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eral movement of the tube, and repeat the operation until
the bubble ceases to run while the partial revolution is made.
To complete the bubble adjustment, level the telescope and
take it out of the Y's and turn it "end for end." If the
bubble remains in the center of the tube, the second adjust-
ment is complete. If it runs to one end, bring it half way
back by means of the check nuts provided for raising or
lowering one end, and the rest of the way, i. e., to the
middle of the tube, by means of the leveling screws. Re-
peat the operation, as the adjustment can rarely be made
with one trial.

1274. Third Adjustment, sometimes called the
"Bar Adjustment." This is to cause the bubble to re-
main in the center of the tube while the telescope is being
revolved horizontally.

Level the instrument, using both sets of leveling screws.
Having centered the bubble carefully with one pair of level-
ing screws, turn the telescope until it stands directly over
the other pair of leveling screws. If the bubble runs, bring
it half way back by means of the locknuts at the end of the
level bar and complete the leveling with the leveling screws.
Repeat the operation, as two or three trials will probably be
necessary to complete the adjustment, so that the bubble
will remain in the center of the tube throughout an entire
horizontal revolution of the telescope.

The adjustments of the level should be tested every day
when in constant use, as any defect in them will detract
from the value of the work done, and a serious defect will
necessitate a repetition of the work.

The cross-hairs are placed at right angles to each other,
one of which should be vertical and indicate to the leveler
whether the leveling rod is being held plumb. If the verti-
cal cross-hair is "out of plumb," adjust it by loosening the
capstan screws which hold the ring, to which the cross-hairs
are fastened. Suspend a plumb-bob at a suitable distance



SURVEYING. 661

from the level, and having sighted to it, tap the capstan
screws sufficiently hard to cause the cross-hairs to move.
In this way the vertical hair can be made to coincide with
the plumb line, which is a true vertical.

1275. Sensibility. The sensibility of the level de-
pends directly upon the radius of the curve of the bubble
tube.

The graduated scale placed directly over the bubble tube
measures the movement of the bubble. The sensibility of
the level may be determined as follows: Having leveled
the instrument, take a reading on the rod held say 200 feet
from the instrument. Suppose this reading to be 5.61 feet;
with the leveling screws cause the bubble to move over one
division of the scale. Suppose the rod then reads 5.63 feet.
Denote the radius of the bubble by x, Fig. 289, the distance

^5.63 f




FIG. 289.

of the rod from the instrument by d, the difference of rod
readings by h, and the movement of the bubble by 5. From
the approximately similar triangles we have // : S :: d : x,

2 00
or .02 : .01 :: 200 : x, whence x = = 100 feet, the radius

of the bubble tube.

1276. Use and Care of the Level. The level
should not be used in rainy weather if it can be avoided.
Moisture obscures the lenses and is otherwise injurious to"
the instrument. When rain is unavoidable, wipe the lenses
frequently with a soft linen handkerchief, and when re-
turned to the office or camp, thoroughly wipe, finishing with
a piece of dry chamois skin and place in a warm, dry place
so that every particle of moisture may be removed. Never



662



SURVEYING.



carry the level with the spindle clamped. This rule is
especially important when working in a wooded country
where underbrush is dense. When undamped, the level
turns freely upon the spindle and yields readily to any pres-
sure. A blow which would inflict no injury upon an un-
damped instrument might seriously damage one while
clamped and rigid.

1 277. Power and Definition. The power of a tele-
scope is measured by the apparent nearness to which the
image of the object is brought to the eye of the observer.

The definition of a telescope is measured by the degree of
clearness of the outline of the image.

1278. Target Rods. Target rods are divided into
two classes, viz., those which are self -reading, or speaking

rods, and those which are not self-reading.
Railroad work is done chiefly with a self-reading
rod. That in most general use is called the
Philadelphia rod, and is shown in Fig. 290.
It is in two sections held together with brass
clamps A and B, one section sliding over the
other. When closed, the rod measures 7 feet,
sliding to 12 feet. It is graduated to feet,
tenths, and hundredths. The feet are marked
in large red figures, half above and half below
the marks of division; tenths of feet are
marked in black figures from 1 to 9, the lines
of division reaching half way across the face
of the rod; hundredths are marked by lines
T ^ T of a foot in width, alternating white and
black, and extending about one-third the way
across the face of the rod. The target is either
circular or elliptical and divided "into quarters,
alternating red and white. The division lines
are so arranged that when the rod is held in a
vertical position one of them will be horizontal
FIG. 290. and the other vertical. The target C is fast-
ened to a collar which slides up and down the rod, and is



SURVEYING. 663

fitted with a screw D, which clamps it at any desired point.
An opening more than one-tenth of a foot in length is cut
in the face of the target. A vernier is fastened to the target
whose zero point exactly coincides with the line which
divides the target horizontally. It lies within the opening,
on the face of the rod, and reads to thousandths of a foot.
To prevent wear, the foot of the rod is shod with brass. Rod
readings under 7 feet are usually taken with the two sections
closed, and the target moved up or down until the horizontal
line on the target coincides with the horizontal cross-hair of
the telescope. When readings of more than 7 feet are taken,
the clamp at B is loosened and the sliding section moved
upwards until the horizontal line of the target and the hori-
zontal cross-hair of the telescope coincide. The rod is then
clamped, and is called a long or high rod, and can be
read to thousandths with the vernier attached to the collar
at B. In setting the target, the leveler should read the rod
as closely as he can with the level, calling the reading to the
rodman, who sets the target at the given reading and holds
the rod up for a check reading. Four times out of five
the leveler's reading will be the correct one, even to thou-
sandths. More mistakes are made in reading the number of
feet than the number of tenths. The leveler by first calling
the reading to the rodman will be certain to prevent such an
error, as it would at once be detected in the check reading.
An experienced rodman can hold a rod practically plumb,
and for all ordinary work his care is considered sufficient.
For work requiring the greatest possible accuracy, such as
bridge foundations, a hand level, which fits closely to the
angle of the rod and carries two small spirit levels, is used to
accurately plumb it. In using a rod which is not self reading,
all readings are taken with the target.

1 279. Examples in Direct Leveling. The princi-
ples of direct leveling are illustrated in Fig. 291.

Let A be the starting point, which has a known elevation
of 20 feet. The instrument is set at B, leveled up, and
sighted to a rod held at A . The target being set, the reading,



664



SURVEYING.




8.42 feet, called a backsight,
is the distance which the point
where the line of collimation cuts
the rod is above the point A, and
is to be added to the elevation of
ihe point A . 20. 00 + 8. 42 = 28. 42
is called the height of instrument
and designated H. I. The instru-
ment being turned in the opposite
direction, a point C is chosen,
which must be below the line of
sight. This point is called a turn-
ing point, and is designated by the
abbreviation T. P. Drive a peg
at C or take for a turning point a
point of rock or some other perma-
nent object upon which the rod is
held. The reading at this point is
a foresight, and is to be sub-
tracted from the height of the
instrument at B to find the ele-
vation of the point at C.

Let the rod reading be 1.20 ft.
As this reading is a foresight, it
must be subtracted from 28.42,
the height of instrument at B;
28.42 1.20 = 27.22', the ele-
vation of the point C. As
the ground rises abruptly,
the rodman should slide the
to its full length, being
careful to keep it
on the same point
C. The leveler car-
ries the instrument
to D, which should
be of such a height
above C that when



rod



SURVEYING. 665

leveled up the line of sight will cut the rod near the top.
The backsight to C gives a reading of 11.56 ft., which, added
to 27.22 ft., the elevation of C, gives 38.78 ft., the height of
the instrument at D. The rodman then goes to E, a point
where a foresight reading is 1.35, which, subtracted from
38.78, the H. I. at D, gives 37.43 feet, the elevation of E.
The level is then set up at F, being careful that the line of
sight shall clear the hill at L. The backsight 6.15 ft.
added to 37.43 ft., the elevation of E, gives 43.58 ft., the
H. I. at F. The rod held at G gives a foresight of 10.90 ft.,
which, subtracted from 43.58, the H. I. at F, gives 32.68, the
elevation at G. Again moving the level to //, the backsight
to G of 4.39 ft. added to 32.68, the elevation of G, gives
37.07 ft., the H. I. at H. Holding the rod at K a foresight
of 5.94 subtracted from 37.07 gives 31.13, the elevation of
the point K. The elevation of the starting point A is
20.00 ft.; the elevation of the point /if is found by' direct
leveling to be 31.13 ft., and the difference in the elevations
of A and K is 31.13 20.00= 11.13 ft. ; that is, the point
K is 11.13 feet higher than the point A.

1 28O. A Datum Line. A datum line is the base
line to which the elevation of every point of a series is re-
ferred. Thus, in Fig. 291, the datum line or plane is 20 feet
lower than the point A, and the elevations of the points
A, >, C, D . . . .K are their elevations above this datum line.
Such a series of elevations is called a line of levels.

1281. Turning Points. Turning points, men-
tioned in Art. 1 279, are the points where backsights and
foresights are taken. The backsights are plus (-(-) readings,
and are to be added; the foresights are minus ( ) readings,
and are to be subtracted. The rodman should make a peg
of well-seasoned oak, or other hard wood, about 9 inches jn
length, 1 inch in diameter, sharpened at one end and
rounded at the other end, which is the turning point. For
driving the peg he should carry in a leather scabbard a
light hatchet. A point for a foresight having been deter-
mined, the rodman drives the peg firmly in the ground and



666



SURVEYING.



holds the rod upon it. After the instrument is moved, set
up, and a backsight taken, the peg is pulled up and carried
in the pocket until another turning point is called for.
Turning points should be taken at about equal distances
from the instrument in order to equalize any small errors in
adjustment. In smooth country an ordinary level will per-
mit of sights of from 300 to 500 feet. A good rodman is as
necessary to accurate and rapid leveling as a good leveler.
A man who is inattentive to the work in hand, or averse to
rapid movement, is not fit for either place. In most locali-
ties, a line of levels of any considerable length will have
enough rough places in it, i. e., places where considerable
changes in elevation occur, to retard progress, however
diligent the level party may be. Laziness or carelessness
merit immediate discharge, and usually receive it.

1282. Bench Marks. On railroad surveys, perma-
nent points called bench marks should be established at
intervals of from 1,000 to 2,000 feet, depending upon the
nature of the country. Any permanent object, such as a
stone door sill, a tree, or point
of large rock, will serve for
a bench mark. Where trees are
available, they are always used,
the point being cut on a pro-
jecting root. On preliminary
lines they should be as near to
the line as possible. A tree
with a large exposed root is
chosen, the bench mark is cut
into the root in the form of a

pyramid, a tack is driven into the apex and the rod held
upon it. The tree is blazed smooth and the letters B. M.,
together with the elevation of the mark, written with red
chalk. A bench mark of this kind is shown in Fig. 292, the
point being at A and the elevation recorded at B.

1 283. Check Levels. Check levels or test levels are
taken for the purpose of checking and proving the accuracy




SURVEYING. CG7

of a line of levels before their adoption as a basis for con-
struction. Usually intermediate points or stations are not
taken, but only the turning points necessary to cover the
line. Readings are taken at all the bench marks, and the
correct elevations marked. The adjustments of the instru-
ment should be frequently tested, and the rodman should
carry a rod level to insure the plumbing of the rod.

1284. Water Checks. When the line of survey fol-
lows the shore of a body of water having no current, such
as a lake or pond, its surface can be used as a check, since
its level for any ordinary space of time will remain un
changed. The sea, whose level is constant, is the base
for all barometric leveling, and at all seaports for direct
leveling.

1 285. Rapid Work. The rate of progress is limited
by the transit party. If the country is open and rolling,
where long sights are frequent and chaining easy, the level
party will not keep up with the transit party. If the
country is smooth and open, both parties can make about
the same progress. If, however, the country is thickly cov-
ered with underbrush or heavy timber, the level party will
have much idle time. A good day's work will vary, accord-
ing to conditions, from three to eight miles.

The target should be set by signals given by the leveler.
An upward movement of the hand is the signal for raising
the target, and a downward movement the signal for lower-
ing it; a circle described by the hand is the signal for
clamping the target, and a wave with both hands indicates
that the target is properly set.

All intermediate readings are read by the leveler, whose
signal "All right" is a single outward wave of the hand,
the rodman being careful to keep the rod at full length. *

The rodman should always call out the rod reading, giving
first the number of feet, or, if the reading is less than 1 foot,
call the figure " naught," never "ought," then pausing a
moment, call the decimal part of the reading. If the rod is
being read to hundredths only, the number, 8.40, is read:



608 SURVEYING.

eight-four, naught; if 8.04, it is read: eight-naught, four.
If the rod is to be read to thousandths, the number, 8.401,
is read: eight-four, naught, one; if 8.410, it is read: eight-
four, one, naught.

The distinctness of a call is in no way proportional to the
amount of noise in it. A few days' practice will enable a
rodman with moderate effort to call a reading so as to be
distinctly heard at a distance of 500 feet. Should a high
wind be blowing, the sights will be shorter, owing to the
vibration of the instrument, and the rodman's work propor-
tionally lessened. The rod reading should always be re-
corded before moving the instrument. The leveler may
check the reading as he passes the rodman. In general,
however, the leveler relies entirely upon the accuracy of the
rodman's readings. If he can not be trusted, his place
should at once be supplied by one who can be trusted.

In taking levels on preliminary railroad surveys, frequently
the turning points, "as well as intermediate stations, are
read by the leveler without being checked by the target.
The rodman has still plenty of occasion for the use of judg-
ment, as the rate of progress depends largely upon the care
shown in the selection of turning points.

1286. Sources of Error. The principal sources of
error are defects in adjustment, which are the fault of the
leveler, and failure of the rodman to plumb the rod, and
wrong target readings, which are the fault of the latter.
Poor levelers and poor rodmen usually go in pairs. Haste
or hurry are poor helps to progress. One can do rapid and
accurate work without haste, but can not hurry and be
either rapid or accurate.

The sun shining directly upon the object glass confuses
the sight. To prevent this, most instruments are provided
with a sun shade, which fits the end of the telescope,
projecting over the object glass.

If the sun shade is lacking, the leveler can hold his hat so
as to shade the object glass.

Wind is also a source of error, as it causes the instrument



SURVEYING. 669

to vibrate, thus preventing the "accurate setting of the
target. The leveler should wait for a lull in the wind, dur-
ing which, if his rodman is alert, he can get a close shot.
At a second lull, he can check the target and feel safe in
moving ahead.

Individual errors, called "personal equation," are defects
in vision peculiar to the individual, so that two persons may
set a target for the same rod, each giving a different read-
ing; but as this personal equation, or error, is constant for
the same person, it does not materially affect the accuracy
of work.

1 287. Necessary Degree of Accuracy. In prelim-
inary railroad work an error of .10 of a foot per mile is
allowable. Time spent in reducing such inaccuracies is
wasted. That painful degree of accuracy termed "hair-
splitting " is no recommendation, and the gain in accuracy
is more than balanced by increased cost and loss of time.
It is a well-known fact that small inaccuracies tend to bal-
ance each other, and that a line of levels covering 20 miles,
taken with a self-reading rod, will closely check a line taken
with target readings and rod level.

1 288. How to Keep Level Notes. Forms for keep-
ing level notes are various. One of the best forms, rarely
or never seen in print, and yet one which is in general use
among engineers, is shown on the following page :

The distinguishing feature of this form of level notes is a
single column for all rod readings. The backsights being
additive and the foresights subtractive readings, they are
distinguished from other rod readings by the characteristic
signs 4- and . The turning points, whose foresight read-
ing is , are further designated by the abbreviation T. P.

1289. How to Cbeck Level Notes. There is one^
method of checking level notes which is in universal use. It
provides for checking the elevations of turning points and
heights of instrument only, which is sufficient, as all other
elevations are deduced from them. The method is very
simple and depends upon the fact that all backsights



670



SURVEYING.



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SURVEYING. 071

are additive or -(- quantities, and all foresights are subtrac-
tive or quantities. The level notes described in Art.
1288 are checked as follows: The elevation of the bench
mark at Station is 100.00 feet, to which all backsights or
-f- readings are to be added, and from this sum all foresights
or readings are to be subtracted. The sum of the +
readings or backsights together with the elevation of the
bench mark at is 122.59. The sum
Thus 100 10~2<> f the ~ readin g s or foresights is

5 61 2 52 2427 ' and the difference 98 - 33 feet
is the elevation of the turning point

f



57 227

level notes is filled, the leveler should



check them, placing a check mark ^
qo-qnT at the last height of instrument or

elevation checked. When the work

of staking out or cross-sectioning is being done, the levels
should be checked at each bench mark on the line. At the
close of each day's work, the leveler must check on the near-
est bench mark.

1 29O. Profiles. A profile represents a vertical sec-
tion of the line of survey. In it all abrupt changes in
elevation are clearly outlined. Vertical and horizontal meas-
urements are usually represented by different scales. Irreg-
ularities of surface are thus rendered more distinct through
exaggeration. For railroad work profiles are commonly
made to the following scales, viz., horizontal, 400 feet =
1 inch ; vertical, 20 feet = 1 inch.

A section of profile paper is shown in Fig. 293. Every
fifth horizontal line and every tenth vertical line is heavy.
By the aid of these heavy lines, distances and elevations are
quickly and correctly estimated and the work of platting
greatly facilitated. The level notes described in Art. 1 288
are platted in Fig. 293. The elevation of some horizontal
line is assumed. This elevation is, of course, referred to
the datum line, and is the base from which the other eleva-
tions are estimated. Every tenth station number is written



672



SURVEYING.



at the bottom of the sheet under the heavy vertical lines.
The profile is first platted in pencil and then inked in black.



1291. Grade Lines. The principal use of a profile
is to enable the engineer to establish a grade line, i. e.,

a line showing the relative proportion of excavation and
embankment in the proposed work. The rate of a grade
line is measured by the vertical rise or fall in each hundred
feet of its length, and is designated by the term per cent.
Thus, a grade line which rises or falls 1 foot in each hundred
feet of its length is called an ascending or descending 1 per
cent, grade, and written -f 1.0 or 1.0 per hundred. A rise
or fall of one-half foot in each hundred feet is called a five-
tenths per cent, grade, and written -4- .5 or .5 per
hundred. The grade line having been decided upon, it is
drawn in red ink.

EXAMPLE. The elevation of Station 20 is 140.0 feet; between Sta-
tions 20 and 100 there is an ascending grade of .75 per cent. ; what is
the elevation of the grade at Station 71 ?

SOLUTION. To obtain the elevation of the grade at Station 71, we
add to the elevation of the grade at Station 20, 140 feet, the total rise
in grade between Stations 20 and 71. Accordingly, 71 20 = 51 ; .75
foot X 51 = 38.25 feet; 140 + 38.25 = 178.25 feet, the elevation of grade
at Station 71.



SURVEYING.



073



TOPOGRAPHICAL SURVEYING.
1292. General Definition. Topographical sur-
veying is the location and representation of the inequalities
of any portion of the earth's surface. The portion surveyed
is conceived to be projected upon a horizontal plane, called
a plane of reference, upon which all inequalities of sur-
face as well as all conspicuous objects are shown in their
true relative positions. The simplest and most generally
used method of representing the topography of a given
surface is by means of contour lines. A map containing
an outline of a given surface, together with the contour
lines representing its inequalities, is called a contour map
of that surface.




Let A B C, in Fig. 294, represent the outline of a hill, and
suppose this hill to be gradually submerged in water, the.,
water rising in successive heights of 10 feet. The flow, or
shore line, at each successive rise is a contour line. The
horizontal lines correspond to the surfaces of the successive
elevations of the water. The points where these horizontal



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