable, and remain sensitive many
years if kept dry. The weight of this camera with its double slides and
100 films is about 1 lb. There is ample room for it in the ordinary
theodolite case.
FOOTNOTES:
[15] Digges's _Pantometria_, see p. 2.
[16] Gardiner's _Practical Surveying_, p. 59, 1737.
[17] Adam's _Geometrical Essays_, pp. 217-229, 1803.
[18] _Civil Engineers' Pocket-Book_, by J. C. Trautwine, C.E.
CHAPTER VIII.
SPECIALITIES IN MODERN AND IMPROVED FORMS OF TRANSIT THEODOLITES FOR
SURVEYING--RAILWAY WORK--EXPLORING.
416.--The description given in the last chapter of a 6-inch transit
theodolite gives all particulars of the original Old English form,
which in a general way comprises the constructive principles of all
others. When we consider modern instruments the details are found to
vary greatly, but most particularly in the direction of uniting in
solid castings many parts that may be shaped out by machinery in a
manner impossible by hand-work, which avoids the instability of the
work being screwed together in many pieces, and makes it at the same
time lighter, more rigid, and less liable to jar out of adjustment.
This direction of construction is also followed in the best modern
work on the Continent and in America. It would extend this work beyond
convenient limits to offer details of the wide variations employed in
practice, but as the author has made this subject a life study, and has
embraced, modified, and endeavoured to improve this class of work in
all its details, freely adopting any improvement he has observed, his
own instruments will represent largely his present ideas of the best
forms, with the economy of having engravings for illustration to hand.
Transit theodolites of portable form will be considered here, leaving
larger stationary instruments to another chapter.
[Illustration: Fig. 169.--_Stanley's patent new model theodolite._]
417.--=New Model Transit Theodolite.=--In this instrument the
principles of construction are the same as in the ordinary transit
theodolite fully described in the last chapter, but the distribution
of materials and details are very different. The general arrangement
of a 5-inch instrument is shown in Fig. 169. One important difference,
as before mentioned, is that the work is not built up so much in
separate castings and pieces as is usual, but every possible casting is
shaped out of the solid to the finished form. The vertical axis is of
nearly the same construction as the ordinary transit, except that the
central axis is about double as strong, being of once and a half the
ordinary diameter. It is made in one casting with the upper framework.
The vernier plate is formed of thin hard hammered gun-metal, which
is screwed upon the axis. This plate has not in this construction to
support the superstructure as in an ordinary theodolite, but has only
to hold the two axis bubbles, which are thereby brought distinctly
in view, and the clamp and tangent motion, which is also placed
conveniently for use upon this upper plate, in a position where there
is less risk of accident than when it is placed upon the outer edge of
the limb.
418.--_The Readers_ to the horizontal limb are jointed to turn up
against the standards and adjust for reflection, as shown Fig. 131. In
this manner the readers do not need detachment to place the instrument
in its case.
[Illustration: Fig. 170.--_Section of standards of new model
theodolite._]
419.--_The Central Axis_ and the standards are made in one casting
in hard gun-metal. The standards are of light cylindrical and ribbed
section. This construction, although of only about one-half the
weight of the A-frame arrangement with its attachments, described in
the last chapter, was found upon testing to have more than double
the rigidity in resisting deflection, with perfect certainty of
avoiding the accidental occurrence of imperfect fitting of parts, or
of screws jarring loose, Fig. 170. The making of the vertical axis
and the standards in one piece was in a certain sense an experiment.
It has been found in practice of many years now to give much greater
resistance to all ordinary strains and jars, and ensure the instrument
keeping in order and adjustment when jolted by carrying over the
shoulder, just as the same principle acts in the dumpy level; but at
the same time, in cases of violent accident, such as the fall of the
instrument from a height, it renders repairs somewhat more expensive,
as this entire part might have to be reinstated instead of the axis
only, the axis of the theodolite being generally made very weak that it
may go first, often indeed with a slight jar. Many details are the same
as the transit theodolite before described, adopting what is thought to
be the soundest principle in all cases.
420.--_The Compass-box_ in this instrument is attached under the limb.
It is of the trough form shown Fig. 32, page 74. The magnetic north
is set to zero. The tribrach is of the form described for levels,
illustrated Figs. 72 and 73, p. 128.
421.--The weights of transit theodolites of this construction are about
6-inch in gun metal 14 lbs., aluminium, 8 lbs.
5-inch " 11 " " 6 "
4-inch " 7¾ " " 4½ "
This pattern embodies all the essential features of a thoroughly
reliable and convenient instrument for all-round general surveying.
It has no unnecessary elaborations and is a strong, light and compact
instrument suitable for continuous hard wear. It has fewer pieces than
any other design and is packed in its case complete in one piece ready
to screw upon its stand upon being taken out of its case.
[Illustration: Fig. 171.--_Stanley's simple sliding stage for tribrach
theodolite._]
422.--The author has devised a special arrangement for displacement of
axis for this theodolite, which does not interfere with its valuable
quality of standing the tribrach on a wall or flat surface, Fig. 171.
In this scheme the arms of the tribrach are slightly elevated by the
foot screws. A flange is formed on the top of the head with a leading
tube through it to the upper surface of the lower tribrach plate; upon
this tube an upper flange is screwed, so that the plate comes between
the two flanges, where it may be fixed by means of rotation of the
flange by a thumb-piece. The engraving shows the arrangement with the
axis displaced to its extreme point, about ¾ of an inch from the
centre.
[Illustration: Fig. 172.--_Stanley's 4-screw sliding stage._]
[Illustration: Fig. 173.--_Stanley's solid round form tripod with
sliding head._]
[Illustration: Fig. 174.--_Stanley's telescopic tripod with sliding
head._]
A somewhat similar arrangement is made for four-screw levelling
instruments shown on page 250 at Fig. 172, but in this the sliding
motion is fixed to position by the action of the levelling screws. It
is sometimes preferred to have the sliding adjustment upon the tripod
head instead of upon the instrument, and in some cases for getting a
greater range of movement, on both, and for this purpose the reviser
has designed the two tripods shown at Figs. 173 and 174, the former
being of the round solid pattern, and the latter having adjustable
sliding legs. A somewhat similar arrangement is made for a sliding head
to a framed tripod as shown below, Fig. 175.
[Illustration: Fig. 175.--_Stanley's framed stand with sliding head._]
423.--=Improved Transit with Adjustable Axis.=--This instrument, Fig.
176, in general, resembles that last described, except that it has
a larger telescope and it is mounted on a sliding stage with screw
adjustments, which is particularly described below. It is frequently
provided with a tacheometrical eye-piece for giving horizontal
distances by subtense taken on the incline, which will be described in
Chapter XII.
[Illustration: Fig. 176.--_Stanley's patent new model theodolite, with
mechanical stage._]
424.--_The Mechanical Tribrach Stage._--This important addition to the
theodolite above described permits exact adjustment over a station. The
upper plate of the tribrach with the movable stage is shown in Fig.
177. A dovetail slide is fitted upon the base of the stage adjustable
for wear by a slip-piece with two screws at the narrow part. The slide
is adjusted to position in the direction of its dovetail fitting by
a large milled screw so as to move the whole instrument above it for
centring in this direction. An upper slide acting in the same manner,
with dovetail fitting pieces at sides moves for an equal distance for
centring transverse to the lower slide by a milled head. This gives the
same kind of motion of displacement that we have in the slide rest of
a lathe or the mechanical stage of a microscope, except that in this
case we have a kind of three-point bearing surface. The motion given
to the screws permits the perfect adjustment of the theodolite over a
point on the ground corresponding with the suspended plummet, after
the instrument is set up to nearly its true position by movement of
the tripod legs. The range of motion is from ¾ to 1 inch, a quantity
quite sufficient for final adjustment, but which does not materially
affect the equilibrium of the instrument upon its rigid tripod, as it
has in this case a broad solid base even in the extreme positions of
the slide.
[Illustration: Fig. 177.--_Stanley's patent tribrach mechanical stage._]
This movement being _above_ the levelling screws, the adjustment of the
instrument for level is not affected by its use, as in the case of all
sliding arrangements _below_ the levelling screws. Suitable means are
provided for taking up any wear that might occur in the slides.
425.--The above stage is supported upon three foot screws, the female
fittings being specially long to give plenty of bearing surface to
prevent wear; they are sawn down on one side so that they spring
lightly upon the screws, and are provided with cross capstan screws for
tightening up when necessary. This plan gives the screw about ¾ inch
of thread, and permits adjustment for comfortable movement and for wear
without any risk of shakiness. The screw in larger instruments of this
class has a cap to exclude dust. The foot of the screw has a ball which
rests in a slotted tube before described, Fig. 71, p. 127.
426.--This theodolite with mechanical stage is generally fitted with
illuminated axis for tunnel work, art. 383. The lamp is not shown in
the illustration.
The weights of this make of theodolite are about
6-inch in gun-metal 18¾ lbs., aluminium 10 lbs.
5-inch " 13¾ " " 7½ "
4-inch " 9½ " " 4½ "
427.--=8-inch Transit Theodolite.=--For ordinary surveying the smaller
instruments are sufficient. For opening a survey in new countries the
8-inch instrument, Fig. 178, or a larger one, is generally used for
the superior triangulation, particularly for observations at night of
distant lights when greater light-grasping power is demanded of the
telescope. The larger circle gives a more exact reading of the limb,
which is generally divided to read clearly to ten seconds of arc, and
by estimation sufficiently near to obtain five seconds reading very
approximately with the verniers. When instruments exceed 8-inches, the
reading is by means of microscopes, the application of which will be
described further on.
[Illustration: Fig. 178.--_8-inch transit theodolite._]
428.--The 8-inch transit illustrated is of the author's model. It is
in general structure similar to the 6-inch just described, except in
certain specialities. The instrument does not clamp upon the vertical
circle, but a similar circle is provided upon the opposite side of
the axis. This answers two purposes, it balances the pressure upon
the pivots and obviates disturbance of the division by the clamp.
The principal bubble is supported upon the vernier frame, as special
exactness is not required for the instrument to be used as a level.
The base support is upon the Everest tribrach system, which will
be described in the next chapter. A long compass is shown, but a
telescopic compass is sometimes used. The instrument is shown with an
axis-lamp and diagonal eye-piece for star observation.
[Illustration: Fig. 179.--_Stanley's quick-setting transit theodolite._]
429.--The vertical axis of this instrument is sometimes pierced for a
look-down telescope to sight its vertical position on the ground to the
centre of a peg. This will be described with geodetic instruments in
the next chapter. It is an expensive refinement, seldom necessary, as
the axis with plummet may easily be brought within the tenth of an inch.
Theodolites of eight inches and over are uniformly packed in two
cases. The lower part is packed in a case by itself, the upper parts
connected with the vertical circle and all the accessories, eye-piece,
plummet, etc., forming the contents of another case, each part being
sufficient for one man to carry without the tripod. The weight of the
entire instrument is 29½ lbs.
430--=Quick-setting Theodolites.=--The demand for instruments with
quick-setting arrangements has greatly increased of late years. They
save a great deal of time in setting up, and also save wear of the
levelling screws, as the instrument may be instantly set nearly level
by its means, so that less than half a turn of the levelling screws
will bring it to true level. An ordinary transit is shown Fig. 179,
fitted with a similar arrangement to that described art. 240, p. 132.
[Illustration: Fig. 180.--_Railway theodolite._]
431.--=Railway Theodolite.=--There are objections made to the transit
theodolite by some civil engineers that it is a large and heavy
instrument, only to be accepted for its perfect convenience over
lighter forms. To meet this objection the author has made a special
transit theodolite, Fig. 180, which is sufficient for railway work and
general surveying upon moderately level country. The transit principle
is conserved by balancing the telescope on its axis to permit it to
transit over the eye-end only. The vertical arc is omitted as being
unnecessary for railway work. The instrument is constructed especially
low and of great rigidity and solidity, with light weight. The compass
is of the trough kind. The limb is covered for protection. It is
extremely portable. Weight of 4-inch, 7¼ lbs.; 5-inch, 9½ lbs.;
6-inch, 12¾ lbs. Being constructed for rough, hard wear and local
use it is not made in aluminium.
432.--For tunnelling underground railways the mining theodolite
described further on will be found the most valuable for railway
engineers.
433.--=Mountain Transit Theodolite.=--This instrument, Fig. 181, is
designed for geographical exploration, and making sketch surveys. It
embraces the transit principle for the convenience of taking zenith
stars. It is made in 3-inch and 4-inch sizes. It has two verniers to
the horizontal limb reading to minutes, and a single vernier to the
vertical circle. It has been made by the author in aluminium alloy
only, the total weight being 2¼ lbs. for the 3-inch and 3½ lbs.
for the 4-inch. The eye-piece reads direct or diagonally. It has
clamp and tangent adjustment to both circles, and a trough compass.
The tripod slides up to half length, each leg being adjustable to fix
to any length within the range of the slide to accommodate it to the
surface of inclined rocks.
434.--_A Mountain Theodolite_ is a term applied to any very small or
light theodolite. These are generally made to order, very frequently to
a reduced model of a larger theodolite, 3 inches being a common size.
The telescope is occasionally placed upon the side of the horizontal
axis to transit. Theodolites of this class generally weigh much more
than the above-described instrument, a common weight being 5 to 7 lbs.
[Illustration: Fig. 181.--_Stanley's mountain transit theodolite._]
435.--=Improved Solar Attachment.=--The reviser's improved solar
attachment admits of a full vertical circle being employed; it also has
a clamp and tangent to the hour circle and declination arc and quick
acting clamp and fine adjustment to the solar arm. An instrument so
fitted is shown on p. 260, Fig. 182.
[Illustration: Fig. 182.--_Stanley's solar attachment._]
436.--=Micrometer Reading Theodolites.=--The favor with which the
smaller micrometer reading theodolites have been received and the ever
increasing demand for them owing to their much greater accuracy has
induced the reviser to introduce a whole range of these instruments. In
many cases where greater accuracy is required for horizontal than for
vertical angles, the micrometers are only fitted to the horizontal
circle and the vertical circle has verniers as usual. A useful
instrument for general surveying without any unnecessary elaboration is
shown below at Fig. 183.
[Illustration: Fig. 183.--_Stanley's micrometer transit theodolite._]
The 6-inch instrument reads to 5 seconds of arc on the horizontal and
to 10 seconds of arc on the vertical circle, and the 5-inch instrument
to 10 seconds of arc on the horizontal and to 20 seconds of arc on the
vertical circle.
A full micrometer reading instrument is shown at Fig. 184, the 6-inch
reading to 5 seconds of arc on both circles and the 5-inch reading to
10 seconds of arc on both circles.
[Illustration: Fig. 184.--_Stanley's full micrometer transit
theodolite._]
A specially light form of micrometer reading transit (Fig. 185) has
recently been designed by the reviser which has met with much favour.
It has a 4½-inch horizontal circle reading by micrometers to 20
seconds of arc which may be approximately read by mental subdivision to
5 seconds, and a 4-inch vertical circle reading by verniers to single
minutes. It is also made micrometer reading to 20 seconds of arc to
both circles. The compass is of circular form reading by microscope to
¼-degrees, which may easily be estimated to a third of this.
[Illustration: Fig. 185.--_Stanley's light micrometer transit._]
An example of a more refined micrometer transit is shown at Fig. 186.
This instrument was designed by Dr. E. Deville, LL.D., Surveyor-General
of Canada, and is arranged for latitude determination by Talcott's
System, and general geodetic work.
[Illustration: Fig. 186.--_Dr. Deville's transit._]
It has a 6-inch horizontal circle reading by micrometers to 5 seconds
of arc, and a 4-inch vertical circle reading by vernier to 1 minute,
with zenith spirit level graduated to 2 seconds of arc, detachable and
interchangeable with a small level for ordinary use. The telescope is
14 inches solar focus with large object glass so that observations
may be taken of stars up to the sixth magnitude; it has a revolving
micrometer diaphragm and electric illumination to micrometers, to
diaphragm both front and back, and to zenith spirit level, it has also
a rheostat for regulating the amount of light, and a dynamo generator
for supplying the current.
[Illustration: Fig. 187.--_Stanley's patent universal transit
theodolite._]
The universal transit designed by the reviser is shown at Fig. 187.
This is an 8-inch instrument, reading by micrometers on both circles to
2 seconds of arc, and is fitted with all the necessary arrangements for
universal work.
A description of the larger geodetic instruments is given in a
subsequent chapter.
CHAPTER IX.
PLAIN THEODOLITES IN WHICH THE TRANSIT PRINCIPLE IS NOT
EMPLOYED--THE PLAIN THEODOLITE--IMPROVED CONSTRUCTION--EVEREST'S--
SIMPLE--ADJUSTMENTS AND EXAMINATION OF THEODOLITES.
437.--The plain theodolite is of nearly its original form as invented
by Sisson. It still retains a limited popularity, which is principally
due to its portability, being of less bulk and weight than the transit
of equal diameter of circle. If we consider the railway theodolite
described in the last chapter as a simple form of transit, this must
be considered as an exception with regard to the bulk and weight, not
being greater than that of the plain theodolite.
438.--=The Plain Theodolite.=--For the general description we may
follow that given in Chapter VII. for the 6-inch transit for all parts
of the instrument below the vernier plate, and for the compass-box
above this plate. The construction of the instrument varies from
the transit in having a half vertical circle only, with a single
vernier, and in the differences in the arrangement of the fittings
connected with the telescope. A single microscope is generally used on
the horizontal circle, and this passes in a groove from one vernier
to another, instead of having two microscopes on arms jointed upon
the vertical axis, as in the better construction of transits before
described.
The standards or A-frames in the ordinary plain theodolite are attached
to the vernier plate, but not generally to the compass-box. The
pivots of the transverse axis, which are made exactly equal in size,
rest on coupled bearings on the tops of the standards, which are in
construction made together, and therefore exactly alike. The transverse
axis is not adjustable, as in the transit theodolite previously
described; the standards have therefore to be adjusted to height in
the manufacture by filing, with the application of a special striding
level, until the transverse axis is brought permanently perpendicular
to the vertical axis.
[Illustration: Fig. 188.--_5-inch plain theodolite._]
439.--The vertical arc is fitted over the transverse axis; that is
constructed with a turned flange to which the arc is firmly screwed.
The arc is divided to 30′ and reads with a vernier to minutes. The
vernier is fixed directly to the vernier plate, and reads generally
with a microscope jointed on the transverse axis, but sometimes with
a loose magnifier for economy. Divisions for difference of hypotenuse
and base are occasionally divided on the back of the arc. The vertical
arc has a clamp and tangent placed at the back, therefore this cannot
be shown in the engraving. Along the bar above the vertical arc a
stout plate is attached by screws. From this a pair of Y's with clips
and eye-pins, as described for the Y-level, art. 192, supports the
telescope.
The telescope is of the same construction as that described for
Y-levels, with turned collars. The diaphragm is cross-webbed. For
economy a simple cap is generally put to the telescope instead of the
better plan of a ray shade. The principal level is fixed to collars
fitted round the telescope, to which are attached one slot-piece
for lateral adjustment of the level, and one screw-piece for linear
adjustment by means of two capstan nuts. The level is placed under the
telescope for compactness.
440.--The parts of the plain theodolite below the standards are the
same as those already described for the transit theodolite, except that
the vernier plate carries one level only at right angles to that of the
telescope. The telescope is therefore set to zero by the vertical arc,
and the two levels are then used as the pair upon the vernier plate
of the transit. The means provided for the adjustment are the same as
those of the Y-level, but the Y's are adjusted firmly by the maker by
fitting them down upon the Y-plate in the manufacture.
441.--The plain theodolite, except where price is the first
consideration, appears to be going gradually out of use, being
superseded by the transit. It has had a long day since its first
conception by Sisson about 1730. For 4-inch and 5-inch instruments
the makers still find a small demand. The 6-inch is rarely enquired
for. The plain theodolite cannot compete with the transit for perfect
utility, but it holds the merit of less weight and of greater
portability. The weights of the three sizes in general use are as
follows:--
_Weights of Plain Theodolites._
Instrument. Case. Outer Case. Tripod.
4-inch 7 lbs. 7½ lbs. 3½ lbs. 8 lbs.
5-inch 11 " 8½ " 4 " 9 "
6-inch 17 " 10 " 5 " 11 "
Very light 3-inch and 4-inch plain theodolites of from 5 lbs. to 7 lbs.
complete are made occasionally for travellers.
[Illustration: Fig. 189.--_Stanley's new model plain theodolite._]
442.--The author has recently modified the plain theodolite, Fig.
189, for which there still remains a small demand in the Colonies,
by making the construction much more solid by shaping the work out of
single castings in gun-metal for parts formerly screwed together in
many pieces, which formerly was necessarily arranged to permit facility
of construction by hand-work. There are also in the new instrument
some improvements made in detail. The limb dividing is covered for
protection. The readers are joined through the vertical axis and are
hinged to turn up. The compass has an aluminium ring with a microscope
which permits it to be read at a convenient height and much more
accurately. The tangent screws are covered to exclude dust, and some
other improved details.
[Illustration: Fig. 190.--_Everest's theodolite._]
443.--=Everest's Theodolite=, Fig. 190, designed by the late Sir
George Everest, and used for details of the great trigonometrical
survey of India, is built up very much upon a well-known common French
model. In service in India it has proved an excellent instrument. The
horizontal circle or limb of this instrument consists of a single
plate, upon which the silver is inlaid flat upon the surface, upon the
plan shown, Fig. 128. In place of the ordinary vernier plate three
arms are extended from the central axis, which carry each a vernier at
its end, reading to a fiducial edge, Fig. 127, p. 186. The verniers
trisect the circle, and are marked _A_, _B_, and _C_. A fourth arm,
proceeding from the same relative position of the centre as the arms of
the vernier, carries a clamp and tangent which is similarly constructed
to that of the ordinary theodolite described. The instrument has also
an under clamp and tangent for setting the telescope to bearing, or for
repeating, as in the ordinary theodolite.
444.--The horizontal axis carries the telescope in a cylindrical
fitting as in the transit theodolite, terminating in two pivots which
are set to permanent position as in the plain theodolite. The pivots
rest in bearings upon short standards carried out from the centre
upon a flat horizontal bar to which a spirit level is attached for
adjustment of the pivots to horizontality. Vertical angles are read off
upon two arcs which have a horizontal axis as their centre attached to
the telescope, so as to move with it in the vertical plane, with clamp
and tangent adjustment. An index, upon the same centre carries two
verniers and has a spirit level attached to it. The verniers are read
by a pair of microscopes. Upon the upper side of the telescope a trough
needle is placed.
445.--This instrument has been used in military surveys by the Royal
Engineers. The objections that civil engineers have made to Everest's
theodolite are that the working parts are made very open, so that the
wet and dust intrude; further it lacks the general convenience of the
transit principle, which is necessary for astronomical observations.
The tripod is sometimes made of the ordinary solid section, art. 216;
but for India, where carrying labour is cheap, a heavy framed stand is
used, which is special, as follows:--
[Illustration: Fig. 191.--_Everest's locking plate tribrach._]
446.--_Everest's Tribrach._--The upper part of the engraving, Fig.
191, shows this tribrach that supports the upper part of the instrument
directly upon its vertical axis. The three arms of the tribrach carry
each a milled-headed adjusting screw, the nut of which is formed in
the arm. The arm is sawn up to admit of adjustment, that the milled
head may turn softly but without any shake. The lower points of the
milled-headed screws, technically _feet_, fall into V-grooves in the
head of the tripod. The V's are not shown in the engraving. Above the
upper surface of the tripod head, a thin, three-armed plate of metal,
termed the _locking plate_, is centred upon the hollow axis of the
head, so that it will move laterally. The locking plate has a _hole
and slot_ at the end of each of its arms, the holes of which admit
the toes of the feet of the tribrach into the V-grooves formed in the
head of the tripod. The locking plate when moved laterally locks all
the toes in at once, so that the instrument is secured by this means
to a certain degree from accident. This locking plate has commonly a
milled-headed screw clamp which fixes it in its locked position. The
head of this screw is under the tripod head, and consequently cannot be
shown in the engraving. It is a defect of this locking plate that the
screws, unless they fall perfectly in the V-grooves have a tendency to
_ride_. To avoid this the author has for many years made the ball feet
fall upon a plain surface, being at the same time held in their places
by a slotted plate which fits over the neck of the balls. This plan,
which is not shown in the engraving, is now adopted by other makers.
The author uses also his patent tribrach sometimes on this instrument.
[Illustration: Fig. 192.--_Stanley's Everest theodolite._]
447.--_The Framed Tripod_ of Sir Geo. Everest's design is made of
straight-grained mahogany, each leg being formed of two _side-pieces_,
with one or two cross-pieces. The engraving, Fig. 191, shows the head
of a tripod of this construction. The side-pieces are spliced together
at the lower ends, where they form a rather obtuse point, which is
shod with a gun-metal shoe. The upper ends of the side-pieces carry
_strap plates_ that receive a bolt which holds them firmly by means of
winged nuts to the tripod head. The legs can be detached after use and
the tripod head be placed in the case with the instrument in a packing
provided for it. Some modification of this form of tripod is generally
used for all large field instruments. The author's improved Everest
theodolite is shown at Fig. 192.
[Illustration: Fig. 193.--_Simple theodolite._]
448.--=Simple Theodolite.=--The plain theodolite being of the cheapest
construction may be stripped of its superior functions, which are used
for testing its adjustments, and be made into a simple angle measurer
for laying out or plotting small parcels of ground, small estates in
building ground, local sewage, gas and water works, and many other
cases of small surveys, for which purpose it will be found sufficient,
with a saving of about half the cost of a perfect theodolite. The
instrument shown above, Fig. 193, was designed by the author to meet
the above cases. In this instrument there is no vertical arc. The
telescope has a socket axis carried upon a single standard. The axis
cannot be seen in the figure from interference of the telescope placed
in front of it. The telescope is arranged to be fixed in a level
position by means of the loose pin being pressed in a pair of holes. It
may then be used as a level by means of the spirit level shown on the
vernier plate. The horizontal circle is divided to read with a single
vernier to 3′ of arc by means of a hand magnifier which is placed in
the case with the instrument. There are internal and external axes,
each provided with clamp and tangent motions to the horizontal circle,
as with the plain theodolite. It is supported on a tribrach, the legs
of which are upon the plan, art. 249.
If it be made with two verniers and divided upon silver it becomes
a useful light instrument for filling in details of superior
triangulation. Weight, about 4¾ lbs.
449.--=Examination and Adjustment of the Theodolite.=--The description
given of a transit theodolite, arts. 369 to 389, will show that
this instrument is provided with means of adjustment in every
requisite direction. Larger transit instruments possess the same
means of adjustment, but in some parts these have greater refinement.
Plain theodolites have the like methods, except in the case of the
transverse axis, which is adjusted once for all by the maker. It will
be necessary, therefore, to limit our space to a discussion of the
examination and adjustments of the transit theodolite only, of which
we have given a full description, arts. 369 to 389, noting only where
variations occur from partial differences between this and others.
450.--A theodolite as it comes from a respectable maker is usually
carefully adjusted in all its parts. If it has travelled a long journey
it is, however, well for an experienced surveyor to put it through its
various adjustments. The corrections, if any are required, will be
generally very small, and these in all probability will be of the same
kind as will occur in the use of the instrument and in the accidental
conditions to which it may be subjected during conveyance from place to
place upon a survey; therefore it is well to be familiar with them.
When a new instrument is received from the maker, it is necessary to
observe attentively the manner of its packing as it lies in its case.
It is well at first to lift the parts a few times gently out of the
case and replace them, so that this may be done at any future time
with certainty and without any risk of strain upon the instrument,
remembering always that an instrument in conveyance is much more liable
to be thrown out of adjustment by carelessly replacing it in its case
than from its ordinary use, art. 42.
451.--For examination or adjustment of the theodolite the tripod
stand should be at first firmly fixed with legs extended to an angle
of about 70° to the ground, which should be solid and hard. As the
telescope has to be brought to the height of the observer's eye, it
is well to mention his stature in ordering an instrument. The tripods
that are made for tall men are often awkward and unsteady if the legs
are extended to bring the telescope down to the height of a short
person. They may always be cut down and refinished by the maker. When
the tripod is set up the toes should be each separately pressed down,
so that future slips are impossible. This being done the instrument
is taken from its case and grasped firmly by the body part under the
horizontal circle, and placed on the tripod at once, then screwed
firmly but not too tightly down upon its bearing surface. With a
6-inch transit theodolite the upper part is sometimes detached and
packed separately in its case. Where this is so, after the body part
is fixed on the tripod, the cleats on the top of the standards must
be opened out, and the upper part of the instrument, lifted by its
telescope, be slowly lowered into its bearings, being particular at the
same time that the clips under the telescope embrace their stay-piece
on the standard. The cleats must then be closed over the pivots. The
instrument being set up to position, all levels may be adjusted to the
centres of their runs, and every part clamped sufficiently to make the
instrument firm, but in no case using violence to produce a strain
in any part. The clamps or other fittings are afterwards separately
released as they are required for examination or adjustment of the
parts to which they relate.
452.--_Examination for Coincidence of Exterior and Interior Vertical
Axes._--The theodolite being set up solidly, and all clamps fixed as
above described, unclamp the lower or exterior axis clamp and set
the vernier plate levels parallel with opposite pairs of parallel
plate screws if the instrument adjusts on the parallel plate system,
art. 193, or one level parallel with one pair of foot screws if it
is made on the tribrach system, art. 234. Now adjust both levels.
Turn the instrument half round (180°) and observe if the levels keep
the centres of their runs. If they do so they are in adjustment to
the exterior axis. If found imperfect, the adjustment by the capstan
heads of the levels is set, by means of the _tommy_ or pin which is
provided in the instrument case, for half the error as it appears by
the bubble, the other half being given by readjustment of the parallel
plate or tribrach screws. In these adjustments it is necessary to be
particular _always_ to observe the bubbles after the hands have left
the instrument, _not during the adjustment_, which produces strain upon
the instrument. Now clamp the lower clamp and note if this clamping has
disturbed the levels. If the levels are very sensitive it will do so in
a slight degree, but the disturbance should be very small if the clamp
is perfect. Now unclamp the vernier plate and note again if this clamp
disturbs the levels: this should also affect them very little. Now
observe the levels if they stand exactly as they did when the exterior
axis was unclamped at their present position, and also at right angles
to this. If they remain as before the axes are truly concentric. If
they do not, there is no remedy except at the hands of the maker. The
vertical axis to which the above examination applies is considered
the most important part of the instrument, and the work should be
thoroughly well done; nevertheless, if the levels are very sensitive,
which they seldom are, such minute faults may be detected, that a small
allowance may be made for imperfection of work, and the instrument
still be considered a sound one. In the use of the instrument it is
always well, after the circle is set either by the magnetic compass or
by sighting a distant point for direction, to clamp the lower clamp and
readjust the levels to the vernier plate. In this way the axis that
will afterwards be used for triangulation will be vertical, and small
errors due to want of coincidence of axes be eliminated.
453.--_Examination of the Azimuthal Level._--This level, which is
placed over the telescope, being made of superior sensitiveness to the
vernier plate levels, is much more accurate for adjusting the vertical
axis, but much slower in operation for testing. The verniers of the
vertical circle should be accurately set to zero, in which position the
run of the bubble should exactly agree with those on the vernier plate
when placed parallel with them in any direction, but this level may
also be considered by itself. Assuming the circle and verniers correct,
or otherwise, it may be reversed over the axis by half turns in all
positions over the parallel plate or tribrach screws, and adjusted by
the capstan heads half the error, as before described, for the vernier
plate levels.
454.--_Examination of the Divisions and Centring._--The vernier plate
being unclamped, the verniers, if two, should be brought approximately
to 0° (360°) and 180°, and then the plate be lightly clamped. The
microscopes or readers are then to be set truly radial with the zero
reading of the verniers, and the tangent screw adjusted to make one of
the readings, say the 360°, exact. The opposite reading, 180°, is then
carefully examined, and the error discovered, if any, is due to the
imperfection of centring, assuming the dividing perfect. At this point
it is well to record the amount of difference. The same examination
is then repeated with the 90° and 270°. In a properly centred and
accurately divided 5-inch or 6-inch theodolite this difference will not
amount to more than 1′ error, in larger instruments proportionately
less. Owing to the difficulties at all times of reading the circle
correctly from difference of direction of light, and what is termed
personal error, it is well to entirely repeat this examination, turning
the instrument half round. It is also well to repeat the examination
at what are termed the _half points_, 45°, 235°, and 135°, 315°. This
will sometimes detect the error of centring, if there be any, in its
true direction. The purpose of the two verniers is to discover this
error. In practice the two readings are always taken, and the mean is
considered as the true reading. Where there are a greater number of
verniers exactly the same principle is followed, but the mean of three
or more readings is taken, which of course assures great accuracy.
Examination of the telescope has been discussed arts. 107 to 115.
455.--_Testing an Instrument for its Stability._--The stability of an
instrument will depend principally upon the quality of the workmanship;
but the same test will also indicate, at any time, whether the
instrument has been submitted to sufficient wear to need the repair
of the optician. For this examination the eye-piece of the telescope
requires to be focussed against a piece of white paper held obliquely
in front of the object-glass so as to throw a soft white light into
the telescope. After the eye-piece is focussed, any distant point may
be taken for a sighting object upon which to direct the telescope.
This point should be focussed by the telescope so that its image falls
centrally upon the intersection of the webs. The eye should then be
shifted up and down or sideways within the range of clear vision of the
webs in the eye-piece to ascertain that there is no parallax, that is,
that the adjustments of the eye-piece and the telescope are in true
focus upon the webs. This preliminary arrangement being made, which
will serve in future examination for other adjustments, all parts of
the instrument should be examined to see that the clamps are firmly
clamped. The object to be used as an index or sighting point should be
brought by the clamp and tangent motions exactly upon the intersection
of the webs as they appear in the telescope, when the following
examinations are to be made.
456.--_Tripod Head Examination._--The telescope being sighted upon an
index point, and all clamps screwed down and the tripod firmly fixed on
the ground, take the tripod head of the theodolite in both hands and
give it a twist of about a pound pull in one direction; then examine
the telescope to see if the index point is displaced in the telescope.
If it still stands correct give a like twist in the opposite direction
and again examine the telescope. If it stands these opposite firm
twists retaining its position the stand is good and in good order. If
it does not, assuming good construction of stand, the remedy may be
found in tightening up all its screws; but if its construction is bad
it will not, even after this tightening, keep in order. There is no
doubt that more inaccurate triangulation is caused by defective tripods
than from any other cause whatever. A perfect instrument is useless on
a bad tripod.
457.--_General Examination of Fixed Parts._--The stand being found
good by the above process, the general fittings of the instrument may
be examined, after clamping all parts and directing the telescope to a
distant point, by taking a quill pen by its root or pipe and pressing
its feathered end upon one side of the eye-piece of the telescope
sufficiently to bend the quill, and afterwards examining the telescope
to see that the webs are not displaced from the index point. This may
be done first to the right hand and then to the left. If the webs
still cut the same object it is clear that the whole of the centres,
fittings, clamps, and tangent screws of the horizontal circle are
correct. If any displacement be discovered, the amount of difference
between the right and left handed twists will be the total error due
to imperfection of work or wear as the case may be. In exactly the
same manner, but by pressing the eye-piece upwards and downwards, the
transit axis and its fittings may be examined. If the instrument be
not generally sound enough to bear the above tests, other critical
adjustments become necessary. For the correction of faults that may be
included in the above operations, the parts of the instrument must be
separately examined.
458.--_Examination of the Transit Axis._--The best means of adjusting
this axis in a theodolite is by a _striding level_, art. 400. When
this is not provided with the instrument, and it is often omitted for
economy, the axis is generally better to be left as it is adjusted in
this particular, by the maker. To adjust the transit axis the vernier
plate bubbles are set exactly true by reversing angles of observation.
The cleats are opened and the striding level is mounted above the
instrument resting upon the pivots. The telescope is placed exactly
over an opposite pair of parallel plate screws, or parallel with
two screws if the base adjustment be on the tribrach principle. The
striding level is then carefully observed and reversed on the pivots.
If there be any difference in the run of the level bubble the transit
axis is adjusted by raising or lowering the movable V on which one
pivot rests by turning the capstan nuts until it is quite correct, if
the instrument has this old-fashioned arrangement, or if not, by a few
strokes of fine emery paper upon the V which is higher. This adjustment
is almost superfluous, as the axis is generally set right at first,
and is not subject to change, especially if solid without an adjustable
V.
459.--For larger theodolites of 12 inches and over, the transit axis
is much better adjusted by means of an artificial horizon, which will
be described further on. By the use of this instrument in the northern
hemisphere the pole star is first observed directly by the telescope,
and then by its reflection from the horizontal surface of clean mercury
placed on the ground at 12 feet or so from the instrument. If the star
and its reflection cut the webs equally in directing the telescope by
movement of its transit axis only from the one to the other, this axis
must be truly horizontal. If the vernier plate be then turned a quarter
of a revolution and the exterior axis a quarter of a revolution, the
telescope transitted and observation be repeated, the verticality
of the principal axis may be adjusted with perfect certainty. The
principal axis should be moved one-eighth of a revolution all round and
the bubble examined at every position to assure perfect adjustment.
With the plain theodolite, Everest's and some others, the transverse
axis is fixed to position by the maker, therefore cannot be adjusted.
460.--_Examination and Adjustment of Webs, Lines on Glass, or
Points._--The ordinary manner of webbing the diaphragm of a theodolite
was shown Fig. 23. Horizontal angles are taken by the upper
intersection of the diagonal webs or lines. A single web is placed
horizontally for taking vertical angles: it is necessary that this
should be nearly true. When the theodolite has its axis vertical, as
shown by the vernier plate bubbles being in the centre of their runs,
if one end of the horizontal web or line be set to cut a small distant
object by sight in the telescope, the same object should keep on the
web while the tangent screw of horizontal circle is moved a distance
sufficient to traverse it, the hand being always taken from the screw
while the observation is made. If it does not do so, the collimating
screws should be lightly tapped with the back of a penknife in the
direction to set it right. These screws have a slot in the body of the
telescope, under the loose covering plate, sufficient to permit of this
small adjustment.
461.--_Adjustment of the Telescope to Vertical Collimation._--The
eye-piece is first focussed as before against a piece of white paper
held obliquely in front of the object-glass until the webs are
sharply seen. The axis of the telescope is then examined for vertical
collimation error. The method of doing this has been already described
for a telescope placed in Y's, as it is in the Y-level, and the plain
theodolite, art. 200. The only difference with the transit theodolite
is that instead of turning the instrument in its Y's, the telescope
is _transitted_, as it is termed, over on the transverse axis exactly
half a revolution, or 180° as seen by the vernier reading; and the
horizontal circle is moved also half a revolution, so that the
telescope points again on the same distant point which is used for an
object. If the webs or lines still cut the same point or small object,
they are in vertical collimation, or truly in the optical axis of the
telescope, as regards the vertical direction which this adjustment is
intended to secure, presuming the circle has been correctly divided and
centred and the verniers accurately set. If the webs or lines do not
cut the same point, half the error is corrected by the top and bottom
collimating screws near the eye-piece. This process is repeated until
it is exact, being particular to observe, as before mentioned, that
there is no parallax. This adjustment cannot be made with the plain
theodolite; but the zero of altitude may be examined on both sides of
the arc.
For the transit theodolite, adjustment by means of a collimator, art.
229, is much more convenient and exact, as lateral and vertical errors
in the position of the webs can be detected in one operation. When a
Y-level is at hand, this may be used as a collimator if it is first set
to solar focus.
462.--_Examination for Perpendicularity of Transit Axis and
Telescope._--The whole of the lower part of the instrument retaining
its position with all clamps firm, open the cleats upon the top of the
standards so as to release the transit axis. Now release one of the
clip screws and gently lift the upper part of the instrument out of
its bearings. Turn the telescope the reverse way upwards, which will
be in this case bubble downwards. Release the clamp and turn the clips
to the reverse position of the telescope, and reverse the position of
the pivots in their bearings. If the telescope be now directed to the
same point as before, and the webs still fall upon it, the telescope
adjustment is at right angles to the transit axis.
463.--_Examination of the Magnetic Needle._--If the needle be placed
in a circular box, as shown in the engraving, Fig. 30, it admits of
no adjustment. If it is placed in a trough, Fig. 161, it admits of
adjustment generally by lateral screws to a portion of its division.
If the needle is used for a survey, it is set to the zero of the
horizontal circle by clamping the vernier plate and bringing the
northern vernier to zero, then releasing the exterior axis and bringing
the needle by the motion of the lower tangent screw to the zero of
its circle. The corrections of the needle for giving true north have
been discussed, art. 132. It is difficult to read an ordinary edge-bar
needle correctly, it is also difficult to mount it perfectly true. It
may be read at both ends, and if the 0° and 180° points cut the line
fairly it is considered correct; if not, the mean of the difference may
be taken. In some instruments a microscope is mounted over the needle
point that the needle may be adjusted to a web; but British surveyors
seldom feel confident of surveys by the magnet, and for triangulation
generally prefer to employ a certain number of distant fixed points,
the bearings of which are at first as accurately ascertained as
possible, for referring objects, rather than to refer frequently to the
magnet. When the needle is out of use it should always remain lifted
off its centre. When the instrument is put by for a long period it is
better to place it in a vertical position and free the needle, so that
it rests in the magnetic meridian, in order to preserve its magnetism
as much as possible.
464--=Use of the Theodolite.=--In setting up a theodolite, place the
tripod nearly over the position in which it is to be used. This is
frequently the socket hole formed in the earth by the removal of a
ranging pole or _picket_, to be described Chapter XVII. Then, after
it is set up, suspend the plummet from the hook, which will be found
inside the head of the tripod. If the ground be solid and level, then
by shifting the toes of the tripod slightly, and firmly pressing them
down one by one, the centre of the plummet may be brought easily within
about ·25 of an inch of its true position. The theodolite is then
placed on its tripod, observing that the telescope is in a position
easy to be used. The centre of the picket-hole, when this is used for
a station, is generally taken by guess-work, which is considered near
enough. It may be taken with a little more refinement by placing, in
the same hole, a short false picket of nine inches or so in length,
but of the same diameter as the ordinary picket, the top of which is
cut off smooth and polished, and has lines sawn across its centre
inlaid with ebony, described in Chapter XVII. The false picket is
carried about with the theodolite. With Everest's and many other forms
of theodolites the hook is fixed under the axis of the instrument. In
this case it is usual to set the theodolite before adjusting it to the
station, as there is no separate hook to the tripod, which also occurs
with all framed stands.
465.--Where there is no hook to the tripod an excellent plan is to have
a false centre, which may be a piece of turned wood with a hole through
it, to fix on the top of the tripod head. The plummet cord adjusts
through the hole. This false centre is also convenient where the axis
adjusts to position by a mechanical stage. Fig, 194 shows a false
centre formed of a piece of ivory with two slots to permit the cord to
loop over and yet hang centrally.
466.--It may be observed that if the tripod be set up out of level,
which it must necessarily be in many cases, the hook, if attached
to the stand and following its inclination, will not hold the cord
at a truly vertical position to the axis. Surveyors commonly allow
a little for this inclination. It is much more accurate to have the
cord suspended directly from the axis of the instrument when it is
constructed to admit this. Then if a false centre be used the plummet
should be suspended a second time from the axis hook. With the kind
of runner shown in the figure this need take little time, as it is
instantly detached and replaced.
[Illustration: Fig. 194.--_False centre for a tripod head._]
467.--After the tripod is fixed with the theodolite upon it, the
readers are set to exact focus. The horizontal circle is then brought
to zero by the vernier plate clamp and tangent, and the compass brought
to magnetic north, if all angles are taken in reference to this as
a check, by means of the lower clamp and tangent. The vernier plate
clamp is then released. The eye-piece is correctly focussed upon the
webs, lines or points against the northern sky, or upon a piece of
white paper held obliquely if this is preferred. The telescope is
then ready to be directed towards a picket or other station mark to
be observed, and set correctly to focus this, after which the eye
is moved to the right and left, to the extent of clear vision in the
eye-piece, to see that the object appears to remain fixed upon the
intersection of the upper V of the webs, or does not _dance_, as it is
sometimes termed. The observation, if of a picket, should be taken as
near the ground as possible, as it may not be set quite upright. If
the telescope is directed to objects where the sun's rays would enter
it, the ray shade should be pulled out sufficiently to quite shield
the object-glass. The initial reading to be recorded is always taken
on the _face_ of the instrument, in which position the upper tangent
screw is always on the right-hand side. When the observation is clear
and satisfactory, it is recorded in the field-book. If the sight lines
taken are to be measured by the chain, the amount of inclination is
taken by the vertical circle reading to the top of the picket if this
is the 6-feet ordinary length, or to a marked band if this is longer.
The inclination may be taken exactly to angle by vernier, or roughly
by scale of difference of hypotenuse and base if this is engraved on
the vertical circle, or by both of these--the one as a check upon
the other. It is common to take the upper inclination as a plus (+)
and the lower as a minus (-). Inclination observation is recorded at
the same time as the horizontal position. Other observations of the
various positions or pickets are taken in a similar manner at the same
time. When a stadia or tacheometrical diaphragm is used the angle is
recorded and the stadia reads the distance. It is thought well when
the theodolite is in position to take as many exact observations as
possible in all directions of intended stations. It is also convenient
to take a number of observations, which from the circumstances present
may be inexact, such for instance as the angles subtended by trees,
gates, rough buildings, or even sometimes the corners of fields, as
from such observations these objects may be placed nearly enough for
ordinary plotting by the angles they subtend from this and another
station upon the plan. In any case they form a check to positions if
taken with pickets afterwards more definitely. These may be marked in
the field-book _inx._ for _inexact_.
468.--=Field Book.=--This book is generally made 8 inches by 4 inches,
covered with red leather, with elastic closing band and sheath for
pencil, as an ordinary pocket-book. It contains about 100 pages of good
stout writing-paper. Two lines are ruled in red ink, ¾ inch apart,
vertically down the centre of each page. The column between the lines
is used for distances measured by the direct chain line at which hedges
are crossed, stations, offsets, or other measurements are taken. In the
right and left columns observations are made of objects desirable to be
recorded or triangulated.
469.--For superior triangulation, definite and prominent fixed objects
are taken at as great distances as possible, so as to include the
details of measured triangles within a superior triangle. A church
steeple, for instance, is a favourite sighting object. This cannot,
however, generally be made, a station for future triangulation unless
a scaffold is built up around it. Generally the most convenient method
on fairly level ground, if the survey is large, is to have an ordinary
scaffold pole, 20 feet or so in length, carefully straightened by a
village carpenter with a stretched chalk line and then painted white.
This may be squared at the end and fixed vertically in a socket formed
of crossed boards to a depth of about 3 feet in the ground, with long
crossing tail pieces rammed firmly with the soil to keep it steady.
When this is used for a triangulating station, the pole is taken
out of its socket and its exact position is centred for placing the
theodolite. Flags are sometimes used to indicate stations: their defect
is that the wind may blow them from or to the observer and thus render
them invisible. Other methods will be found in practical works on
surveying. This subject will also be reconsidered in Chapter XVII.
[Illustration: Fig. 195.--_Diagram bisection of circle._]
470.--_Elimination of Instrumental Errors during
Triangulation--Changing Face._--It is generally advised to _change
face_ with the theodolite after angles are taken in the ordinary way,
that is, to take first the initial angles reading from the _face_
vernier with the tangent screw on the right hand, and then to take the
same angles with the back vernier, the telescope being transitted.
This, of course gives a reading on a different part of the circle and
corrects the error of position of the vernier, or centring, in the
following manner:--In Fig. 195 let a be zero (360°), the reading of
the face vernier. Let the opposite reading (180°) be at _a′_. Suppose
at 180° on the left-hand side of the instrument the 180° reads at
_b_, then observe by the telescope an object that cuts this reading,
or place a picket to do so. Change face; then the same arc will come
to _c_, and the telescope must traverse _cb_ to come to the first
direction. The instrumental error is half _bc_, which bisected in _a′_
gives 180° exactly. The same principle of repetition with changed face
may be made an any part of the circle, and the mean will be the correct
reading.
471.--_Repeating Angles._--This is performed by taking all parts of
the circle for reading a given angle, so that errors of division
and centring of the instrument are eliminated. The process is as
follows:--Take the angular positions of two objects in azimuth,
commencing with the zero of the horizontal circle, say the two objects
subtend from the centre of the instrument 36° 10′; then turning the
telescope back from its advanced position at 36° 10′ by releasing the
lower or axis clamp, we may bring the first reading to the original
zero position. Now clamp the lower clamp and release the vernier plate
clamp and take again a forward reading. If this reads 36° 10′ + 36° 10′
= 72° 20′, the circle and centring appear so far correct; but it will
probably read 72° 21′, and the corrected reading would be the mean 36°
10′ 30″. If we continue this system round in ten pairs of readings
the whole circle will be embraced, then the mean of the sum of the
minutes divided by the number of pairs of observations will give the
true reading of the minutes. By taking the readings of two opposite
verniers separately, the circle would be encompassed by five readings.
This plan is followed in all important triangulations where the work
is submitted to calculation. Such refinement is scarcely necessary for
direct plotting with the protractor.
472.--It may be observed that if the horizontal circle is placed with
its zero constantly to magnetic north--not necessarily for taking
angles in reference to this--that the same part of the circle will
always be used in the same direction; so that the sum of errors of the
whole circle must necessarily tend to tie, even if the division is to
a certain degree imperfect, provided also that the protractor used in
plotting is also kept in one direction. This plan has otherwise no
inconvenience, as any arc or angle may be taken by the difference of
the circle reading in any position in which it may happen to fall.
This does not mean that it is advisable to survey above ground by
the needle--it is quite otherwise. It is best to have some distinct,
sharply defined object to which all angles are referred, and therefore
called a _referring object_, as the general index. The magnetic bearing
need only be the initial position of the horizontal circle of the
instrument.
473.--With larger, what are termed geodetic instruments, to be
described in the next chapter, constructive errors are not permissible;
but these instruments are observed under altogether different
conditions, which are suitable to the precision demanded. A large
theodolite is generally fixed upon solid rock, or masonry with good
foundation, or upon a very firm solid framed stand, and is protected
from wind, sun and rain. Where it is necessary on level ground to
elevate the instrument for more extensive view, a proper structure is
built, in which the theodolite is isolated from the outer walls or
enclosure carrying the stage upon which the observer works, so that
no vibration or deflection of this, caused by the wind or the weight
of the body, affects the instrument. Under such conditions angles are
read on various points of the circle by micrometer microscopes so as
to obtain a sufficient number of means, that personal and instrumental
errors may be reduced to a minimum.
CHAPTER X.
LARGE THEODOLITES USED ONLY FOR GEODETIC SURVEYS--STANLEY'S 10- AND
12-INCH--14-INCH ALTAZIMUTH--COLONEL STRANGE'S 36-INCH THEODOLITE.
474.--Large theodolites employed upon geodetic surveys. Where the
complete survey of a country has to be made, a system of large
triangles is formed over the country from convenient positions which
are naturally or artificially elevated so as to obtain distant views
with the telescope. These triangles are correctly measured by angles
subtended from a very carefully measured base or bases set out upon
approximately level planes, which are generally of a mile or more in
length. Where measurements are derived from such bases by constant
intersection of angular positions extended therefrom to large triangles
or other polygons, it becomes important that the theodolite employed
should measure such angles with great accuracy. In this case the
vernier reading does not possess sufficient refinement, and the
divisions representing the degrees have to be magnified to appear wider
apart, so that they can be more finely subdivided for the reading to be
taken by means of a micrometer microscope capable of subdividing the
divisions made upon the instrument even to single seconds of arc. The
theodolites used for the superior triangulation of Great Britain were
Ramsden's 36-inch and 18-inch, which, although constructed in the last
century, remain excellent instruments.
475.--The construction of large theodolites is varied very
considerably according to the conditions present in the country to
be surveyed. This subject if carried into detail would extend much
beyond the intended limits of this work. This chapter will therefore be
limited to the description of a 10- or 12-inch instrument, and to two
historical instruments which have been used successfully for geodetic
work.
[Illustration: Fig. 196.--_Stanley's 10-inch transit theodolite._]
476.--=10- or 12-inch Theodolite.=--These instruments approach the
limit in size of portability for movable stations in triangulation. The
illustration, Fig. 196, is of the author's latest 10-inch model, the
patterns of which were made for a theodolite with vernier readings to
be used for the construction of a spiral tunnel through the Andes, now
completed. The object in its construction was to obtain great rigidity
with moderate weight. To this end the gun-metal of which it is made
is shaped out from castings as comprehensive in unity of parts as
possible. It has a framed mahogany stand (not shown) which is braced in
every way and provided with a very rigid head. In general construction
of the instrument illustrated it has a mechanical sliding stage and an
extra powerful clamp and tangent arrangement for the lower limb, the
adjusting screws being all covered to exclude dust. The circles are
divided to 5 minutes, and are read by micrometers to single seconds
of arc. Two vertical arcs are used, the second one carrying the clamp
and tangent arrangements, which also serves to balance the trunnion.
The 10-inch instrument carries a 16-inch telescope with 2-inch
object-glass, and the 12-inch instrument an 18-inch telescope with
2-1/8-inch object-glass. The tangent screws all act against springs to
avoid loss of time.
[Illustration: Fig. 197.--_14-inch altazimuth theodolite._]
477.--=14-inch Theodolite.=--For this description a modern instrument
is taken which Colonel A. R. Clark selected for illustration in his
excellent article on Geodesy in the ninth edition of the _Encyclopædia
Britannica_, to the publishers of which the author is indebted for the
illustration, Fig. 197. The instrument is a combination of a transit
theodolite with special arrangements as an altazimuth instrument
with fixed base, one side of the vertical circle being divided to
place the zero in a direction coincident with the polar axis. The
construction as a simple fixed transit theodolite for support upon a
pedestal in stone wherein the axis remains permanent for a principal
geodetic station, and therefore requires only a single setting to
bring it to true north and south for zero, renders change of position
of horizontal limb unnecessary for a permanent station. For this the
instrument is well adapted, and will be discussed here. The telescope
is of 18 inches focus, with 2 inches clear object-glass. The axis
pivots are of hard steel: one is perforated for illumination by a
lamp. The vertical circle is placed almost directly upon the side of
the telescope, and the tangent arm on the opposite side is of nearly
equal weight, so that there is no counterbalance necessary. There are
three Ramsden eye-pieces giving powers of 17, 35, and 54, and one
diagonal eye-piece. A level is attached inside the standard, divided
to read 10″ of arc: this has cemented ends, art. 177, and is enclosed
in an outer tube for protection. Two other exactly similar levels are
attached to the exterior axis of the instrument. The circle is divided
to 5′ of arc and reads by two micrometer microscopes to single seconds.
The vertical axis of the instrument is of steel. It is placed with
the apex of the cone upwards, and terminates on a triangular spring
with three adjusting screws by which any portion of the weight of the
upper part of the instrument can be relieved from the axis, so that the
whole instrument moves quite freely. The horizontal circle reads with
three micrometer microscopes on the upper circle to single seconds.
Originally the light was thrown down on the divisions by three ivory
cones placed over the fronts of the microscopes, as shown in the
illustration; but these have been changed in the present instrument for
concave swivelled reflectors, which may be set to any angle convenient
to throw sufficient light upon the circle. The microscopes are
supported from the body of the instrument upon hollow conical arms upon
the same excellent plan originally used by Ramsden. The microscopes
have adjustments in three directions, so as to bring them exactly into
place for trisection of the horizontal circle. The clamp and tangent
motion is placed directly upon the divided circle, and has adjustments
to secure freedom from strain; but this is not perfect--it is perhaps
the worst feature in the instrument, some modification of the plan
shown, Fig. 196, being much better for large instruments.
The whole instrument is mounted on a tribrach frame, which is adapted
to stand upon a portable table or upon masonry. The screws have lateral
adjustment to prevent loss of time by wear.
478.--It is a common custom with this class of instrument to make the
axes of hard steel. This plan is no doubt very satisfactory as it
leaves the optician's hands, but the author very much prefers good hard
bell-metal. When he saw the above described instrument at Southampton,
there was quite sufficient evidence of rust on the pivots to destroy
all perfection of centring, and this could scarcely have occurred
with bell-metal. Of course the brittleness of bell-metal would be
objectionable where the instrument might be subjected to severe jar
in carriage from place to place; but the author has obviated this by
a plan he would strongly recommend for general adoption--of having
the axis of good gun-metal, and to silver-solder a ring of bell-metal
thereon where the fitting surfaces occur. If the gun-metal is pure
it will bear the average reliable strain of hardened steel, which in
hardening and tempering is not with certainty always free from flaws;
and the average wear of pure bell-metal is perhaps quite as good as
steel.
479.--=36-inch Theodolite=, Fig. 198, was designed by the late Colonel
A. Strange and constructed by Messrs. Troughton & Simms for the Great
Trigonometrical Survey of India. It is probably the most complete
and perfect theodolite ever constructed. The leading characteristics
of this important instrument only will be given. It has a horizontal
circle 36 inches diameter, and a vertical circle 24 inches diameter.
The telescope has a focal length of 36 inches: the aperture of the
object-glass is 3·25 inches.
[Illustration: Fig. 198.--_36-inch theodolite--Great Indian Survey.
From a photograph._]
480.--_The Stand_ has three massive mahogany legs _AA_ braced together
with horizontal and oblique wrought-iron bars _B_. Each leg is divided
vertically, and contains a long, gun-metal, square-threaded screw _C_
which is made to rotate by means of a worm-wheel and endless screw
worked by a winch handle _D_, and capable of being firmly clamped
after adjustment at points about 15 inches apart _E_. The upper ends
of these screws are conical, and fit into three inverted radial grooves
formed in the lower side of a cast-iron circle or table, which is thus
supported by the three screws without being attached to them, and is
therefore free to accommodate itself to expansional changes without
restraint. The upper surface of the cast-iron circle is turned flat and
true to receive the tribrach of the instrument. The three screws _F_
which pass through the side of this circle are intended to adjust the
centre of the instrument over the station mark. A lever _G_ also passes
through the side of the circle and actuates three rollers, which, when
in action, support the greater part of the weight of the instrument,
and so enables the horizontal zero to be set without difficulty. As
the instrument weighs over 400 lbs., it will be seen that some such
arrangement is absolutely necessary to enable it to be moved on the
cast-iron circle. When the correct position has been obtained, the
lever is thrown out of action, and the instrument remains immovably
seated upon its circular frame.
481.--_The Foot Screws_ are tapped through the ends of the tribrach
arms in the usual way, but have a range of motion not exceeding 1/10
inch. This range may appear small, but is really much more than is
required, as the upper surface of the cast-iron circle can be levelled
by the long screws in the mahogany legs before the instrument is placed
on it, so that not more than about 1/100 inch of motion is required.
The foot screws do not rest directly on the cast-iron circle, but on
the extremities of an intermediate three-armed plate, securely bolted
to the centre of the instrument, the distance between the tribrach
and the plate being about 1/10 inch. The object of this arrangement
is to obviate the disturbance of level and azimuth which arises from
clamping foot screws of the ordinary construction after adjustment,
as well as that due to looseness of the foot screws in the tribrach
arms. The arms of the spring plate, being of considerable width, have
great horizontal rigidity, but being comparatively thin are easily bent
vertically. The outer ends of the arms rest on the cast-iron circle or
stand; the foot screws pass through the tribrach arms, but not through
the spring arms. It is evident therefore that when the foot screw is
turned inwards with the screwing motion, the solid end of the tribrach
will be raised and the slit between the two arms widened; but since the
end of the screw does not rest on the stand, but on an intermediate
arm, which is actually a portion of the tribrach itself, it is clear
that if a lateral pressure be applied to the tribrach no motion will be
caused thereby, however loose the screw may be, so long as the pressure
is less than the lateral rigidity of the intermediate arm. The lateral
pressure caused by turning the instrument in azimuth when taking
observations is greatly within this limit. This plan is perfectly
successful, but it is only available where a moderate range of vertical
movement is needed. In the present instance, as the cast-iron ring
or stand on which the instrument is supported is always first made
practically level, the vertical range of the foot screws need not be
more than a small fraction of an inch. Another point with regard to the
foot screws is their delicacy and certainty of action. This is attained
by applying to them a clamp and tangent screw arrangement _H_ very
similar in principle to that sometimes applied to circles. Although
the foot screws themselves are rather coarse, having only about eight
threads to the inch, the arrangement is such that one entire revolution
of the slow motion tangent screw alters the level only about one second
of arc. Hence the foot screws in this instrument, though coarse and
strong enough to bear great weight are probably for the first time made
in keeping, in point of refinement, with its most delicate parts.
482.--_The Horizontal Circles._--The inner or working circle is 36
inches in diameter. It is very finely divided on silver to 5 minutes,
and is read by five equidistant micrometer microscopes to tenths of
a second of arc. It is fixed at the centre to the tribrach, but
everywhere else is perfectly free. The outer or _guard circle_ consists
of a second horizontal circle exterior to and concentric with the inner
circle. There is a space of about 1/10 inch all round between the two
circles, and the upper plane of the outer circle stands about the
same quantity above that of the inner or principal circle. The guard
circle is supported by radii of its own, quite independent of those of
the inner circle. This circle has several functions. It protects the
working circle from accidental injury; it helps to distribute changes
of temperature uniformly over the circumference of the working circle;
it receives the clamp and tangent screw, leaving the working circle
absolutely free from contact at all times; and it bears a strongly-cut
set of divisions, more visible to the naked eye than those of the
working circle, which are exceedingly fine, and therefore would be
inconvenient for setting the instrument approximately in azimuth.
483.--_The Horizontal Tangent Screws._--It will be seen at _II′_ that
there are two clamps and two tangent screws to the horizontal circle.
It is necessary to have both, on account of the large size of the
circle. In use, of course, they are not both used at the same time.
In the present position of the instrument the clamp and tangent screw
on the left-hand side of the illustration would be employed; but on
reversing the telescope this clamp would be released and the one on
the opposite side made use of. It is necessary with this, as with
smaller instruments, to avoid loss of motion in the tangent screws.
Many methods have been employed to obviate this loss of motion, but
while they are suitable to small instruments they are not so effective
with large ones, such as that under consideration. The plan adopted in
this case is that known as the _divided nut_ principle. The block into
which the tangent screw is tapped is divided transversely and the two
halves are forced asunder, and therefore act against the contrary sides
of the screw threads by four internal spiral springs. The tension of
these springs is necessarily constant, and therefore not subject to
the disturbance and slow recovery of elastic force unavoidable in an
external spring. Means are supplied for regulating the tension of the
four springs, which must be a little in excess of the force necessary
to move the revolving mass, without taking the parts to pieces.
484.--_The Vertical Axis_ is a truncated cone of steel with its base
downwards. It is about 6·5 inches high and 3·3 inches and 2 inches in
diameter at the base and summit respectively, the flange being about
4·5 inches in diameter and constructed on the isolated principle. The
vertical axis socket and the five horizontal microscopic arms are cast
in one piece of aluminium bronze, the elliptical table carrying the
telescope supports being bolted to the central boss in which the socket
of the vertical axis is formed. The vertical axis and the elliptical
table are both perforated in the centre so as to allow of a look-down
telescope being employed in adjusting the instrument accurately over
the station mark.
485.--_The Telescope_ is furnished with two separate eye-ends, carrying
respectively a vertical and a horizontal parallel wire micrometer _J_.
It is also supplied with both bright and dark field illumination, the
latter being employed when faint stars are observed. The vertical
circle _K_ is divided on silver similar to the horizontal circle, and
is read by two opposite micrometer microscopes when the instrument
is used for terrestrial work: but when required for astronomical
purposes four micrometers can be used, and they can be shifted to any
part of the circle on which they are clamped. In the illustration the
four micrometers are shown in position. The two rods or handles seen
parallel with the telescope at _LL′_ are attached to the middle of the
transit axis where the telescope passes through it, and are intended to
raise or depress the telescope without touching it by hand. These rods
are also used for carrying adjustable counterpoises, the instrument
being so balanced in every part that the equipoise is as nearly
perfect as practicable through any diametrical section of the vertical
axis.
486.--_The Spirit Levels_, both horizontal _M_ and vertical _N_, are
very delicate. They are constructed so that the divisions on their
scales represent as nearly as possible one second of arc. The scales
are divided to twenty per inch. The glass bubble tubes are mounted
on V bearings, and are kept in position by light springs in such a
manner that they are free to adapt themselves to changes of temperature
with perfect freedom. They are also enclosed in external cylindrical
glass covers to protect them from sudden changes of temperature. The
arrangements for adjusting the levels are such as to obviate strains
without risk of shake, and to ensure delicacy of action.
487.--_The Five Micrometer Microscopes O_ for reading the horizontal
circle are carried by the same number of equidistant radial arms
branching from the central boss which carries the whole of the
instrument above the horizontal circles. These micrometers are made on
Robinson's principle, Fig. 199, that is, with a short bow spring _S_
having a central nut tapped through it to keep the tension between the
bearing of the micrometer screw on the end of the outer box and the
slide which carries the webs constant with whatever part of the screw
may be in use. The radial arms each carry a vertical socket which is
bored out cylindrically to receive the microscope. These sockets are
slotted vertically, and have three clamping screws at the side to
hold the microscopes firmly in position when they are once adjusted.
The two webs in these micrometers are placed parallel to one another,
and at such a distance apart that when in proper adjustment they are
a trifle wider apart than the width of one line on the circle, as
shown in Fig. 200. The micrometer heads are divided into sixty parts,
and the whole is arranged so that in practice ten revolutions of the
micrometer screw traverse the webs over ten minutes of arc or two
divisions on the circle. Each division therefore on the micrometer
head represents one second of arc; and as the divisions are clearly cut
on silver and about one-tenth of an inch apart, there is no difficulty
in reading to the tenth of a second, which, on a circle of 36 inches in
diameter, is equal to the ·00000872 of an inch, or the three-thousandth
part of one division of the circle; this, as before stated, is equal
to five minutes of arc, or the ·02616 of an inch. The illumination
of the microscopes, or rather of the divisions of the circle, is a
most important matter. When such exact measures are to be taken it
is effected by means of perforated silver reflectors attached to the
micrometer arms and mounted quite independently of the micrometers
themselves. The axis of each reflector coincides with the axis of
its microscope. All the reflectors have both vertical and horizontal
movements, and are therefore readily adjustable to the best position
for securing effective illumination under the varying conditions in
which the instrument may be employed.
[Illustration: Fig. 199.--_Robinson's micrometer._]
[Illustration: Fig. 200.--_Webs of micrometer._]
488.--_Relieving Apparatus._--It will be readily understood that the
moving parts of so large an instrument must necessarily be very heavy.
In this case the telescope, vertical circle, pillars, elliptical table,
horizontal micrometer arms, and vertical axis socket weigh nearly 300
lbs. It would of course be impossible to take horizontal angles with so
much friction on the flange of the vertical axis as this weight would
produce, hence the necessity for some form of relieving apparatus. That
employed in this case is a system of forty spiral springs, each of a
definite length, which when adjusted support about 6·25 lbs. The spiral
springs are mounted on a flat ring in two circles with projecting pins
to keep them in position. The upper ends of the springs support a
steel ring with a circular groove on its upper surface, between which
and a corresponding groove in the outer part of the vertical axis
socket three equidistant, nearly frictionless steel rollers run; so
that by this means about 250 lbs. weight is taken off the flange of
the vertical axis, the remaining weight being sufficient to allow of
the instrument moving with the necessary freedom, and at the same time
giving all the stability requisite for accurate levelling.
CHAPTER XI.
MINING SURVEY INSTRUMENTS--CIRCUMFERENTORS--PLAIN MINER'S
DIAL--SIGHTS--TRIPOD STAND--ADJUSTMENTS--HENDERSON'S DIAL--LEAN'S
DIAL--ADJUSTMENTS--HEDLEY'S DIAL--ADDITIONAL TELESCOPE--IMPROVED
HEDLEY TRIBRACH AND BALL ADJUSTMENT--REFLECTORS--CONTINENTAL
FORMS--THEODOLITE SOUTERRAIN--TRIPOD TABLES--STANLEY'S MINING
THEODOLITE--PASTORELLI'S AND HOFFMANN'S ADJUSTABLE TRIPOD HEADS--
MINING TRANSIT THEODOLITES--STANLEY'S PRISMATIC MINING COMPASS--
HANGING DIAL--HANGING CLINOMETER--SEMI-CIRCUMFERENTOR--MINING LAMPS.
489.--=Miner's Circumferentor.=--In the original form of theodolite,
as it was at first designed by Digges, open sights took the place of
the telescope. The sights in this case were extended on arms. The
compass-box, afterwards added, was placed over the axis and made as
free from obstruction as possible, so that the needle, upon which
general surveying formerly depended, could be read correctly by
placing the eye vertically to the plane of the horizontal circle of
division against which the needle read. After the introduction of
the telescope to the theodolite this old form of instrument took the
general designation of the _circumferentor_; and subsequently, being
best adapted to underground surveying, it became, with some slight
alterations, the _miner's dial_.
490.--Upon this original circumferentor improvements have been made in
the various mining dials we possess, in all of which the large open
compass is still preserved. This prominence of the compass does not
indicate that the modern scientific mining engineer has any desire to
depend upon it for taking horizontal angles, but that in close and
tortuous workings it provides the nearest and often the only possible
means of taking angles having regard to the extreme difficulties of
observation of any kind. Where workings are open and fairly plane
the telescope and circle with vernier reading can be used, so that
at the present time the better instruments possess the means also of
taking observations of angular direction by vernier reading. Several
other very important factors specialize mining from ordinary surveying
instruments, which may be stated as follows:--1. That there shall be
means of shortening the tripod for work in strata of small depth. 2.
That the instrument shall be low and compact in itself, that the head
of the surveyor may be placed above it if possible, even in shallow
workings. 3. That great extent of adjustment of the compass-box to
horizontality shall be given in the fittings of the instrument, on
account of the difficulty of extending the legs at all times for tripod
adjustment and from the extreme inclination of the floor of the working
in some cases. 4. That it is desirable in mining survey instruments
that the telescope, if there is one, shall take sights at all angles
upon the surface of the earth in the locality in which the instrument
is used, as also about a vertical position, so as to be able to sight
lines from the top to the bottom of the shaft, or _vice versa_, to
set off angles in the same azimuth as those taken at the surface by
direction of stretched wires or otherwise. This last contrivance will
also give the means of sighting a perfectly vertical point beneath the
centre of the instrument placed at the top of the shaft, to make a
concurrent station below during ventilation, when the plummet would be
disturbed. The devices by which these various requirements have been
met more or less perfectly will go far to explain the specialities of
construction found in mining surveying instruments, which will now be
described, commencing with the oldest and most simple specialised form
upon which improvements have been made in many directions.
491.--=Plain Miner's Dial.=--The original simple form of specialized
miner's dial is shown in Fig. 201. It consists of a compass, divided to
single degrees, read by a finely pointed edge-bar needle mounted on a
jewelled cap. The needle has a sliding rider placed upon it, art. 130,
so that it may be carefully balanced to horizontality in any locality
in which it is used. The divided compass is raised on a step, and the
upper surface of the needle is made to be quite level with the division
when the compass is horizontal. In erecting the instrument with the
needle correctly balanced, the compass may therefore be brought to
horizontality by the coincidence of the upper surface of the needle
with the plane of the divisions, without the necessity of having spirit
levels.
[Illustration: Fig. 201.--_Mining dial._]
[Illustration: Fig. 202.--_Cover to the same._]
[Illustration: Fig. 203.--_Sight._]
[Illustration: Fig. 204.--_Section of ball and socket joint._]
492.--The compass-box is extended in one meridian, north to south, by
strong arms that carry a pair of sights hinged to turn down to the
surface of the cover for portability. The compass-box and arms together
are termed the _limb_. The limb of the instrument is mounted upon a
_ball and socket joint_ to be described. The socket is slotted down on
one side to permit the limb to be turned to a vertical position. In
this position the level shown on the front of the instrument is used
for levelling by means of the sights: this level is not, however, put
on all plain dials.
493.--The cover of the compass-box, Fig. 202, is fixed on the box to a
given position by a stud and slot. It has an arc divided upon its outer
surface, which is centred from a small hole placed near the outer edge.
A line from the centre of the hole to the zero of the arc is made
perpendicular to the central indices of the sights. A piece of silk or
a horse-hair carrying a small plummet is fixed to hang from the hole.
By this means when the limb is turned down in the slot of the socket
and the silk or hair stretched by the plummet to permit it to hang in
front of the arc, it will then cut the divisions, and thus form a
reading index to the arc, giving thereby approximately the vertical
angle at which the sights are set to degrees.
494.--The instrument is mounted on a simple jointed tripod to be
described. It will be seen by the above description that this
instrument is cheaply made, and is not designed for very exact work.
It is now giving way for more exact instruments, but it forms the
groundwork on which mining survey instruments are most generally
constructed. The height of this dial with sights erect is 11 inches;
weight, 6 lbs. Some of the separate parts above enumerated, which are
common to many other forms of mining instruments, will now be more
particularly described.
495.--_Sights_, one of which is shown separately, Fig. 203, are common
to mining instruments. They are constructed essentially in two parts,
technically termed the _slit_ and the _window_. The _slit_ _A_ is a
narrow parallel cut made through the metal upon the inner surface
of the sight, which is turned towards the centre of the instrument.
The thickness of the metal is hollowed away on the outer side which
comes next the eye, so as to present a thin edge only for the sighting
slit, as shown in section at _A′_. In some instruments the slit is
formed of two thin plates fixed to the sight by screws in slots, which
render it adjustable both to width and position; this is the better
way if machinery be not used for cutting the slit. The _window_ _B_
is an oblong opening, across which a hair wire or a thin plate placed
edgewise is fixed in line with the slit. The hair or wire is laid in a
deeply engraved line, so that it is in the same plane as the centre of
the slit. The ends of the hair are held firmly by drawing them through
small holes and fixing them therein by means of dry, conical, pinewood
pins pressed tightly in the holes. When a thin plate is used edgewise,
this is soft-soldered into the top and bottom of the window. In the
pair of sights the window of one sight is placed at the lower position
and the slit in the upper. In the fellow sight the positions of these
parts are reversed, the observation being always taken from the slit
through the window. The duplication of parts in each sight permits it
to be used in either direction.
496.--_In the use of the Sight_ the point or object to be observed from
the slit should appear to be bisected by the hair in the window at the
same time that it appears to the eye to stand in the centre of the
slit. For this reason it is not necessary that the slit should be very
narrow. It is generally more comfortable to take the sight with the eye
at the distance of 10 to 12 inches in front of the slit to obtain clear
vision of it. In this case if it be made too narrow it shuts out the
field of view.