taken that the metal is quite free from iron, and that no iron comes
near it. On this point the maker cannot be too guarded. The author has
in several instances found the compass-box of perfectly free metal; but
a single foul screw made of commercial brass wire, being used to fix
the ring or the rose plate, has by its influence entirely destroyed the
value of the compass.
140.--In the construction of the compass-box the author has found the
most certain method of getting the divisions correct with the centre is
to make the division directly from the standing-point of the compass,
and not to try to get this point correct to the divisions afterwards.
The standing-point may be fixed directly to the box by screwing, or
be attached to a brass plate before fixing. It is adjusted to the
compass-box by bending until the needle turns freely, but at the same
time nearly touches the circle. The needle is then removed and the
circle is divided with the point as its centre. Where the divisions
read to the point of the needle, or to a line upon it without a
magnifier, the divisions of the circle may be made directly upon the
lathe by a lever to the slide-rest if the lathe has a well-divided
headstock. When the divisions are magnified and require great accuracy,
or where a floating ring is used upon the needle, the circle should be
divided upon the dividing engine, which will be described further on,
the centre used being still the point or pivot on the bottom of the
case, from which the divisions are to be made radially.
141.--=Preservation of the Magnetism in Needles.=--It is most important
that the magnetism of the needle, particularly in mining-dials where
so much depends upon it, should be preserved to near saturation in
order to secure certain direction in opposition to the friction of
the centre, necessarily always present. This is often much neglected
from carelessness, or want of knowledge of the principles of magnetic
action. In the first place we know that a bar of soft iron, possessing
no evident magnetism, if it be placed in the magnetic meridian with
proper dip, will after a time manifest strong magnetic properties.
Thus, such a bar in London placed due north and south, with a dip of
67° to the north, becomes a weak magnet. From this we may also infer,
and this experiment shows, that a needle placed in this position will
not lose its magnetism. But what is most important to observe is that
if the needle is placed in a _contrary direction_, as, for instance,
with its northern end towards the south, it is in constant opposition
to the influences of terrestrial magnetism, and will certainly become
weaker. Therefore, although it is necessary to lift the needle when
carrying the instrument, which must necessarily place its poles in
all directions, it is not at all necessary that the needle should be
lifted when the instrument is put by out of use. Indeed, magnetism is
_materially preserved_ by releasing the lift to let the needle take
its true bearing. This does not at all injure the standing-point, as
there is no movement upon it to cause wear. Of course if the needle is
at first magnetised beyond its permanent condition it will lose this
surplus magnetism, but the residual magnetism in this position will
remain nearly constant.
142.--A valuable precaution for a needle in constant wear is
occasionally, say twice a year, or much oftener if it is used in a
dusty mine, to take it out of its box and wipe out the cap with the
point of a small sable brush. The standing-point may at the same time
be sharpened if necessary by gently rubbing it all round with a slip of
oiled Arkansas stone at its former pointing angle. The sharpness of a
needle is easily ascertained by sliding the thumb-nail over the point
at an angle of about 30° to it. If the point sticks and holds the nail,
it is sharp; if it glides upon it, it is dull. The author has often had
compasses of various kinds sent to him for remagnetisation whose only
fault has been dulness of centre.
143.--=Ring Compasses.=--In modern theodolites, levels and prismatic
compasses, the magnetic needle carries a light divided circle,
which is now generally made of aluminium on account of the extreme
lightness of this metal. A broad needle is used of about ¼ inch in
width and 1/18 inch in thickness. There is considerable difficulty
in mounting the circle to get it truly concentric and correct for
bearing, therefore ring compasses are often found to be inaccurate.
The author has followed two methods of construction, either of which
answers fairly well:--The one is to leave a bar across the compass when
cutting out the compass ring from a plate of aluminium. In this case,
when the outer edge of the ring is chucked in the lathe to be turned, a
centre hole is also made in the cross-bar which exactly fits over the
cap of the needle, so that the adjustment for centre is practically
secured, and attention is only necessary to get the adjustment correct
for bearing--that is, the 0° at true magnetic north to the axis of
the needle. Another method, which was suggested to the author by the
late Mr. Thos. Cushing of the India Office, answers perfectly, and
only entails a little extra trouble in setting for dividing. This
is to permanently mount the ring on the needle without any means of
after-adjustment, and to divide the circle from a point placed in the
axis of the dividing engine, upon which the ruby centre is placed,
being of course particular that the zero line 0° cuts the magnetic axis
true north in the graduation.
144.--=Mariners' Compasses=, and an inexpensive class of prismatic
compasses, are made with a paper disc in place of the ring above
described answering the same purposes. The paper disc is generally
made in two thicknesses with a thin sheet of talc placed between them.
Mariners' compasses have frequently the divisions painted directly
upon talc for transparency by lighting from beneath, also for general
lightness combined with stiffness.
145.--The reading of mariners' compasses, and the compasses on levels
where the needle carries a divided ring, is taken from a line drawn
vertically up the inside of the box or a pointer. This _lead_ line in
the mariners' compass gives the direction of the head of the vessel; a
pointer in the level compass gives a direction in line with the axis of
the telescope. In high-class theodolites, a microscope is used by the
author reading to a spider's web in the diaphragm.
146.--=Trough Compass=, sometimes termed a _long compass_. Where an
instrument possesses a double vertical axis and a divided circle, as
the theodolite, the division of the circle may take the place of the
divided ring of the compass and save the repetition of the graduation,
at the same time the needle may often be made longer, as the bulk of
the compass-box is proportionately less. In fact in all cases where the
magnetic north only is required the trough compass is to be preferred.
The ordinary construction of this compass is in the form of a narrow
box, Fig. 32, _A_ representing a plan, and _B_ a parallel section taken
through it horizontally. About 10° are graduated on each side of the
meridian line, _aa_ being adjusting screws to bring the scale true with
the needle.
[Illustration: Fig. 32.--_Trough compass for attachment to an
instrument._]
147.--=Magnification of Reading.=--With the trough compass it is very
common to have some form of microscope for reading the needle more
exactly. This may be done by a Ramsden eye-piece being placed directly
over the needle, as is common in some German instruments. A much
more convenient plan for certain instruments is to read the needle
longitudinally. This is generally done by means of a transparent scale
being placed across the end of the needle which is divided upon glass
or horn. This may read to either the near or distant point of the
needle. A very good form of needle reading is found in some French
instruments. This is shown Fig. 33, where the compass is shown entirely
enclosed in a tube _C_ which protects it from dust. The needle _N_ has
a vertical point fixed upon its end at _P_ which reads pretty closely
to a scale of 10° divided upon glass at _G_ by the eye-piece _E_. It
has a lifter _L_ pressed up by a milled-head screw _M_. Fig. 34 shows
the graduated glass. This compass is attached beneath the limb of a
theodolite, or in any other convenient position upon an instrument.
The author has placed a compass constructed upon this principle in
a telescope, in such a manner that the needle may be read with the
eye-piece, so as to cut a line with a distant object coincident with
the line cut by the principal telescope of the instrument at 0° of its
graduation. This plan will be more fully explained with tacheometers,
Chapter XII.
[Illustration: Fig. 33.--_Needle with reader._]
[Illustration: Fig. 34.--_Scale at G._]
148.--=The Prismatic Compass=, shown Fig. 35, was invented by Charles
August Schmalcalder in 1812. It is the most convenient portable
instrument for reading magnetic bearings. Angles may be taken with
great rapidity within about 15′ of arc by holding the instrument in
the hand, or perhaps within 5′ if the instrument is of 4 to 6 inches
diameter and placed on a stand. It is a most valuable instrument for
filling in close details, such as may occur among buildings, trees,
etc., after the principal points have been laid down from observations
taken with the theodolite. The principles of the reflection of a prism
were discussed, art. 55, Fig. 3, p. 29.
[Illustration: Fig. 35.--_Ordinary prismatic compass._]
[Illustration: Fig. 36.--_Section of the same, but with mirror._]
149.--_Prismatic Compasses_ are made from 2½ to 6 inches in
diameter. The compass needle is sometimes made to carry a card
dial for the 2½-inch size; for larger sizes the ring is now made
uniformly of aluminium. The reading of the compass ring is effected by
means of a glass prism, Fig. 36, _P_, which is cut to 45° upon one face
and 90° for the two others, one 90° face being worked convex, so as to
give magnifying power simultaneously with reflection of the ring at
right angles, so that the reading of the compass appears to stand erect
before the user of the instrument, and to be considerably magnified.
As the reading is made on the side of the ring nearest the observer,
the figures on the ring are engraved right to left. The prism is placed
in a box, with a vertical sight slit _SS_ over it, which cuts a line
with the centre of the top of the prism. The box with its prism moves
upwards or downwards in a sliding fitting _SL_ by means of a _thumb
nail_ stud, which adjusts the prism until it is in exact focus with
the divisions on the ring. The back of the prism-box has a hinge _H_,
so that this box may be closed down to the level of the compass-box
to render it portable when out of use. On the opposite side of the
compass-box to that upon which the prism is placed a long vertical
window _SV_ is attached, having a central hair placed so as to cut a
direct line from the slit _SS_ in the prism-box across the axis of the
needle. This window-piece is jointed to turn down upon the face of the
compass-box and simultaneously to lift the compass needle off its
centre by a part of it pressing the outer end of the lifting lever _L_.
To prevent too great a continuity of the oscillation of the compass
needle and the ring, through unsteadiness of the hand in holding it, a
pin is placed at _S_, through the compass-box under the window, which
carries a light spring _B_ that just touches the ring lightly when the
pin is pressed in, and thereby brings the compass ring to rest, or
fixes it for reading with some degree of certainty. An open ring under
the prism-box is sometimes used for placing a piece of ribbon through
it, to attach it to some part of the person to save dropping the
compass accidentally when it is used in the hand. When the instrument
is out of use a metal cover is provided to protect the glass. The
instrument is uniformly carried in a leather case with strap to pass
over the shoulder. As these instruments are often carried by military
surveyors, they are better made of a stiff aluminium alloy, which makes
the instrument less than half its ordinary weight.
150.--=Additional Parts= commonly provided with the prismatic compass
are a mirror and sunshades, shown only in section Fig. 36. The mirror
_M_ is carried in a frame attached with a sliding piece to the
window, upon which it can be placed either upwards or downwards. It
is jointed with a hinge so as to be set at any angle. By reflection
from the mirror, bearings in azimuth are taken much above or below the
horizontal plane. Sun-glasses are also provided in front of the prism,
which are used for taking the sun's place either with or without the
mirror, a single sun-glass being also used very comfortably for working
towards the sun at all times. The sun-glasses, which are simply small,
dark-coloured glass circles in frames, are not shown in the engraving.
151.--=To Prepare to take Observations with the Prismatic Compass.=
After the window and prism are opened out, the prism is adjusted
to read the divided ring sharply when the compass is about level,
by raising or lowering the prism _P_ by pressure of the thumb and
forefinger of the right hand upon the stud placed upon the prism slide
fitting, shown below _SL_, until the divisions appear clear.
152.--=In Using the Prismatic Compass=, the compass-box is held with
the thumb of the right hand under the prism at _SL_ and the forefinger
upon the stud _S_. The object which it is desired to observe is sighted
through the slit _SS_, cutting the left-hand side of the hair in the
window _SV_, while the division which comes opposite the reading point
at its edge by the reflection from the prism is noted. The ring when
free oscillates for a time, but is easily brought to rest for reading
by gently pressing the pin _S_ upon which the forefinger is placed.
153.--Where objects are observed for taking their bearings above the
horizontal plane, the length of the window will be sufficient to
take in a vertical angle of 20° to 30°; but for such altitudes it is
necessary to take very great care that the compass is held level,
to get magnetic angles even approximately true. Below the horizon,
angles can be obtained with somewhat greater certainty by means of
reflections from the mirror. Altogether, except for taking nearly
horizontal angles, or for very close work in filling in after the
theodolite, it is much better to have the prismatic compass mounted
upon a tripod stand. With a stand, where the angle in azimuth is much
above or below the horizontal plane, it is better to have a small glass
level, described further on, art. 181, to place across the compass
when setting it up. If the compass ring is very carefully balanced
across 90° to 270° two bright wire points may be placed inside the
compass-box, level with the compass ring, which will answer for the
cross levelling.
154.--=Stands.=--The author has made a very simple and inexpensive
tripod stand for the prismatic compass, the head of which consists of
a ball and socket only, clamped by a large milled-head screw. An axis
through the ball permits horizontal adjustment, shown in section, Fig.
37.
[Illustration: Fig. 37.--_Improved prismatic compass stand._]
[Illustration: Fig. 38.--_Hutchinson's prismatic compass._]
155.--=Hutchinson's Prismatic Compass=, Fig. 38, is now very generally
used by military men. In this compass the metal cover is fixed on the
top of the compass-box, and a glazed opening is placed in the cover,
occupying about one-eighth of its area, near the prism. This opening
gives sufficient light to the compass card to permit it to be easily
read, and the loose cover is dispensed with; besides which, the cover
being fixed, this, as well as the whole instrument, may be made much
lighter, while retaining equal rigidity for wear. This compass is not
fitted with shade and mirror arrangements as before described. Size,
2½ inches diameter, ¾ inch in thickness; weight, only 8½ oz. in
brass; 3¼ oz. in aluminium.
156.--=Captain Burnier's Military Compass.=--This portable compass is
more generally used on the Continent than other forms. It is generally
combined with a clinometer, therefore the illustration is deferred,
_seq._ with clinometers. The compass ring is set up vertical to the
plane of the needle, and is read by an index point by means of a
cylindrical lens. It has a pair of sights formed of a slit near the
eye-piece, and a hair in the window as in the prismatic. When this
instrument is held horizontally, at about a foot distance from the eye,
the sight line and the index line read distinctly into the graduations
of the ring. A lifter is provided to raise the compass off its
centre, as with the prismatic compass, and a spring clutch to prevent
continuity of oscillation. It is adapted to be set up on a plain rod
stand, the socket fitting to which is held in the hand when it is used
as a hand instrument.
[Illustration: Fig. 39.--_Sketching protractor for use with prismatic
compass._]
157.--=Surveying with the Compass only.=--In modern practice very
little surveying is performed with the compass, except for sketch or
exploring maps and filling in details, wherein the prismatic compass is
useful. The magnetic needle was formerly much used for surface work,
and depended upon almost entirely for underground work; but this has
been found practically in many cases unsafe, from the uncertainty of
magnetic variations, local and other, in the district surveyed. Mining
compasses, or _dials_, as they are termed, are now in modern practice
made with means of taking angles with the compass, and independently
of it. This subject will therefore be deferred to a future chapter on
mining instruments.
158.--=In Plotting Military Sketch Surveys= from angles taken with
the prismatic compass, the paper employed is ruled lightly all over
with parallel lines an inch or less apart. The angles taken with the
prismatic compass from 0° to 360° (northern zero) are set off with
an ivory military protractor, which has lines to correspond with
latitudinal lines drawn over its face at 90° to its base, so that the
protractor may be placed transverse to any line drawn on the paper with
its centre in any position. Particulars of this method are given in
every detail in Major Jackson's _Course of Military Surveying_, and in
my work on Drawing Instruments. The military protractor is shown Fig.
39.
159.--For making a sketch plan with the prismatic compass, a very
convenient way is to use the tee-square, the upper edge of the blade
of which represents magnetic east to west, the upper end of the board
magnetic north and the lower end south, according to the reading of
the compass. The bearings taken from any starting-point are set off
on the plot by a semicircular protractor with its base resting along
the tee-square. The northern angles are raised with the square at the
left-hand side of the board and the southern with it at the right. The
distances from the station for all bearings are measured and set off by
scale.
160.--It is indifferent how many stations are taken by the prismatic
compass. The measurements in any direction may continue all round an
estate, and will be found fairly correct if carefully made, as the
small personal errors in reading the prismatic, which may be _plus or
minus_, tend to correct each other on the whole, and to tie up the
lines.
161.--The rolling parallel rule may replace the tee-square, if it is
thought desirable to place the plan in a direction other than that
erect to magnetic north with the paper, or that it is inconvenient to
use the tee-square. In this case a few parallel lines may be at first
drawn correctly across the paper, at about equal distances, with a
sharp pencil E. to W. for references to reset the parallel rule at any
position desired.
[Illustration: Figs. 40, 41, 42, 43.--_Pocket magnetic compasses._]
162.--=Pocket Magnetic Compasses.=--The subject of compasses will
scarcely be complete without mention of the small pocket compasses
which are so useful and universal. Several well-known forms are shown
in the next illustration. The square form shown first, Fig. 40, will
be found the most useful for very rough sketching. The edges may be
sighted for the direction of roads, etc., or the box may be placed
against a wall for taking the magnetic direction of a building. In like
manner also the compass-box may be laid on a drawing and lines drawn
along by the edges of the box to the magnetic directions taken. This in
most cases is sufficiently accurate for architectural work, in which
the exact direction is not generally thought to be important. Fig. 41
is a French form of compass with step reading level with the upper
surface of the needle. Fig. 42 is an old English form with enamelled
dial, with lifter under the bow of the handle. Fig. 43 is the same make
in a hunter case. In this the lifter rises upon the case being closed.
[Illustration: Fig. 44.--_Trough form "Egyptian compass."_]
163.--The author has made a small pocket magnetic compass, which is
represented in the illustration above. The needle is placed in a long
box. It reads at its point into a single line when the needle is
exactly parallel with the sides of the box. The lid turns up endwise.
The needle is lifted by closing the box.
[Illustration: Fig. 45.--_The author's under slide for setting off
variation._]
164.--In a form of compass similar to the above, the author has added a
thin slide to the under side of the box, by means of which the magnetic
variation may be adjusted, as shown Fig. 45. This slide moves out just
the amount of magnetic variation, the stud _S_ being made concentric
for this adjustment. If the slide box be made of ivory a few useful
scales may be divided upon it. The compass slips into a light leather
case, and is the most portable for its length of needle of any compass
made. The edges of the box are used as directing lines, as above
described for the square form. The illustrations show a compass made
for Great Britain, but a similar instrument is also made universal.
In this case the box is a little wider, with the centre of the slide
in the middle, so that the magnetic variation can be set off west or
east. A rider also on the needle enables it to be balanced in southern
latitudes.
[Illustration: Fig. 46.--_Barker's luminous compass._]
165.--=Barker's Luminous Compass=, with floating dial of
mother-of-pearl, one-half of this being engraved with black figures
and the other half painted black with the figures left white, permits
magnetic direction to be observed in the dusk and by moonlight. These
compasses, Fig. 46, are much used by travellers. Mr. Francis Barker has
also designed a compass in which the needle carries a bar coated with
luminous paint.
FOOTNOTE:
[2] See _Phil. Trans._ 1896, vol. 188, Map. 9.
CHAPTER IV.
LEVELS--METHODS OF ASCERTAINING--LEVEL TUBES--MANUFACTURE--CURVATURE--
SENSITIVENESS-TESTING--READING--CIRCULAR LEVELS--SURVEYORS' LEVELS--
Y-LEVEL--PARALLEL PLATES--ADJUSTMENTS OF Y-LEVELS--SUGGESTED
IMPROVEMENTS--DUMPY LEVEL--TRIPOD STANDS--ADJUSTMENT OF DUMPY--
COLLIMATOR--IMPROVEMENTS IN DUMPY LEVEL--TRIBRACH HEAD--DIAPHRAGMS--
CUSHING'S LEVEL--COOKE'S LEVEL--CHEAP FORMS OF LEVEL--HAND LEVELS--
TELESCOPIC LEVEL--REFLECTING LEVELS--WATER LEVELS.
166.--=A Level Plane= is understood technically to be a plane truly
tangential to the theoretical spheroidal surface of the earth, as
represented by any spot upon the mean surface of the ocean or of still
water free from local attraction. The importance of having the means of
constructing efficient instruments that can be conveniently employed to
obtain the correct relative altitudes of points or stations upon the
earth's surface, in relation to such a plane or _datum_, can scarcely
be overrated. Such instruments are not only used for topographical
surveys of countries, but also in designing and carrying out public
works adapted to the local conditions of natural inclination of the
land surface, for railways, drainage, irrigation, canals, water-works,
and other constructions.
167.--The force constantly at our command to enable us to ascertain
relative altitudes and to form mentally or graphically local level
lines on the earth, is that of _gravity_; and it is only a question
in any case how the action of this force shall be employed. There are
four principles which we may accept as data for employing gravity, each
depending upon a natural phenomenon:--(1) The open upper surface of a
liquid unaffected by currents of air, or the influence of solid objects
in close proximity causing capillary action, or local attraction of
solid masses, represents a level plane. (2) The line of a plummet
unaffected by currents or lateral attractions forms a vertical line
to which the level plane is everywhere at right angles. (3) The
atmospheric pressure, from the approximated equality of its density due
to its weight in proportion to its height over limited areas, gives
pressure according to its gravity--therefore altitude or difference
of level relatively to lesser pressure compared with a lower datum.
This pressure is measurable with a barometer or other form of pressure
gauge. (4) The resistance to ebullition in a liquid is inversely
proportional to the weight or pressure of the aërial fluid resting
upon its surface. This is measurable by the temperature at which
liquids boil under varying atmospheric pressures. Various instrumental
refinements have been discovered to render these natural phenomena
available in practical use for ascertaining difference of height. The
first and most exact method employed for this purpose, by means of the
liquid plane, will be considered in this chapter. The other methods
will be deferred to later pages.
168.--In taking the level of a liquid surface contained in a vessel,
we have, as just stated, to keep this surface free from the disturbing
influence of air currents, and to surround the surface with equal
conditions of capillary attraction, or to make these conditions equal
in the direction in which we desire to ascertain our level. This is
found practically to be best performed by means of a sealed glass tube,
in which the liquid will by gravitation naturally occupy the lower
place, and any air or lighter fluid contained therein the space above
this.
[Illustration: Fig. 47.--_Level tube (bubble)._]
169.--=Level Tubes=, _or Bubble Tubes_, as they are technically
termed, are used as a part of nearly all important surveying
instruments. One of these is represented Fig. 47. The glass for the
construction of these tubes is drawn at the glass-houses in lengths of
about 6 feet, and may be ordered of any desired size and substance. The
tubes are drawn of as nearly straight and equal bore as possible. They
are, nevertheless, found to be, when examined after annealing, curved
more or less in various directions at different parts of their lengths.
They are found also generally to be slightly tapering from end to end
and of slightly unequal substance. In the manufacture of level tubes
parts of the tube are selected with approximately regular longitudinal
curvature, and these parts are cut off into the required lengths by a
triangular file dipped in spirits of turpentine, to be ready for the
future operations of grinding, sealing, and dividing. After the tube
is cut off and carefully examined to get its most concave internal
surface upwards, this is then marked by a _test mark_, with the flat of
a file, near one end for future work and reference. The grinding of the
inside of the glass tube to true curvature is performed by passing it
over a brass mandrel or _core_, which is employed to grind the glass by
means of fine emery. The _core_ is turned slightly barrel-form to the
longitudinal curvature intended for the upper surface of the finished
tube. It is made of full three-quarters the diameter of the interior
of the tube, and a little longer than the entire tube. This core is
attached by its ends to two stiff but flexible wires of brass, about 8
B.W.G. for a tube of ·7 inch diameter, and these wires are held firmly
by their ends in two vices, so that the core is slung, as it were,
to permit a certain amount of flexibility under the pressure of the
hand used in grinding. Some good makers do not use a mandrel core,
but only a strip of brass on the mandrel, extending about 60° of the
circumference. In this case the strip has to be corrected for curvature
during the grinding, which plan is sometimes preferred for certainty.
The grinding of a tube cannot be commenced with coarse emery, such as
is used in the grinding of lenses, as the cut of a coarse emery will
quickly split the tube. After the glaze is removed there is not so
much risk, so that a little time may be saved by passing a current of
hydrofluoric acid gas through the tube; but more careful testing is
required afterwards, as the cut of the grinding tool is not so evident
at sight when the glazed surface is removed.
170.--The operation of grinding is very much the same as that described
for lenses, p. 17. The surface is required to be traversed in every
direction longitudinally and transversely, which is effected as far
as possible by a twist of the hand alternately to the right and left.
The tube should also be frequently taken off and turned end for end.
Slight variations of curvature are readily made by differences of
pressure of the hand on parts of the tube; and a little _coaxing_ is
allowed to get the centre of the tube _quick_ where the tube is to be
used for levelling only, and not for measuring small angles, so that
in this case the finished tube is slightly parabolical. The finishing
touch is produced with wash-emery. The inside should be left smooth but
not polished, as the slight roughness of a fine ground surface assists
the capillary action by causing better adhesion of the spirit, and
gives a quicker run to the bubble. Where the tubes are required of a
given radius they are tested frequently, during the grinding, upon the
_bubble trier_, by placing two corks in the ends of the tube, which is
nearly filled first with water for rough trial, and then with spirit
for final correction.
171.--=The Bubble Trier= is a bar or bed 12 to 20 inches long, with
two extended feet ending in points at one end, and a micrometer screw
at the other, the point of which is a resting foot, thereby forming
a tripod. This stands on a cast-iron or slate surface plate. The
micrometer screw has a fine thread, and a large head with divisions
upon it to read seconds of arc. The tube is supported on the bar by two
Y's, which are adjustable for distance apart, according to the length
of the tubes to be tried.
172.--_The Sensitiveness of a Level Tube_, the upper curvature and
ground surface being equal, depends very much upon the capillary
action due to its internal diameter, the larger tube, from the freedom
of restraint by capillarity, being the more active. As regards the
ultimate settling to gravitation equilibrium, perhaps there is no
difference, but small tubes are sluggish and take time to work. The
following are about the usual dimensions of the interior of sensitive
tubes--8 inches × 1 inch diameter, 7 inches × ·9, 6 inches × ·8, 5
inches × ·7, 4 inches × ·6, 3 inches × ·5, 2½ inches × ·45, 2 inches
× ·4, 1½ inches × ·35, 1 inch × ·3. The larger the volume the
greater the expansion of liquid with heat; the longer the tube the less
torsion it is liable to suffer from sealing, so that if possible, as
expansion is a serious defect, it would be better to have short tubes,
if these could be sealed without disturbance of curvature. Much shorter
tubes are used in America than in Great Britain.
173.--_The Curvature of a Level Tube_ is worked to radius according to
the delicacy of the work to be performed with it afterwards. The radii
of curvature of different level tubes used for scientific purposes
vary from about 30 feet to 1000 feet or more. The radius of any curve
may be conveniently measured by the relation of its versed sine to
its chord of arc, the chord being the length of the tube. If this is
first calculated out, a piece of shellac may be attached by melting it
down upon the centre of the edge of a parallel glass straight-edge, to
represent by its thickness the versed sine. The spot of shellac may be
brought to the exact height required from the straight-edge by filing
and stoning, at the same time taking its protuberance by a calliper
gauge provided with vernier or micrometer to read ·001 inch. The versed
sine of a given radius is formed for a given chord--
_versed sine_ = _rad_ - √(_rad_^2 - (½ _cho_)^2).
174.--The general instruction, however, given to the maker is the
distance of run of the bubble that is required to give seconds or
minutes of arc; and perhaps this is after all the best test for
accuracy of the tube which, like all other articles in glass submitted
to the process of grinding, is subject to a certain amount of local
error. By this method the local error is discovered by testing with
the bubble trier. When the run is given, the radius of the curve of
the tube may be found if desired by the use of a common multiplier, as
follows, very approximately--
_Arc equal to radius expressed in minutes, 3437·74677._
_ " " " seconds, 206264·80625._
The run of a good sensitive tube is frequently made 1/30 inch to the
second, here (omitting decimals)--
arc sec (206264·8) × 1/30 inch = 573 feet radius nearly.
For scientific purposes a millimetre run per second is commonly used,
then--
arc sec (206264·8) × ·001 metre = 206·264 metres radius
or 680 feet nearly.
For an ordinary 12-inch dumpy level the tube is divided into minutes at
about 1/10 inch apart, radius 28 to 30 feet; for a sensitive 14-inch
Y-level of good construction the same divisions may represent five
seconds, radius of bubble tube about 170 feet.
175.--_The Divisions upon Ordinary Level Tubes_ are made after the tube
is finished, but with the highly sensitive ones the divisions are made
first. The run is taken from ten to thirty divisions on each side
of the centre of the tube, where it is lightly marked with a marking
diamond. These spaces are then equally subdivided and etched in with
hydrofluoric acid or marked with a hard steel edge dipped in turps. If
further refinement be required, the errors of run in relation to the
divisions are tabulated from the testing of the tube with the bubble
trier. A less careful method is employed by some makers of leaving the
level tube undivided and fixing an independent metal or ivory scale
over it.
176.--Level tubes are generally filled with pure alcohol for ordinary
purposes; for trade purposes with methylic alcohol, which is much
cheaper. For very delicate tubes sulphuric ether or chloroform is
used. The sensitiveness of the bubble depends very greatly upon the
mobility of the liquid with which it is filled, and to the quality
of adhesion of the liquid to the glass. The relative mobility of the
above-mentioned liquids is found by delicate tests with the bubble
trier for small distances under the microscope at a temperature of 60°
Fahr. Taking water as 100:--we find commercial methylic alcohol 22,
absolute alcohol 13, sulphuric ether 5, chloroform 3,--that is, for
equal small runs taken in 15 seconds of time. All bubbles appear to be
more or less affected by temperature, particularly where the spirit is
not nearly absolute. In the higher temperatures the bubbles are more
active. The objection to chloroform, where it is likely to be subject
to great changes of temperature, and where there is no provision made
for regulating the size of the bubble--the means of doing which will be
presently discussed--is that its expansion from heat is so great that
it is very liable to burst its tube. It can therefore only be used with
ordinary sealed tubes where these are small and strong. Sulphuric ether
has the same fault, but in a lesser degree.
177.--The sealing of ground tubes requires the skill of a very
experienced glass-blower, and is a technical matter on which no written
instructions would be of value under any conditions. A little strain
is unavoidably put upon the tube in sealing with the blow-pipe, so that
the curvature to which it is worked is more or less disturbed. For this
reason level tubes which are required to be of the highest degree of
accuracy are sometimes left as they are ground, and closed at the ends
by small discs of glass grooved to the end surfaces. These are fixed
on with glue, and when the glue is set are bound over with silk and
finally varnished; but this plan is much too delicate for instruments
for use in the field.
[Illustration: Fig. 48.--_Colonel Strange's level tube._]
178.--=Colonel Strange's Level Tube.=--These tubes, Fig. 48, are blown
with an outward bead at each end of the tube, two outwardly screwed
collars, _F_, being first placed over the tube before the blowing.
The tube is then ground to curvature. A plug, _S_, is formed for each
end of the tube from a plano-convex lens, ground to a bevel on the
plano side, and also ground into the end of the tube as a stopper. A
cap, _C_, is screwed over the end upon the collar. The springiness of
the cap keeps the stopper always tight. As there is no blow-pipe used
after the grinding, the tube remains constant as it is ground, or it
can be adjusted by grinding to any desired sensitiveness. This cap,
for security, is better covered with silk tied over it, and afterwards
well varnished. In this class of tube there is always a little risk of
evaporation. The system is not adapted to instruments to be used in the
open air.
179.--=Chambered Level Tubes.=--As the run of a bubble varies
slightly with its size, for exact purposes and extreme climates it
is very desirable to be able to adjust the size of the bubble to the
surrounding temperature, so that it shall be kept at about equal
dimensions for all measurements made with it. This becomes particularly
important where chloroform is used, from the expansion being very
great. A general way of doing this is to have a stopper ground into one
end of the tube, which is itself a small bottle, on the under side of
which a hole is ground, so that by turning the tube over and raising it
more or less, any amount of the highly rarefied air it contains may be
taken to form the bubble that may be desired. The stopper is fixed with
thin glue. The general construction is shown below, Fig. 49. Of course
where such a tube is used there must be means of tipping and turning
the instrument in which it is fixed over, or the bubble itself must
have separate fixings. The portability of a surveyor's level admits
readily of the necessary tipping; with theodolite levels at right
angles to each other upon the vernier plate it would be impossible.
[Illustration: Fig. 49.--_Bubble with supplemental air-chamber._]
[Illustration: Fig. 50.--_Colonel Scott's patent protected bubble._]
180.--=Extra Strong Level Tubes.=--Colonel Scott's very ingenious
device of enclosing a level tube within another tube of thoroughly
annealed glass will be found valuable in all cases where the tube is
much exposed, or where it is difficult afterwards to procure a new
tube in the case of accident. These tubes are at present only made
by the author for Scott's telescopic gun-sights, which are nearly
like small theodolites. The level tube, Fig. 50, is made as stout as
possible to be soundly sealed after filling. It is then enclosed in
an annealed tube, _CC′_, of about ·08 to ·12 inch in thickness, the
interspace between the two tubes being filled with Canada balsam. It is
then plugged with elastic marine glue, _KK′_ and cemented over _PP′_.
The annealed tube is of great strength, so that the complete naked tube
thus formed will bear dropping on the ground, and also when attached to
a large gun will bear the violent vibration of firing without injury.
[Illustration: Fig. 51.--_Artificial horizon level._]
181.--=The Level Tube may form a Complete Instrument in itself.=--In
this case the lower surface is ground to a flat plane to rest on any
plane surface. This level is generally contained in a small pocket
case, and is most convenient for adjusting instruments to level. It is
commonly used with the black glass artificial horizon, to be described.
182.--=Mounting Tubes.=--Level tubes when applied to instruments are
generally mounted in brass covering tubes. Small level tubes under
2½ inches are conveniently mounted in such tubes with a fixing
of slaked plaster of Paris inserted at each end of the brass tube.
Larger level tubes should be bound round with thin paper pasted round
the ends, which is allowed to get quite dry, to be afterwards fitted
to the brass tube with a file. Fitted in this manner the tubes admit
of adjustment to the difference of expansion of the metal and glass
by change of temperature without distortion. There is no objection,
however, to thoroughly fixing one end of the tube with plaster if
the other be left free, and this is perhaps advisable for portable
instruments.
It is convenient in mounting level tubes to place white glazed paper
under the bubble to reflect the light that passes through it to ensure
better observation.
183.--=In Fixing Undivided Level Tubes=, or replacing them in
instruments, it is important to observe that the side with the _test
mark_, which is a small ground facet, should be placed on the _top_.
184.--_In the Use of Level Tubes_ generally, it is not well to have
them of greater sensitiveness than the general construction of the
instrument upon which they are placed permits. Thus the centre of a
surveyor's level that may be under constant strain from the unavoidable
inequality of the pressure of parallel plate screws, will appear never
to reverse properly if it has a very sensitive bubble, the cause of the
irregularity being entirely due to the distortion from the strain on
the vertical axis of the instrument. The same irregularity occurs in a
lesser degree with the Y's of a theodolite, where these and the collars
become corroded by exposure. The optician often gets an undue amount of
credit for perfecting such instruments when he has merely replaced the
sensitive bubble by a dull one--that is by doing what is really in this
case the best for the instrument.
185.--When an instrument that depends entirely upon the level for
its possible working is to be used abroad, an extra tube should be
taken, as the level tube is very generally more exposed and is more
delicate than any other part of the instrument. The tube may not only
be accidentally fractured with a slight bow, but even the heat of the
sun's rays will sometimes burst it.
186.--=Reading the Bubble.=--The exact position of the capillary
concave surface of the spirit in the tube is liable to deceive the
observer by the difference of refraction and reflection it gives,
whether the light is towards the right or left hand. To avoid this
cause of error it is better, in sunlight, to hold a piece of white
paper at a short distance over the end of the bubble during the
observation taken of its terminal reading into the divided scale on
the tube. It is also important to note that the observer should stand
at right angles to the tube to see the position exactly where the
upper capillary line of the spirit cuts, as the tube itself refracts
the light unequally. It is not at all difficult to read the bubble if
the observer stand over it; but generally, as it is mounted upon an
instrument, it is at the height of the eye. In this position the hollow
surface round the bubble, caused by the adhesion of the liquid to the
sides of the glass tube, reflects the light in a manner that the hollow
may be taken for the end of the bubble, and a false reading made. It
is better if possible to take the convenient side reading first, and
afterwards get a glance at the upper surface reading for certainty.
In some cases this may be much assisted by the employment of a small
mirror of about the size of a spectacle eye, which is carried open in
the pocket, or, as the author has made it, it may close in a horn case
with a pocket lens, as in the Fig. 52 shown below. _C_ sheath, _M_
mirror, _L_ lens.
[Illustration: Fig. 52.--_Pocket lens and mirror._]
187.--=Circular Levels= have been made tentatively for a long period.
They consist of a worked concave lens fitted into a brass cell with
indiarubber seating, the glass being secured by burnishing over a
bezel. This construction answers very well when new, but the spirit
the level contains is certain to evaporate slowly under every possible
care. Mr. J. J. Hicks has patented a hermetically sealed circular
level, in which he has succeeded in working the upper surface of
the glass to curvature. These levels, of course, are not subject
to evaporation, and are very useful and portable for approximate
levelling--as for plane tables, cameras, etc.
[Illustration: Fig. 53.--_Hicks' patent circular level._]
188.--=Surveyors' Levels=, of which there are many forms, consist
essentially of a telescope with a diaphragm at the mutual foci of the
objective and the eye-piece, the axis of the telescope being placed in
a direction truly parallel with the crown of a sensitive level tube.
The telescope with its level is mounted upon metal frame-work, carried
up from a vertical axis upon which the telescope rests. The vertical
axis is adjustable in relation to the axis of the telescope, so that
they may be brought perfectly perpendicular, the one to the other. The
whole instrument is also adjustable to a position of verticality of
its central axis, and the horizontality of the telescope in relation
to the surface of the earth in what is termed the _setting-up_ of
the instrument; so that when it is set up in this position levels
may be taken from it in any horizontal direction from one point of
observation, by rotation of the telescope about the vertical axis.
Having these essential objects in view in the construction of the
level, the form of the instrument may be varied as to details according
to the mechanical skill and taste of the maker and the special demands
of the civil engineer.
189.--In the design of a surveyor's level very important considerations
are:--That the metal should be so distributed that every part is as
light as possible, consistently with sufficient solidity to take a
moderate amount of accidental rough usage, and ensure freedom from
vibration; that the whole structure should be in equilibrium about its
vertical axis when the telescope is extended at mean range, that is,
at about the focus of three chains--this is a quality often neglected;
that there should be sufficient light in the telescope, and that it
should possess a firm and durable stand. Every form of level should
embrace these qualities.
190.--_The Oldest Form of Surveyor's Level_ is that termed the Y-level,
so named from the telescope being supported in Y-formed bearings.
This instrument was originally invented by Jonathan Sisson, a leading
instrument maker of the 18th century. It was much improved and brought
nearly to its present state of perfection by Ramsden, to whom practical
opticians owe so much for many advancements of their science, and to
his liberal publication thereof. This instrument is now very little
used in Great Britain; but it still maintains its original position,
to a certain extent, on the Continent and in America. In the eyes of
the optician it is still the most perfect level, possessing all the
instrumental refinements of adjustment he can desire. The reasons for
its partial abandonment by the profession will be discussed further on.
191.--=The Y-Level= in a modern form is represented in the engraving
below, Fig. 54. The Y's are shown at _YY″_ edgewise. They are
supported by standards _SR_ upon the limb _L_. The telescope is
surrounded by two collars which are soldered upon it at positions
exactly corresponding with the Y's. The collars are turned perfectly
cylindrical and parallel on the surface with the axis of the telescope,
and ground in a gauge-plate to exact size so that the telescope may be
turned end for end in the Y's without altering the linear direction
of its axis in reversing it. The telescope is held from shifting
longitudinally in its Y's by a pair of flanges placed on the inside of
the collar pieces.
[Illustration: Fig. 54.--_Surveyor's Y-level._]
192.--The Y's are erected upon the limb, to which they are each fixed
firmly by a clamping nut _R_ at one end, and a milled head clamp at
_M_. The telescope is held down by strap pieces, each of which has a
joint at one end and a loose pin at the other, _PP_. The pin is secured
from dropping when out of use by a piece of cord attached to a part of
the instrument and to a loop through its head. At the top of the inner
side of the strap-piece under _YY″_ a piece of cork is inserted in a
cave. The cork by its elasticity keeps an equal but light pressure upon
the collar of the telescope. It will be seen that by the above plan of
holding the telescope, it is so far free that it may be revolved on its
axis, by which perfect adjustment of the diaphragm to the axis of the
instrument may be made in any direction.
[Illustration: Fig. 55.--_Section of parallel plate and vertical
axis--arrangement of Y and other levels._]
193.--=Parallel Plates= as a mode of adjustment of the vertical axis
will be first described, as they present the oldest form of setting
up adjustment. The vertical axis of the Y-level was formerly carried
tapering downwards, and the upper parallel plate was placed at about
the centre of the socket. Under this construction the socket was more
liable to strain from the use of the parallel plate screws. It is more
general now to construct the axis as represented in the illustration
above, Fig. 55, for Y and other levels with parallel plates. This
construction also renders the instrument more portable, as the parallel
plates and axis may be detached and lie closer in the case; the plan
is nevertheless open to many risks, which will be referred to in
discussing a three-screw arrangement. The general construction is shown
in the figure, of which the left-hand side is a half-section. _A_ is
a screw by which the parallel plates are attached to the limb of the
instrument; _M_ a large _milled head_, by means of which the screw
can be brought up firmly to its collar; _SS′_ the _socket_ which is
ground to fit the cone _C_; _C_ forms a part of the _upper parallel
plate_ _UP_; _B_ a _ball pin_ which screws firmly into _C_; _LP_ _lower
parallel plate_, part of which forms the ball socket, so that the whole
instrument rocks about the ball _B_ as a centre, by the action of the
parallel plate screws _PS_; _B′_ female screw for fixing this part,
which is called altogether the _parallel plates_, to the tripod head. A
clamping screw is sometimes placed upon the axis for slow motion. The
parallel plate screws are _tapped_, that is, have female threads cut
into the upper plate _UP_, and their points press the lower parallel
plate _LP_ at certain points, there being a _stop-piece_ placed round
the point of one screw to prevent rotation. The pressure upon the
screws can be increased as desired by means of the milled heads, and
the instrument made rigid in proportion; but it is very undesirable
that the pressure should be greater than that just necessary to support
the instrument firmly, as it is easy by the power of the screws to
disturb the figure of the axis and thereby derange it.
The diaphragm of the telescope of the Y-level is generally webbed with
plain cross webs. The diaphragm and webs were described arts. 99 to 106.
194.--_The Setting-up of the Y-Level_ is necessary to be understood
before the instrument can be adjusted. The same description which
answers for the setting up for adjustment will also answer for
the setting-up of the instrument in the field for actual work. In
this description it will be convenient, therefore, to consider the
instrument as being in this case _in adjustment_ as it leaves the hand
of the maker. The after adjustments will be presently taken as from the
original state of the instrument, as the maker has to do them in the
first instance. Practically, the civil engineer has only to make slight
differential adjustments at any time, as an instrument, by the solidity
of its construction, will retain the general adjustment nearly, upon
which further adjustments take more the nature of final corrections,
which become necessary only from accidental causes.
195.--_Setting-up of the Y or other Level with Parallel Plates._--The
tripod stand is opened out so that the legs stand, if on level ground,
inclined towards the centre of the instrument at an angle of about 70°
to the horizon. The toes of the legs are each separately pressed into
the ground sufficiently to make the instrument stand quite firmly. The
instrument is then taken from its case and screwed down tightly upon
the tripod head.
196.--_The Eye-piece is Adjusted_, art. 108, by sliding it gently in
and out until the webs can be seen most distinctly. On a bright day a
white pocket-handkerchief may with advantage be thrown singly over the
object-glass to prevent any confusion from objects in the field of view
during the focussing of the eye-piece. For the setting-up adjustment of
the telescope, it is brought in position to lie directly over one pair
of parallel plate screws, Fig. 55, _PS_, _SP_.
[Illustration: Fig. 56.--_Diagram plan of parallel plate screw milled
heads._]
197.--The milled heads only of these screws are represented in plan
in the diagram Fig. 56, _aa′_ being the opposite pair over which the
telescope will be assumed to be at first placed. The level tube is now
brought to adjustment by bringing the bubble to the centre of its run
by means of the parallel plate screws _aa′_, by taking the milled heads
of these screws, one between the ball of the thumb and forefinger of
each hand, and rolling them simultaneously the one in one direction and
the other in the reverse. This action tips the axis of the telescope in
one direction or the other. Thus by the screws being rolled inwards, as
shown by the direction of the arrows in the diagram, the left-hand side
of the instrument would be raised. If turned the reverse way, the right
hand end would be raised. The opposite end, from that to which the
bubble runs, always requires to be raised. Where the ground is rather
soft, adjustment when nearly correct may be made partially by pressing
down one or other of the legs; in this case the telescope should be
placed parallel with the toe of the leg which is pressed down and the
axis of the instrument.
198.--When the level tube is adjusted over the screws _aa′_ it is then
placed over _bb′_ and adjusted in a similar manner, returning again to
the position _aa′_ for final adjustment. When the level is in perfect
adjustment the bubble should stand in the centre of its run in making a
complete circuit of the horizon by rotation of the instrument upon its
vertical axis.
199.--In and during the setting-up adjustment it is most important that
the screws should not be made tight enough to cause, by their pressure
upon the parallel plates, distortion of the vertical axis. Should this
occur, the instrument will not level in all positions by the same
setting. The action of the screws also, from the great elasticity of
the metal, should distribute the pressure about equally between the
_opposite pairs aa′ and bb′_. The difficulty of accomplishing this
with certainty makes another form of adjustment, with three screws
only, preferable for setting-up, which will be considered further on.
Where the instrument is set up for use, if the adjustment of the bubble
be fairly correct to the centre of its run, the reading of the staff
may be sighted and the telescope brought to true focus upon it by
moving its milled head until the divisions of the staff are as sharp
as possible, and then moving the eye upwards and downwards to be sure
there is no error of parallax, art. 109. After this the final adjusting
of the bubble should be made, noting particularly that there are the
same number of divisions in its run on each side from the centre if it
is a divided bubble.
200.--_Adjustment of the Axis of the Telescope_ in true parallel
direction with the periphery of its supporting collars in its Y's.
This is performed entirely with the four capstan-headed screws which
adjust the diaphragm, one of which is shown, Fig. 54, _C_. Having
the adjustment of the eye-piece in focus for the webs in the manner
described, arts. 108, 109, the object-glass focussed upon a distant
distinct small object or mark, and without parallax, the instrument
which carries the telescope is then exactly adjusted to make the
intersection of the webs cut the mark. The telescope is now turned
half round on its axis, so that the lower part becomes the upper, and
observation is again made of the distant small object or mark. If the
same intersection of the webs falls on the same point of the object,
the collimation adjustment is perfect. If it does not do so, the upper
capstan-headed screw at _C_, or the under opposite one, is loosened by
means of the small pin provided with the instrument, and the opposite
screw tightened until the webs are brought over a point situated
half-way between the points cut by the first and second observation.
The telescope is again directed to the point first observed, and the
adjustment checked to see if it has been done correctly, that is, if
the level reverses, cutting the same point, or whether it requires
further adjustment by the same process as before. The other web of
the diaphragm, at right angles to the first, is adjusted in a similar
manner, but with the other pair of capstan-headed screws.
201.--It is sometimes inconvenient to adjust out of doors: this may be
performed very well indoors. By daylight a small cross may be made with
ink on a sheet of white writing-paper for the sighting object, which
should be placed at as great a distance as convenient, say 20 or 30
feet. By night a pin-hole may be made through a piece of paper and a
candle or a lamp be placed behind it.
202.--_Adjustment of Vertical Axis._--For this the eye-piece is first
brought to focus on the webs. The telescope is then placed directly
over one pair of parallel plate screws opposite each other, and the
instrument is levelled. The Y's are then opened out; and the telescope
is directed so that the intersection of the webs cuts or covers any
distinct small mark upon a distant object, or preferably upon the
centre reading of a foot line upon a levelling staff. There is no
objection to adjusting slightly to this by the parallel plate screws,
as this adjustment is independent of the level of the instrument. The
telescope is then taken out of its Y's and is turned end for end and
replaced. The telescope is now turned half a revolution on its vertical
axis, and the webs are again brought to read on the staff, if one is
used. If they now fall upon the same spot or foot line, the vertical
axis is perfectly perpendicular to the axis of the telescope in this
direction. If the webs do not fall upon the first reading or point, the
amount of difference of reading is recorded and this space is bisected;
so that now, if the telescope be adjusted by the milled head _M_, at
its bearings upon the limb upon which it is supported, for the webs
to cut the bisection, the axis will be perfectly perpendicular in the
direction of its bearing socket. The same process must now be repeated
with the telescope placed at right angles to its first position,
that is by bringing it over the other pair of parallel plate screws
which were not used at first. There is at all times a certain amount
of disturbance of the instrument due to handling it; it is therefore
necessary to repeat the whole of the above process until the instrument
reverses in any direction, but this final adjustment is better deferred
until the adjustment of the level tube, to be next described, has been
made.
203.--_Adjustment of the Level Tube._--The telescope is placed as
before over an opposite pair of parallel plate screws, and these are
adjusted until the bubble is in the centre of its run. The telescope
is then turned half a revolution, so that it is placed over the same
pair of screws in the reverse direction, and the displacement from the
bubble from the centre is now noted. The capstan-headed bubble screws
at the end of the level _B_ are then adjusted to one-fourth of the
difference observed, and the parallel plate screws are adjusted for
the other fourth, so that by these two adjustments the difference of
the run in the two positions is bisected. The same process is repeated
over the second opposite pair of parallel plate screws. If this be
very carefully done with a correctly divided bubble, the Y's of the
telescope may be opened out and the telescope be reversed end for end
in its Y's, and the bubble remain true. But it is quite as well to go
over all the adjustments a second time, as before recommended.
204.--If the level is to be adjusted by night, this can be done very
correctly by a fine cross drawn on paper placed on a wall, with
a candle or gas burner shining brightly on it at twenty feet or
so distance from the instrument. For this adjustment by night the
instrument must be well constructed, as the tubes require drawing out
to their full extent for focussing near objects. If the tubes are not
quite straight, the object-glass suffers considerable displacement in
the drawing out, or technically _droops_, which is a very common fault
in badly-made instruments.
205.--Where webs are used for the reading, they are liable to become
baggy or dirty, art. 101, and very frequently to break; nothing can,
therefore, be more useful than to be able to re-web a stop in the
evening, with command of the easy and certain means of readjustment
described, when far from the optician's aid.
206.--As the Y-level is so perfect in its arrangement for adjustments,
and so nearly meets the optician's ideal, it will be well to inquire
what are the objections made to its use by the majority of British
surveyors. The first and most important is that it possesses so
_many loose parts_, to which the practical man honestly objects. The
author was, many years ago, when Y-levels were more popular, trying
to persuade a cautious practical surveyor who appeared to be very
anxious for the certainty of his work, and who was going abroad, to
take a Y-level instead of a dumpy one he was selecting, when he had
his arguments stopped by the following question:--"Suppose you were
surveying in a tropical country, thousands of miles and an ocean voyage
from civilisation, where your native porter objected to carry much
weight, and your instrument case had to be left at a back station--when
your umbrella was all the burden you felt you could support. In this
case, suppose your porter, whom you had lost sight of for a short
time, arrived with your level, minus the telescope--lost by becoming
loose, perhaps from having been played with while he was resting--how
would you praise the Y-level?" This gentleman assured me that _he_
did not, and that this was a true account of his experience with the
last Y-level he possessed. Other objections, besides loose parts, are
that Y's and collars do not remain as perfect as when they leave the
optician--that they are liable to wear by friction of constant movement
in being carried about upon the points in contact between them, and
thereby form facets; that the collars become corroded by exposure, and
that they have open spaces that collect sand from flying dust which
fixes itself into the collars and Y's, so that this arrangement loses
the perfection the optician claims for it. Further, that the cross
bubble, which is uniformly placed on the dumpy level, effects a great
saving of time over swinging the telescope backwards and forwards
with every movement of the adjusting screws. Another feature is that
in the dumpy level, to be described, the vertical and horizontal webs
of the diaphragm cannot be disturbed from their position by rotation
of the telescope after the level is once set up; and this verticality
indicates conveniently at once whether the staff is held vertically,
which is otherwise a great difficulty with the ordinary form of Y-level
reading.
207.--=Improved Y-Level.=--The above-described defects the author has
tried to remedy by a modification of the Y arrangement, by forming
the Y's with much broader bearings, and instead of the old loose
pins screw fastenings are fitted, which firmly lock the telescope in
position with the webs vertical. This, so far, obviates the danger
from loose parts, as by this arrangement the telescope also becomes
practically firmly fixed. In adjustment the collars are opened out, and
in closing press a stud into the telescope by which it takes a given
position. This enables a cross bubble, shown on Fig. 57, to be also
placed on the telescope for approximate adjustment, which saves the
frequent disturbance of the telescope by making cross adjustments. The
diaphragm of this Y-level is exactly the same as that of the dumpy, to
be described art. 210. From the limb downwards the author uses the same
construction as he now employs on his improved dumpy level. This will
be described with that instrument further on, art 231, _seq._ Also the
setting-up adjustment with it, which is different from that already
described where parallel plates are employed.
[Illustration: Fig. 57.--_Improved Y-level._]
208.--Perhaps, upon the whole, the conditions which formerly rendered
the Y-level undoubtedly the best practical level have so much changed
that the more solid construction of the dumpy may entirely supersede
it, as it seems likely to do in modern practice, and the optician
will lose his ideal. Some reasons for this may be stated, but whether
sufficient is a question. The manufacture of object-glasses of good
figure and proper centring was formerly understood by a few scientific
opticians, who were principally engaged upon astronomical telescopes,
so that, with the exception of those made by Troughton and Simms,
no very good and accurately centred lenses were used in surveying
instruments. With bad centring alone, in ordinary telescopes, the
webs in collimating were drifted aside, and needed the Y system of
adjustment to make the telescope workable for levelling. In the
modern good object-glass, of which there are now several makers, the
centring is so nearly perfect that the webs in adjustment fall in the
centre of the diaphragm when it is placed true to the cylindrical
axis of the telescope. If the webs are placed as suggested without
further adjustment, no very serious interference is caused by want
of collimation of the axis. With this fact in view, the instrument
maker needs leave little space for adjustment of the webs for centre
displacement to become a source of error to persons not used to
adjustments.
209.--Further, with a well-centred object-glass, as it leaves the hands
of the scientific optician, and a solidly constructed adjustment to
collimation being provided for in the making of a level, true working
may be done even if there is a small error in the collimation. The
late William Gravatt, C.E., was of opinion that firm construction,
compact form, and plenty of light in the telescope were more important
than easy facilities of adjustment. There is no doubt he found the
less open adjustments the better in the hands of the imperfectly
trained assistants who were pressed into service during the railway
mania of 1848. At any rate, at this period we have his invention of
the "Gravatt," or, as it was afterwards termed, the "Dumpy" level,
which has remained with us with slight modifications in its mechanical
parts and with increasing popularity until the present time. The late
Mr. Troughton, recognising the same facts, also made a level in which
there was no adjustment to the supports of the telescope after it
left the hands of the maker. In his level he also left no adjustment
to the bubble tube, which no doubt would prevent tampering, but which
could scarcely be called an improvement; as this tube is liable at all
times to be broken, therefore to need replacing with another tube,
which cannot be made quite similar, and therefore needs easy means of
adjustment for a surveyor to replace it when abroad. This level has
gone out of use, but it is mentioned here, as the old engraving of it
remains in some of our _modern_ text-books.
[Illustration: _Fig. 59.--Dumpy level._]
210.--=The Dumpy Level.=--One of the most important structural
improvements made by the late William Gravatt in his dumpy level, was
the addition of a cross bubble, shown end-view in Fig. 59 at _CB_.
This improvement over the old form of Y-level permitted the setting-up
of the instrument to be completed approximately, without turning the
level a quarter revolution backwards and forwards several times during
the operation, as was necessary in the setting-up of the Y-level. The
compact form, lightness, and large field of view in the telescope
otherwise commended it to civil engineers, when Gravatt had pointed out
the possibility of sufficient practical adjustment without resorting
to the cumbrous proportions of the Y-level as it was then made. Modern
experience has shown that the dumpy form of telescope could very well
be applied to the Y construction, and this has been done, as shown in
the preceding pages; but at the time the dumpy was invented by Gravatt,
the Y-levels were very commonly made 20 inches or more in length of
telescope, and were altogether very flimsy affairs. Gravatt's 12-inch
level was found to be quite equal in power and of less than half the
bulk and weight. A 12-inch dumpy should read the ·01 foot on a Sopwith
staff, which is described in the next chapter, at 5 chains with a
webbed or glass diaphragm, Fig. 61; with a more open reading than
Sopwith's staff a greater distance than this. A 14-inch dumpy should
read the ·01 foot at 10 chains.
211.--=The Dumpy Level= of modern form is represented in the engraving,
Fig. 59. It consists of a telescope, fully described art. 94, which
carries a ray shade _RS_ at the object-glass end, to work in the
field to eastward or westward facing a low sun. The eye-piece _EP_ is
adjustable to the webs in the telescope by pressure in or out. Two
straps or bands are accurately fitted and soldered round the tube of
the telescope; one of these carries a hinge joint, and the other a pair
of locking nuts to support the level tube _GG_, and at the same time
permit its adjustment. The level casing tube has two three-quarter
bands, which slide upon it, pointed at one end _GG_: these adjust to
the length of the bubble for changes by temperature. The lower part
of each strap-piece is left a solid block of metal, to give very firm
support to the telescope as it rests upon the limb _L_ beneath. The
limb may be either a casting with a socket screw only in its centre,
or a compass box may be formed in the centre and the socket screw be
placed under this, as it is shown in the figure at _S_. The attachment
of the telescope support to the limb is made by three screws, two of
which draw the limb down, and one in the centre presses it upwards, as
shown in the section Fig. 60--_CC′_ telescope, _TT′_ drawing screws,
_P_ pressing screw.
[Illustration: Fig. 60.--_Attachment of telescope block to limbs._]
212.--It will be seen that by this means firm adjustment may be made
either by raising or lowering one end of the telescope, as also by
a lateral rocking motion should the web or bubble not be quite to
position. This plan is certainly moderately solid, and little fault
can be found with it, except that a little torsion may be put on the
telescope by unequal screwing, and that it appears slovenly in leaving
an open gap between the limb and block; therefore the author prefers
in his own form of level, which will be presently described, that the
block be solidly fitted down upon the limb, as is shown in the section
Fig. 60, and the telescope be placed permanently exactly parallel with
it. If the vertical axis be once fixed truly perpendicular to the axis
of the telescope as solidly as possible there is very little risk of a
bell-metal centre of ¾ inch or so diameter being bent; therefore all
parts may be closely fitted between the axis and the telescope. Some
makers, instead of screwing down at both ends of the limb, make one end
a rocking centre and adjust only by screw at the other end. This plan
lacks a little of the stability looked for in the dumpy system. The
general construction of the vertical axis is the same as that of the
Y-level already described. The parallel plates, tripod head, and tripod
are also the same, art. 193, Fig. 55.
213.--As the telescope of the dumpy level does not possess any simple
means of determining the accuracy of the fitting of its sliding tube,
it is a very important point in these levels that this fitting should
be good, so that the object-glass does not droop when extended. For
this reason the inner sliding tube of the telescope should be as long
as possible, and its adjustment by the rack sufficient to bring an
object in focus at 15 to 20 feet distance. This point is sometimes
neglected. The author was once amused by a young surveyor bringing him
an invention, which was to fix two points by the side of the telescope
_to enable him to read at short distances_. It was seen on examination
of his own level that his telescope, a badly-fitted one, would not read
at half a chain, hence the ingenuity of his invention. In some cheaply
made levels the solid ring fitting to the telescope, above described,
which connects the limb firmly with the bubble tube, is replaced by
blocks soldered on the telescope with soft solder: the method is very
unsound from risk of imperfect soldering. The blocks are very liable to
become loosened by a jar.
214.--The diaphragm of the dumpy level is generally webbed with two
vertical webs and one horizontal. In use the image of the staff is
brought between the vertical webs, which indicate whether it is held
upright. The upper margin of the portion of the horizontal web between
the two vertical ones is the index of level to which all readings are
made, either for adjustment or for reading the levelling staff in the
field. The somewhat loose and slovenly four-screw adjustment for a
level diaphragm used in rough work with capstan-head screws, shown
Fig. 23, p. 50, which is necessary for the adjustment of the telescope
in Y's, has been abandoned for many years in the better-constructed
dumpy levels by all good makers, and the more solid construction, shown
below, Fig. 61, used in the place thereof. In this plan there is no
lateral adjustment: the diaphragm is carried as a frame in a dovetail
slide, and is adjustable by vertical screws only. The figure shows the
face of diaphragm:--_BB′_ slide pieces, _A_ slide moved by capstan-head
screws.
[Illustration: Fig. 61.--_Diaphragm of dumpy level with webbed stop._]
[Illustration: Fig. 62.--_Same, with stadia webs._]
215.--=Subtense or Stadia Webs.=--It is very advisable in all levels to
have two extra webs, or lines cut on glass, placed one on each side of
the central horizontal web or line, fixed at such a distance apart that
the image of 10 feet of the staff when placed at 10 chains distance may
exactly cut the inner space between the lines. These webs or lines may
be used as a means of measuring distances often more exactly than can
be performed with the chain if the surface of the land is irregular;
or, in any case, they form a good check upon chain measurement. If the
webs or lines are separated so as to subtend an arc whose chord is 10
feet at 10 chains, it is easily seen that 1 foot of the staff will
represent this chord at 1 chain, and that each ·01 of the foot on the
staff will represent 1 link in distance. A diaphragm webbed or lined
in the manner described is shown in Fig. 62. There is some difficulty
in placing webs in exact position, and allowance should be made for the
optical conditions by the addition of a plus factor. This important
subject will be fully discussed hereafter in Chapter XII.
[Illustration: Fig. 63.--_Tripod._]
[Illustration: Fig. 64.--_Section of one turn-up leg of the same._]
[Illustration: Fig. 65.--_Section of tripod._]
216.--=Tripods, or Stands.=--This matter was deferred when
describing the Y-level. The same form of tripod is used for both
Y-level and dumpy. In this country the tripod is generally made of
straight-grained, well-seasoned Honduras mahogany, which stands better
than any other wood. When the tripod is folded up for carrying or for
putting by it forms a cylindrical pole which is bellied out at about
one-third its length from the top, and diminishes downwards and upwards
from this point. For a 14-inch Y-level or dumpy the dimensions of the
tripod are about 3½ inches at its greatest diameter when closed,
tapering off to 2½ inches at both the top and the bottom ends. For
a 12-inch level the section is somewhat less. Each leg of the tripod
takes an equal section of the cylinder, the inner angle meeting in
the axis being at an angle of 120°, as shown in section Fig. 65.
_Shovel-pieces_ are shown in Fig. 59 _AA′_ (p. 110), attached to the
top of each leg by four screws passing from the brass to the wood.
There should be also two screws from a brass plate inside the leg to
the shovel-piece, making connection brass to brass: this is important,
as fixings from the brass to the wood only become loose and shaky by
shrinkage. The shovel-piece is formed into a strong tenon at its upper
end, through which a bolt passes connecting the _book-pieces_ together.
The book-pieces are plates cut to an angle of 120°, so as to fall
true on the tenons of the shovel-pieces. Where hand-work is used for
making the tripod head, the book-pieces are attached by three screws;
where machinery is used, the head is made in the shaping machine out
of a solid casting, which is much better. The tripod head carries a
screw about 1½ inches diameter with coarse thread, which fits into
a socket on the lower parallel plate of the level, whether Y or dumpy.
There should always be a plain piece, technically a _lead_, above the
screw. This holds the instrument steady before it is screwed down,
and also leads the screw directly to its corresponding thread, thus
saving risk of crossing the thread. A common defect in tripod heads is
the thinness of the tenon, so that the leg, if twisted, is felt to be
rickety. This tenon is better made wide, as shown in the staff head in
Fig. 70A, _seq_.
217.--There is a little difference of opinion as to the form of the
woodwork of the tripod for 14-inch levels, some preferring an open
framed stand in place of the solid form shown in section Fig. 65. These
open framed stands are not so compact to carry, and, as the author
thinks, unnecessary for levels of 12 inches and under where the tripod
head is solidly made. They are well adapted for larger levels and for
theodolites, therefore the description of a framed tripod will be
deferred to the discussion of these instruments further on.
A few engineers prefer yellow pine for the tripods instead of mahogany:
this is much lighter for its relative stiffness, but it is rather soft
for the fixing to the shovel-pieces, and therefore scarcely so reliable
as mahogany for durability. Where lightness is important the author
employs cedar, which is as light as pine but harder.
218.--The lower points of the legs, technically _toes_, are pointed to
an angle of about 60°, and are shod on the insides with steel plates
to bite the surface upon which the tripod stands when the legs are
extended for use. Two brass rings slip over and bind the legs together
when the tripod is out of use.
219.--Many years ago the author introduced the plan of having one of
the legs to turn up at about 1 foot distance from the toe. This is
shown Fig. 63 at _A_, and in detail section Fig. 64. The joint is made
perfectly firm by a winged screw at _S_, which screws from a boss cast
on the hinge _J_ to a solid metal shoe _P_. When the leg is turned
up, the screw fixes it in the female screw _S_. This plan is very
convenient for use in mountainous districts, as it enables the level
to be set up fairly well without an uncomfortable angle to any of the
legs, or risk of the instrument toppling over. This plan is now nearly
superseded by a ball joint as a part of the setting-up adjustment.
The tripod head shown under the level of Fig. 59 is by no means the
best, but it is the easiest made therefore, it is the general trade
form in use, both for the level and theodolite. Some very superior
forms will be discussed further on in description of the instruments to
which they are attached.
220.--=The adjustments of the Dumpy Level.=--As this instrument does
not possess the means of revolving the telescope upon its axis as with
the Y-level, the adjustments are somewhat more complicated, and are
performed in an entirely different manner when they are to be made by
the civil engineer. The differences are not so great in the hands of
the optician, as he generally possesses a movable pair of Y's upon
which he can adjust the telescope conveniently for collimation within
his own works, by supporting the telescope tube in Y's at a position
exterior to the bands which surround it. The tools for this adjustment
the author has occasionally supplied upon demand with the dumpy level.
But what is necessary here will be to give the mode of adjustment which
the civil engineer can accomplish at any time without supplementary
apparatus.
The bubble is handier to work with when adjusted to reverse in the
centre of its run, but it does not really matter, as equally accurate
work can be done with it in any other position. Should the bubble
not reverse in the centre of its run, adjust the instrument by the
levelling screws until it reverses in some position. Say you start with
bubble in the centre, and on reversing, it runs towards the eye end
of the telescope six divisions, then alter the levelling screws until
it is only half this, or three divisions towards the eye end, then,
if properly levelled, the telescope will make an entire revolution
with the bubble in that position, which will prove that the axis is
vertical. The bubble can now be adjusted by the opposing nuts at the
one end by means of the tommy pin (provided in the case) until it is
in the centre of its run, and it will then reverse in that position
instead of three divisions towards the eye end.
221.--_Adjustment to Collimation._--Upon a fairly level piece of ground
the staff plate, fully described further on, is trodden well down on
the ground, and the level is set up at say 3 chains from this, in
which position the staff is read as a back sight. Now in the opposite
direction in the same line, at 3 chains distance from the level, a
second staff plate, or in defect of this if the surface be not firm, a
stake or a boulder, is driven firmly down in the earth, and the staff
is placed upon this erect and face to the instrument as a foresight.
The instrument is turned half round and the second station is read.
These readings of the staves taken will be truly level with each other,
if the axis of the instrument has been set up quite vertically, so that
the bubble has kept its centre in all positions. This is true although
the axis may have been out of collimation. This arrangement is shown
in Fig. 66, _L_ the first position of the level taking sights at equal
distance from _S_ and _S′_. Let the level be now removed to _L′_: if
correct it should cut the staves _SS′_ at equal distances above or
below the first readings at _aa′_, which are at equal distances from
_bb′_ readings from _L′_, therefore level and parallel with the first
reading.
[Illustration: Fig. 66.--_Adjustment of dumpy level._]
222.--In the dumpy level, as it leaves the hands of any respectable
maker, the subsequent adjustments required can never be great, unless
the level has suffered a serious fall so as to bend the limb. The
rewebbing the stop, if carefully done, would require only a slight
readjustment; but it may be convenient to give an exact method for
extreme cases, which may be given in detail for clearness, and at the
same time we may also consider the influence of the curvature of the
earth.
223.--_Original Adjustment of the Dumpy Level to Collimation with
consideration of the Curvature of the Earth._--Suppose the readings
of the two levelling staves at 10 chains apart, taken with the level
placed at intermediate distance as before, read 7·50 and 4·50, and that
we now place the level linearly at 1 chain outside the first reading
and it reads the near staff 6·50 and the distant staff 5·50, by the
inclination of the ground, this would be a + and a - reading; but we
require both readings of one sign, and as the distant staff reading is
much too high, it is clear we require - readings for correction. The
correction will be of the difference of reading in proportion to the
distances, calling the lower reading minus--
7·50 - 6·50 = -1, 4·50 + 5·50 = +1, difference = 2.
That is -2′, as our readings are - and as the -2′ is in 10 chains,
at 1 chain the distance of - the near staff = -·2, and 11 chains the
distant staff = -2·2. The correction will therefore be for the near
staff 1 chain distant 6·50 - ·2 = 6·30, and for the distant staff at
11 chains 5·50 - 2·2 = 3·30 = -1·2 below each of the first readings.
If the telescope be now collimated to the near staff reading 6·30, by
adjusting the screws immediately under it for distance between the
limb and the telescope, and the bubble be readjusted to the telescope
without moving the instrument or touching the parallel plate screws,
the adjustment will be perfect, less the small error due to the earth's
curvature in 1 chain. If the telescope be adjusted to the distant staff
3·30, curvature of the earth will be corrected by the level for 11
chains, which is 0·0106 foot or ·01 nearly, the smallest reading we
have on the staff.
224.--It was claimed by the late William Gravatt for his method of
adjustment,[3] which was equivalent to that given above, but more
complicated and with three staves, that the fixed correction for
curvature at 10 chains would be uniform in the working of the level
_pro ratâ_ for all distances. There is some difference of opinion
on this subject: at any rate, a 10 chain correction would only be
applicable to very approximately level ground where average 10-chain
stations could be taken.
225.--Where space is not at command and curvature correction is not
desired, adjustments of the level may be made with care at 1 chain
distance on each side of the setting-up of the level with one staff
only, which can be moved from one stake to the other, and with the
final setting-up of the instrument at 1 chain distance from these
stakes as before, art. 221. For this the staff only requires moving
twice, if the collimation adjustment is to the last reading only
calculated out as above. This close system has a certain amount of
merit, that by reading from one staff only for both stations it is
more accurate, as any inequality between the divisions of two separate
staves is avoided.
[Illustration: Fig. 67.--_Collimator for adjustments to horizontality
of the telescope._]
226.--=Collimator.=--Optical manufacturers in populous districts, and
some observatories, as that of the India Store Department at Lambeth,
adjust by means of the collimator by the exact method due to the late
eminent German mathematician, Carl F. Gauss, which is hence termed
the _method of Gauss_. The collimator consists of any good telescope
permanently adjusted to solar focus, with a webbed diaphragm placed in
the focus, where it may be illuminated by a lamp or by the reflection
of daylight, and provided with means of bringing the telescope to a
level position. As the collimator is generally constructed, it consists
of an 18-inch telescope, Fig. 67, of the same description as that used
for a Y-level, described art. 94, in which the telescope is surrounded
by accurately turned collars formed to rest in Y's. The Y's are
supported upon a heavy cast-iron stand, of somewhat triangular form,
of nearly the length of the telescope, about 6 inches wide at one end
and 2 inches at the other. The stand has two feet extended to the full
width at the wider end, and one foot at the narrower end under the
telescope. Each foot has an adjusting screw. The complete collimator
is supported, at about the height of the telescope of the level on its
stand, on a very solid pier of stone or brickwork in cement capped with
a stout slate slab. The telescope is brought to perfect collimation as
with the Y-level, already described art. 200, and the level is fixed
true with the axis of the telescope, when the collimation is perfect.
227.--A lamp or gas flame is placed at a short distance from the
eye-piece end of the telescope, so as to illuminate the webs that they
may be distinctly seen when looking into the objective end of the
telescope. In bright daylight, if there is a skylight over, a reflector
will answer the same purpose. At the Lambeth Observatory a fine
needle-point hole is used instead of webs.
228.--The instrument to be adjusted may be placed at any convenient
distance from the collimator. For adjustment of a level, where the
collimator is already in adjustment, the level is raised upon its stand
until the axis of the telescope sensibly coincides with the axis of
the collimator; then if the telescope of the level to be adjusted be
focussed into the objective end of the collimator, the illuminated webs
will be clearly seen; and if these webs be brought by adjustment of
the level exactly to coincide with its own webs, the collimation lines
of the two instruments are exactly parallel. In this adjustment it is
only necessary to be sure that the vertical axis of the level is truly
vertical, so that the bubble reverses without displacement, in which
case the whole instrument must then be in perfect adjustment.
229.--It would be very difficult to use this method of adjustment if
it were necessary that the axes of the level and collimator should
_exactly_ coincide. It is only necessary that they should nearly
coincide, on account of the imperfection of object-glasses, which
rarely work so well near the edge as towards the centre; otherwise any
directly parallel position in front of the object-glass would answer,
as the next diagram will show. Let _O_, Fig. 68 be the object-glass of
the collimator, whose solar focus is at _F_. Then the rays _PP_, and
all other parallel rays falling on the object-glass, will be brought
to a focus at _F_; and reciprocally all rays departing from _F_ in
passing through this object-glass will leave in parallel lines _PP_.
Let _O′_ be the object-glass of a telescope to be collimated, _F′_ its
solar focus. Then all rays from _P_ to _P_ departing from _F_ that fall
within the parallel space _P′P′_ will be brought to focus at _F′_. When
the image at _F_ is illuminated by a lamp _L_, the webs or other index
will be clearly seen by the eye-piece at _F′_ when the two telescopes
are exactly parallel with each other. In this position the webs of the
level are adjusted to make this coincidence. It is easily seen that by
this method we eliminate all errors of atmospheric refraction, and are
quite independent of the state of the atmosphere for obtaining distinct
vision for adjustment.
[Illustration: Fig. 68.--_Diagram of collimation by two telescopes._]
230.--When two levels are at command, one a Y-level, or even a dumpy
in perfect adjustment, the one may be used as a collimator to the
other by setting them up at a distance within their focal range on a
firm basement floor. A candle or a lamp will give sufficient light to
illuminate the webs of the instrument, which is used as a collimator,
being certain, of course, that this instrument is first placed in level
adjustment and set at _solar focus_, and that the instrument used as a
collimator has a good object-glass.
[Illustration: Fig. 69.]
[Illustration: Fig. 70.--_Stanley's model 14-inch dumpy level._]
[Illustration: Fig. 70A.--_Tripod._]
231.--=Improved Dumpy Level.=--The writer has made some improvements in
the dumpy level, which have so far met with very general approbation
from the profession, Fig. 70. These improvements are directed to
ensure much greater sensitiveness in the longer bubble, therefore
greater accuracy in the work performed by it; more solidity of
construction without increase of weight; and permanence of reading
index, with some additional matters. In these improvements the
mounting of the longer level tube, instead of being placed in a stiff
joint at one end, or between rigid clamping nuts at both ends, has a
barrel-fitting at one end which is ground into a parallel hole. This
plan admits of circular self-adjustment to the bubble tube, which
the clamping of the nuts can never twist or strain during vertical
displacement; and the joint can be made perfectly sound with certainty,
which saves the risk of accident to the bubble from expansion by heat
and some other conditions. A more recent form of cross level, Fig.
69, shown in perspective near the ray-shade in the engraving Fig. 70
has been designed by the author, in which the level casing is bored
entirely out of the solid. It is supported upon the side of one
telescope strap by three stout pins, the centre one fitting its hole,
and the two outer ones are loosely held by cross screws to permit a
small amount of adjustment, which is all that is necessary. By this
construction the level fixings are made in five pieces only, including
screws, instead of thirteen as usual, at the same time making the level
more portable and solid for hard wear. The telescope straps are fitted
at their stumps solidly down upon the limb, as shown Fig. 60, p. 112.
Adjusting screws are placed under this as in the dumpy level described,
but the pressure screw is not employed except in case of accident far
away from an optician, when it is found to be there ready for use. The
limb is framed out into two edge bars: this gives greater vertical
sectional strength and resistance to torsion without increase of weight
in the instrument. Where a compass is used, this is included in the
frame of the limb, as shown in the engraving. The compass is read
with a prism, this being much more convenient and exact than looking
down upon the divided circle, the instrument being necessarily placed
for use at nearly the height of the eye. The compass ring is made of
aluminium.
232.--The further improvement, which the author considers of the
greatest moment, is that the vertical axis is fixed directly and firmly
upon the limb, and not through a loose screw fitting for separation
at this point as in the ordinary dumpy. This is shown to be important
in that, with the dumpy, where a loose screw is employed, any little
difference of screwing down upon the axis when the instrument is set
up causes so much derangement of a sensitive bubble in relation to the
vertical axis, that the optician is bound to use a rather dull bubble
with the ordinary dumpy. Further, a particle of grit or the slightest
bruise on the collar in replacing the instrument in its case throws
it out of adjustment at this important point. The objection to the
author's plan is that it makes the case for the instrument somewhat
larger; but the advantage of certainty of permanent adjustment appears
to him very far to counterbalance this objection where accuracy is
aimed at.
233.--=Tribrach.=--The setting-up adjustment of the instrument is upon
tribrach limbs with three screws only. These screws can never strain
the vertical axis, which in this instrument is somewhat deeper and
more firmly made than that of the dumpy. In the old form of tribrach
the points of the screws were held down by a spring plate placed above
them. This plate, in carrying the instrument upon its stand over the
shoulder, which is the most comfortable way if the stations are not far
apart, was very liable to strain sufficiently for the screws to get
loose. The author patented a much more solid method, by which the old
spring plate is entirely dispensed with. In this plan each screw has a
ball at the lower end, which is inserted in a tubular fitting formed
in a solid tribrach, made of exact dimensions to take it. The tube is
open on the upper side, as shown in longitudinal section _H_, Fig. 71.
Many years' experience and the fact that numbers of makers have copied
this form since the expiration of the patent, shows this plan to be
perfectly successful. The general construction of the lower part of
this level may be seen from inspection: _L_ limb, fitted with compass;
_C_ axis, in one casting with the limb; _S_ sprang, carrying the socket
and supporting the instrument. _PH_ shows the ball head arrangements to
the screws. A central screw in this part detaches the tripod. One point
is shown at _P_, of which there are three, to support the level upon a
wall or rock in cases where the tripod cannot be used--a most important
advantage in town levelling. The tripod head is made much more firmly
than that of the ordinary construction, by extending two wing fittings
from the top of the shovel-plates as wide apart as possible, instead
of the narrow tenon fitting before described. The shovel-plates are
screwed to the staff by means of a stout nut-plate inside the tripod
_F_. Those who have experienced how much defective levelling is due
to a shaky tripod head will appreciate this precaution. The general
arrangement is also shown in Fig. 70A.
[Illustration: Fig. 71.--_Details of Stanley's dumpy level: half
elevation to left, half section to right._]
234.--As the tribrach system of adjustment is of somewhat recent
adoption to ordinary surveying instruments in this country, it strikes
the stranger to it as being more difficult in use. It is really the
most simple and expeditious system as is clearly explained by the
foregoing diagrams, Figs. 72, 73 of the plan of a level, omitting its
lower parts.
[Illustration: Figs. 72 and 73.--_Tribrach adjustment._]
235.--The bubble of the level is placed parallel with two of the screws
of the tribrach, that is as _B_ and _C_, Fig. 72, and is adjusted to
the centre of its run. It is then placed at right angles to the first
position, so that the screw _A_ comes directly under the bubble, to be
adjusted by this screw only until it again comes in the centre of its
run. Fig. 73 shows this second position with the screw _A_ underneath.
The level should after this read all round true, but it is well to try
it round parallel with the different pairs of screws in all positions
to give small adjustment if required. Where there is a cross bubble
the level may remain for adjustment in its first position, but it is
well to try it all round, as the long bubble is made uniformly the more
sensitive.
[Illustration: Fig. 74.--_Ray shade._]
236.--=The Ray Shade= to the telescope used in the above-described
level has two narrow slits opposite each other at 180°. A zero line
is carried from one slit to a line on the ray shade fitting when the
slits are quite horizontal. Sights through the slits at zero enable an
approximate cross-level to be taken. The edge of the tube of the ray
shade is divided 30° on each side of the zero line to 2°, so as to take
approximate lateral inclines of the surface of the land in levelling.
This useful plan of cross-sighting was originally proposed by Gravatt.
[Illustration: Fig. 75.--_Stanley's platino-iridium point level stop._]
237.--The most important variation from the telescope of the dumpy
level described is in the diaphragm, where webs or lines of any kind
are entirely done away with, and are replaced by a special form of
index. This is represented in Fig. 75. The movable part carrying the
opening of the diaphragm is placed in a sliding fitting, as previously
described, art. 214, for the dumpy level. The index which replaces the
web is a finely-pointed needle formed of platino-iridium (platinum ·75,
iridium ·25). This alloy has about the hardness of spring-tempered
steel, and is, as far as known, perfectly non-corrosive in air or
moisture. A pair of vertical points indicate the position for holding
the staff. It will be found by experiment that the point reading is
much more exact than with the web, as irradiation due to edge reading
of the web is entirely avoided, and also the covering of the object
as it would be intersected by the web due to the angle its thickness
subtends upon the staff, which is very palpable at 10 chains distance.
The iridium point is sufficiently strong to be kept perfectly clean
by touching it occasionally with the point of a camel-hair brush if
it appear dusty. With care this point will last in adjustment for as
long a period as the level itself remains in use. Upon first impression
the point may not appear so fine as a web, but practically it is more
exact, as the previous exaggerated images will show--Fig. 76 is the
image of a division of the staff partially covered by a web _WW′_;
Fig. 77 that of a magnified image of the point _P_ brought towards a
division for reading. It will be readily observed that the fractional
part of the 1/100 foot block, which the point _P_ cuts, is much more
easily estimated than that in which the web _WW′_ covers a part of a
similar block.
[Illustration:
Fig. 76. Fig. 77.
_Difference of reading with a web and a point, shown much magnified._]
238.--In early levels of improved construction, as shown Fig. 70, a
difficulty was experienced in practice in bringing the index point
exactly up to the edge of the line as it is shown in Fig. 77 at _P_.
This difficulty has been obviated in recent highest class instruments
by making a tangent screw adjustment to the axis as shown under the
level in Fig. 78. There was a great objection to the old form of
tangent adjustment by clamping on the axis, as this was found to
disturb the centre. In the plan shown in the illustration the clamp is
left free by jointing to the axis until it grips one of the arms of the
tribrach upon a vertical surface; in this way it cannot disturb the
axis. The level, Fig. 78, is shown mounted on a framed stand, which
is preferred by the Indian Government, and is generally necessary for
rigidity for large instruments of over fourteen inches. This will be
described further on with theodolites, art. 447, on framed stands.
[Illustration: Fig. 78.--_Stanley's improved dumpy._]
239.--=Stadia Points.=--The author commonly makes the points, Fig. 75,
_VV′_ stadia points, by making the distance of the extreme ends of
these subtend an angle, equal to 10 feet of the levelling staff at ten
chains distance, or 1 foot of the levelling staff at 100 feet distance
(+ a constant to be discussed Chapter XII.), by which measurements
of the distance of the staff can by taken or checked by observation
through the telescope only.
[Illustration: Fig. 79.--_Stanley's quick setting-up level._]
240.--=Quick setting-up Tribrach.=--One objection has to be made to the
tribrach over the four-screw system of adjustment, that the four-screw
admits of greater inclination to the tripod, which is important in
hilly countries. To remedy this defect the author designed a ball
arrangement to the axis, which permits the level to be set 15° to
the inclination of the tripod independently of the screw adjustment,
so that the level, when the tripod is set at its best angle, may be
brought immediately to nearly its final position. The arrangement is
shown in the engraving Fig. 79. The axis carries a cup formed in the
metal casting, which can be clamped down upon a ball-shaped recess
formed upon the tribrach by means of a winged nut placed under it,
the wings of which project between the tribrach screws. A very slight
pressure is sufficient to firmly clamp the ball. This form of level is
now very popular with civil engineers. With a point diaphragm and a
tangent screw to the axis, not shown in the engraving, it is, in the
author's opinion, the best practical level he has been able to design.
[Illustration: Fig. 80.--_Stanley's Engineer's level._]
Since the last edition was written the reviser of this work introduced,
in conjunction with Mr. Stanley, his new solid bodied engineer's level,
which has practically revolutionized the form of dumpy level and has
proved such a success that more of this form are now made than all
other forms put together. In this level, Fig. 80, the centre, body of
telescope, object end and bubble fitting are all combined in one piece
of gun-metal, so that although of vastly greater strength and rigidity
it does not weigh as much as the old form of tubular body with its
collar and stage. This does away with many separate pieces which are
usually soldered and screwed together. It thus forms the strongest
and most compact level yet made, and with ordinary care it will last
in perfect adjustment a lifetime. The pinion for focussing is fitted
to the side of the cast body, instead of to a tube, thus greatly
increasing its firmness. Its form is equally adapted to the four-screw
levelling if desired, as shown on next page, Fig. 81, in which it will
be seen the four-screw levelling is of much improved form, giving
greater strength and far more wearing and bearing surface to the
levelling screws.
[Illustration: Fig. 81.--_Stanley's Engineer's level._]
The reviser has also patented a new form of spherical joint, which
has met with equal favour. This improvement consists of a section of
a ball (screwed to fit the stand head) fitted within the lower plate
and a simple means of clamping it in any position, which, when released
allows of sufficient rocking movement in any direction to compensate
for any uneven setting up of the stand. It does not add to the height
of the instrument, may be instantly set nearly level, and less than
half a turn of the levelling screws will bring the instrument into true
position. It is shown fitted to the new engineer's level at Fig. 82
below, but is equally applicable to any other form of instrument.
[Illustration: Fig. 82.--_Stanley's Engineer's level fitted with quick
setting spherical lower plate._]
As ninety per cent. of the orders now for levels are for the form shown
at Fig. 82, the reviser ventures to think that this must be favoured
by the profession as the best practical instrument yet made.
A further improvement has been made by making the diaphragms
interchangeable, so that any form of diaphragm that is preferred may
be instantly fitted without disturbing the adjustment, and when lines
on glass are used it may be removed for cleaning, and replaced without
interfering with the adjustment.
The diaphragms illustrated below, Fig. 83, are usual forms, and it is
recommended that when webs are preferred a glass one should be carried
as a spare in case of accidents.
[Illustration: Fig. 83.
E
_Stadia points for
clamp and
tangent levels._
F
_Stadia points
for ordinary
levels._
G
_Stadia glass
diaphragm._
H
_Webs._
J
_Stadia
webs._]
241.--The further discussion of the subject of high-class levels
becomes somewhat difficult. Leaving out of consideration the levels
sold by the trading optician, who deals in the commercial article
but sometimes superadds a little fad, every genuine manufacturer
has his pet plans of carrying out details, some of which may be
very meritorious, but which could scarcely be described without a
fuller discussion than our space permits. There is also, no doubt,
a great number of mistakes that have been made in the construction
of surveyor's levels. The direction in which the scientific optician
generally fixes his attention is to give the advantages of the Y-level
in the dumpy form, assuming the civil engineer holds a certain amount
of prejudice against the use of the Y, for which, in its old form at
least, the writer must admit that he was fully justified. Whether
the professional man, nevertheless, will ever depart from the solid
construction of the dumpy remains an open question.
[Illustration: Fig. 84.--_Cushing's 12-inch improved level._]
242.--=Cushing's Level.=--The level illustrated above, Fig. 84, by the
late Mr. Thos. Cushing, F.R.A.S., Inspector of Scientific Instruments
for India, would under any circumstances claim attention, from this
gentleman's well-known high technical scientific attainments. It has
also the merit of being in practical use in India at the present
time.[4] The principal improvement in this instrument over the dumpy
form, which it otherwise represents, is in the construction of the
telescope, which is said to possess all the necessary adjustments of
the Y-level. The telescope is firmly fixed in collars soldered to the
tube, as in the dumpy. The tube at each end is formed into a stout
socket collar. These socket collars are exactly alike, and are ground
to fit either the objective or the eye-piece end of the telescope, so
that these parts may be reversed, the one for the other. This
reversing is nearly equivalent to turning the telescope end for end
in the Y-level. The end also rotates in its fitting, which is nearly
equivalent to rotating the telescope half a revolution in the Y-level.
The reversible ends of the telescope are held in their ground fittings
by studs and slides (_bayonet notches_). It is easily seen that by this
plan adjustments may be made of collimation and of fixing the line of
collimation perpendicular to the vertical axis, as with the Y-level,
if the object-glass be originally correctly centred. The stop is of
the slide form described for the dumpy, Fig. 61, and a glass diaphragm
is used. One important arrangement is also made in this part of the
instrument--which is necessary, as glasses become frequently bedewed
in the telescope--viz., that the eye-piece end may be removed from its
ground fitting and the glass cleaned and replaced without disturbing
the adjustment in any injurious degree. The general construction of
the instrument can be seen from the illustration. The supports of
the telescope have a rocking axis at one end, and are adjusted by
capstan-headed nuts at the other. The adjustable support for setting up
the instrument is upon Everest's tribrach system for theodolites, to
be described further on, in Chapter IX. The tripod head has also wider
bearing than is general, which is attained by extending the book-plates
into the form of a socket fitting. The illustration given is of a
12-inch level; in the 14-inch an open framed stand is used in place of
the solid tripod, as in Fig. 78, which will be described further on,
for theodolites. The level is a decidedly good one; but the author has
experienced with it some slight defects when compared with his own Y
form. The ground collars are a little inclined to bite, particularly if
the instrument has been laid by for some time, so that in reversing for
adjustment there is great risk of disturbing the instrument. The glass
index, although permanent, has the same defect as the web--of covering
the image of the staff reading. It also obstructs a little light, and
is subject to dew, which the point system avoids. The weight of the
instrument is increased by the collar fittings.
243.--=Cooke's Level.=--An instrument somewhat equivalent to the above
has been patented by Messrs. T. Cooke & Sons. In this, instead of the
objective and eye-piece ends of the telescope only being reversible in
the collar fittings, as in Mr. Cushing's level, the entire telescope
reverses end for end in an extra outer tube, which is fitted between
the collars. This tube also permits the rotation of the whole optical
parts about the axis of the telescope for adjustment for collimation,
although in a manner more frictional, and therefore more likely to
disturb the instrument than in the simple Y adjustment. In this
instrument, again, it is easily seen that it is the perfection of the
Y-level, without its outward appearance, that is aimed at, and to gain
this the weight is increased by extra fittings and double tubes, which
are liable to become fixed by a slight dent upon the outer tube. Taken
altogether it is not quite so convenient or so simple as the best
constructed Y-level; but if it gives the adjustments the optician holds
to be most important, in a disguised form it may be acceptable to the
civil engineer. We may in this manner, perhaps, from the optician's
point of view, count it a certain gain in the same direction as Mr.
Cushing's level just described; but if we may accept the late Mr. Wm.
Gravatt's ideas, already mentioned, the complication is unnecessary.
244.--A few other structural variations of details may be mentioned, as
these are constantly cropping up as new inventions. The bubble tube is
sometimes placed upon the stage instead of being upon the telescope.
This is thought to protect it. It is not, however, so easy to read it
in this position. The compass is sometimes made a loose part--when
it is not required on the work its weight is saved. Various forms of
locking screws are made to the supports of the telescope; these are
only necessary to correct imperfect work. The axis collar is sometimes
extended to a limb bearing. This is common in French instruments; it
makes the movement stiffer, and is quite unnecessary unless the axis is
made too short. A well-known German firm recently brought out a level
with internal focussing, by means of an auxiliary lens mounted in a
tube inside the telescope, moved by a rack and pinion, but any internal
lens is a source of trouble, as it cannot be got at to be cleaned,
and in hot, damp climates it becomes bedewed. The device is very old,
having been patented in America many years ago and discarded.
245.--=Supplementary Parts to Levels.=--As a rule, supplementary
parts fixed to the instrument, beyond the magnetic compass sometimes
required, are very objectionable if the object of the level is to
be levelling, as these additional parts inevitably increase the
weight which has constantly to be borne in carrying the instrument.
Supplementary parts have been carried, in various schemes, to the
extent of combining the entire level with the theodolite, at the same
time nearly combining the united weights of the two instruments. As a
rule, professional men rarely care for complex combinations; and even
after a limited popularity is granted to extra parts not absolutely
required, these are generally finally abandoned. Mention of two such
parts, therefore, only will be made, as these owe their introduction to
the late William Gravatt, and are found applied to many levels in use,
or at least contained in the case with the instrument.
246.--_Bubble Reflector._--This was formerly placed upon all dumpy
levels. It consists of a small mirror about 2 inches by 5/8 inch fixed
in a frame that is jointed at its lower end to a short piece of tube
partly cut away so as to form only a little over a semi-cylinder. This
tubular part just clips firmly upon the brass casing tube of the spirit
level. The reflector, when placed vertically on the level tube, can be
adjusted by its joint, so that the run of the bubble may be observed
by reflection in looking above the eye-piece to see that it is in
adjustment at the time of taking an observation. Its use was thought
to be a precaution in levelling, particularly on marshy ground. The
observation of the bubble is less exact than by a side reading, and
cannot be relied on.
[Illustration: Fig. 85.--_Compact cheap form of dumpy level._]
247.--_Sight Vanes._--Two sight vanes are placed above the telescope,
either as loose fittings or to hinge down upon the level tube. One vane
has a vertical narrow slit and cross hair; the other has a window with
a vertical horse-hair placed in its centre. This arrangement gives
sight of distant landmarks in line with the direction of the telescope,
upwards or downwards, beyond its field of view. A slider, fixed upon
the window sight, reads at its upper edge into divisions cut on the
vane, by means of which an approximate rate of forward inclination of
the land may be taken. This sighting arrangement adds about half a
pound weight to the instrument. It was useful with object-glasses of
small field of view, but is useless with good modern glasses of wide
angle.
248.--=Lower-class Levels.=--A level is often required by an architect
or a contractor for works of limited area, where it is quite
unnecessary to go to the expense of a civil engineer's level of refined
manufacture. In such cases the level may only be used occasionally
and under favourable circumstances, so that extreme solidity is not
demanded, neither is distant view in the telescope required. The level
generally made for such work is a simple dumpy, without cross bubble,
compass, or any extra fittings, and with one eye-piece only.
249.--The instrument Fig. 85 illustrates the author's newest design for
a simple level. It has a light form of tripod. The legs clamp directly
between angle plates--these are not quite so portable or so neat as
cylindrical legs, but they are easily made, very firm, and will bear
considerable wear and keep in order. A still cheaper form is made with
smaller telescope and turned legs for the tripod.
[Illustration: Fig. 86.--_Contractor's or builder's level._]
250.--The illustration Fig. 86 represents the cheapest form of level
with a tripod stand that has been constructed, which contains the
important factor of a telescope. The telescope has a sliding fitting,
which is moved by a knob outside, this being made more quickly than
a rack and pinion fitting. The level tube is solidly supported in
collars. The adjustment is in one direction only, so that the bubble
must be set and examined at the time of reading the staff. The
instrument is supported on a _sprang_, jointed at one end and held by
a milled-headed screw at the other. Any shakiness of the thread of
screw there may be is taken up by a stiff German silver spring between
the sprang and the limb. It is sometimes made with a ball and socket
joint for first adjustment, but this renders it nearly as costly as a
superior level. The tripod head is of simple construction. The legs are
oak or ash, and are clamped on the head by bolts. This simple tripod
is fairly firm in use. The level is good enough for ordinary building
works, laying short drains, etc., within limited areas. It is much more
accurate than any form of open sighted level without telescope. Sir
George Leach has recently made a modification of this old form of level
by placing a pendulum to rock the axis to cross level position, which
is a refinement, although rather a costly one.
[Illustration: Fig. 87.--_Sighted reflecting pocket level._]
251.--=Sighted Pocket Level.=--This consists of a tube, which is
generally drawn of square section. A pin-hole sight is made in the
closed end of the tube, Fig. 87, at _E_. The field end of the tube is
left open. The sight is taken by looking through the centre of the
pin-hole across the edge of the reflector _R_. A level with a small
bubble is placed or inserted in the top of the tube at _B_. The metal
casing of this is cut away on the upper and under sides to render the
bubble visible from the interior of the tube by means of the reflector
_R_, which occupies one half vertical section of the interior of the
tube. This is placed at 45° to the axis. The reflector is fixed upon an
inner tube so that it may be withdrawn to be cleaned. When the level
is set horizontally, a distant object in the direct sight line is seen
through half the tube, and simultaneously the reflection of the bubble
in the other half appears. A line engraved upon _R_ indicates when the
bubble is central, and when these coincide the distant object and the
eye are level. The instrument is about 4 inches long, and weighs about
8 oz. in its case.
[Illustration: Fig. 88.--_Pocket telescopic level._]
252.--=Pocket Telescopic Level.=--In the above-described pocket level,
where it is made short, the average middle-aged man will not have
sufficient accommodation of vision to be able to see the bubble and
the screen sharply defined simultaneously with the distant object to
which the level is to be taken. In Captain Barrie's[5] level these
objections are avoided by making the reflector and bubble form part of
a telescope, Fig. 88. An achromatic glass of short focus is used, and
the eye-piece is of long focus so as to bring the bubble to focus in
the centre of the mirror, which is made of curved form to decrease the
apparent size of the bubble. The image of the bubble does not give by
bisection a very definite index. The author has found that this level
may be much improved by placing a point in the telescope at the mutual
foci of the object-glass, eye-piece and the bubble. The appearance
of the mirror and point is shown at _B_. The point is shown by a
dot at _P_. The curved mirror _R_. The dotted line shows the path
of reflection from the bubble. This level will work with very fair
accuracy as a hand instrument. Size, about 4½ inches by ¾ inch.
Weight in case, about 8 oz.
[Illustration: Fig. 89.--_Reflecting level._]
[Illustration: Fig. 90.--_The same construction in protecting case._]
253.--=Reflecting Level.=--This simple level, Fig. 89, the invention
of Colonel Burel, is one of the most portable. When it is used with a
fair amount of care it will give good approximate results. It consists
of a piece of parallel glass, which has half the surface silvered to
form a reflector. It is suspended in such a manner that the glass hangs
vertically by gravitation. The position of the mirror to the plain
glass may be that shown in the engraving, or horizontally if preferred.
The mirror, Fig. 89, is inserted in a solid metal frame suspended from
a gimbal, which permits it to hang perfectly free to the action of
gravitation. The centres of suspension are made with slightly-rounded
knife-edges. A ring at the upper part of the instrument is placed over
the thumb or finger to support the instrument when in use. A stout pin
passes through a prolongation of the lower part of the frame, screwed
or otherwise, which permits adjustment by filing to bring the mirror
when it is suspended exactly into a vertical plane. The instrument,
fitted into a neat case, weighs from 5 oz. to 9 oz.
254.--_In using the Reflecting Level_, it is held upon the thumb at
about arm's length, and adjusted by raising or lowering the arm until
the reflection of the pupil of the eye seen in the mirror is exactly
bisected by the line cut by the mirror against the clear glass. The
distant object seen in front, that cuts this sight line and the image
of the pupil of the eye, will then be in true level position with the
eye of the observer, provided the air is still, so that the mirror is
not deflected from verticality. From the natural unsteadiness of the
hand there is some little difficulty of getting this level quite free
from oscillation. This may be obviated, or nearly so, by clutching a
picket or staff with the hand and suspending the level from the thumb
projected out for the purpose, or by resting the hand against a tree or
other firm support. Capt. A. H. East, R.A., has suggested to the author
a very capital device which he employs for hand instruments. This is to
place the handle of a stick (or umbrella) in the waistcoat pocket, to
clutch the body of the stick with the hand which holds the instrument,
and to steady it with the other hand. In this manner the two arms and
the stick form a tripod of surprising steadiness.
255.--=Reflecting Level in Case.=--In windy weather much greater
exactness may be secured by placing the pendulous level, just
described, in a tubular case, Fig. 90. The case is made of double
tubes, so that the aperture cut on one side may by a half turn of
the outer tube close and protect the instrument when out of use. The
transparent side of the inner case is sometimes closed by thin glass
tube of its own internal diameter. It is much better if made with two
vertical sides glazed with parallel glass. When this form of instrument
is used, it may be, if required, made to fit on the top of a light
staff. The eye is then brought with much greater certainty to the
point of bisection on the edge of the mirror, and much greater accuracy
is thus attained in levelling with it.
256.--=Water Levels.=--The antique form of level, composed of two vials
fixed on the ends of a tube and partly filled with water, by which a
level is sighted in looking over the surface of the water, is still
used to a limited extent in rural districts on the Continent; but the
spirit level in some simple form is fast superseding it. The same
principle of level, but with long tube, has been found convenient for
the surveyor in measuring through close buildings, Fig. 91.
[Illustration: Fig. 91.--_Tubular water level with open vials._]
[Illustration: Fig. 92.--_Browne's standard water level._]
257.--=Browne's Water Level=[6] is found to be a convenient instrument
for levelling in close towns. It consists of a pair of glass tubes
of about 2 feet in length, placed in a casing tube for protection.
The casing tube is divided into inches and parts, or the scale is a
detached piece of painted wood, or any rod or rule. A cock at the
bottom admits the water to flow to level in the pair of tubes, one of
which is shown, Fig. 92. There is a handle at the top which unscrews to
fill the level, and a small air cock. It is easily seen that the water
finds its level, and the difference of reading of the two standards is
the difference of level of the surfaces upon which they are placed. By
closing the cocks the level is made portable. In this position it does
not matter how high the centre of the pipe is placed--for instance,
in crossing over a wall--as the water will still find its level when
the cocks are released by syphoning the water from the one side or the
other. It is a very convenient and exact level for laying drain pipes
in open weather, and for making foundations for heavy machinery, etc.,
but of course it will not stand frost.
Platelayers' levels and mechanics' levels generally are deferred to
consider with useful hand tools and apparatus employed by surveyors in
the final chapter.
FOOTNOTES:
[3] See Simms' _Mathematical Instruments_, p. 3.
[4] See pamphlet on _A New Form of Levelling Instrument_, by Thos.
Cushing, F.R.A.S., 1879.
[5] Patent No. 69, Wm. Barrie, 1856.
[6] Patent No. 6742, John Browne, June, 1834.
CHAPTER V.