U. S. DEPARTMENT OF COMMERCE
COAST AND GEODETIC SURVEY _

MANUAL OF |
TIDE OBSERVATIONS

SPECIAL PUBLICATION No. 196
REVISED (1941) EDITION

BATA LIBRARY
- REFERENCE COLLECTION
NWOKIDS HOLE OCEANOGRAPHIC INSTITU; -

U. S. DEPARTMENT OF COMMERCE

JESSE H. JONES, Secretary TGA

COAST AND GEODETIC SURVEY
LEO OTIS COLBERT, Director

Special Publication No. 196 -
Revised (1941) Edition =

| MANUAL
OF TIDE OBSERVATIONS

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__DATALIBRARY
___ REFERENCE COLLECTION
*W#KX IDS HOLE OCEANOGRAPHIC IN STITUTE Ss

74

UNIFED STATES
GOVERNMENT PRINTING OFFICE
WASHINGTON : 1941

For sale by the Superintendent of Documents, U. S. Government Printing Office
Washington 25, D.C. - Price 25 cents (Paper cover)

FOREWORD

This is one of a series of manuals published by the United States
Coast and Geodetic Survey for the purpose of giving the general re-
quirements of the Bureau in carrying on its various activities. This
volume contains instruction for the observations of tides and the usual
reductions necessary for the determination of datum planes and the
nonharmonic quantities published in the Tide Tables. Directions for
taking temperature and density observations are also included for the
convenience of tide observers who take such observations in connection
with their regular duties at the tide stations.

Corresponding instructions for tidal current observations are given
separately in Special Publication No. 215, Manual of Current Obser-
vations. The harmonic computations required for the prediction of
tides and currents are described in Special Publication No. 98, Manual
of Harmonic Analysis and Prediction of Tides.

The present work supersedes a previous edition of the manual
published in 1935 and contains a revised description of the standard
automatic tide gage to include recent improvements in this instru-
ment. This manual was prepared by Paul Schureman, Principal
Mathematician, under the direct supervision of Capt. Paul C. Whit-
ney, Chief of Division of Tides and Currents,

II

CONTENTS

Page
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LEDS MORTAR ES Oe aes § ye I gn, pte pr ag Oe CTS pea erie 5
Standanrdiautomaticstide gages area tere ae eee ee Pee ee 5
Rortablerautomatic tideseagee: ys. Tel see Ne ee 18
rat euIG ers ta tiOmes ole Volga cps b eae de iS sion ee ee Ne Melee 25
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El oastawe lle sae UAC Das hee oe Pa et eg ae ok ee oe, Ee ~ 26
BINT ERIN O US Cen eM tt BERD Rie tay AD a yale CARER WA Re Sub Ei Oe 28
Installation of tide staff___._.__..-._-_-_- Say cP Ls 2 Oe SIN BR ge 29
astallationrof tape cage! 28/2 0h Se oa Be oS eC ye el tony Bi 29
Installation of standard automatic tide PACS oe oe eames here ke Ue 32
MRT cl ali loe Te In yran VE Stet nee Sees es ren ey Naa 0 Ce Nae 37
Operation of tide station EA BS) GT Se AS Se Se ela aa AR 41
Inspectionrofitide station! syns te Veh iee ee eins ye RE 50
DECoOneanyebiGerstabione te n6 25 i oe eo eS te buiieterolbeaM satel 52
linear rrOmpeme eps een Denon tins icles a nS Roane ee apnea aes Mine ae Be Moe 53
Establishment of secondary tide station_____________=_____-______- 54
Installation and operation of portable automatic tide eee sea AR iol Ai 33)
ER CIAE ede Gro I Si aes ee Fete OM rosie eA ey cal ie me «is Sa 59
pEAbmlacionvand rea uehione 22052 4 ST PY ae ope i SO 59
Ege lanai ary WORK eee apr 2 kee Dy is eek a 59
Hoiomrandelow. water tabulation.“ — 2222 62 ce ee ee ee 63
Hourly heishtitabulation 222° oS. e 82 EIS NGOS eae Hk Ape Ons a 66
Intern DOlationse sete oe We ee bli gr wa Aah) oes SU Nee Eee eo lie 68
Dera Tel ead arnt rey eat) Se ee a A eco are aol a 69
Hee inbare GUC liOnsE ee Ak Ce pens pan A A OLE gee i haa ie en ye ee 73
whic aleclat umn sees see ES ee et Se ee 76
mice erecueersOr SOUNGINGR!. 28.3 2 e. tos 2) ee Se ee Bs 18
Lemperature andidensity observations--_2-_. =. = 222) eee 81
“LNEPEOY OES PD DSL Cae a ot We ge da Ne eg GE APL Se 81
IDES aNSST 0 ER TUN PSO NT hey et UME RL ee Se a mF 83
{UG ease a 7 ASA lea Oo pa MN A ge 89
ILLUSTRATIONS
ievhorcable tide stafi;and, supports 22405205822 oe le sae Se 2
DATO CH RUE Cee supe Sinscers pent MUNA re Fig ales EMO CY ake i Mee BR 3
3. Standard automatic tide gage (with cover)_____.-________-_______- 7
4. Standard automatic tide gage (front view) _______________-_______-- 8
5. Standard automatic tide gage (side and back)____________________- 9
6. Standard automatic tide gage (clock connection)__________________- 10
7. Standard automatic tide gage (recording pencils)__________________-_ 11
8. Standard automatic tide gage (right forward end)_________--------- 12
9. Standard automatic tide gage (left forward end)__________________- 13
10. Standard automatic tide gage (paper installation) _________.______-_- 14
Pie eeortable automatic tide gape (front). 12. 2-20 eet Te 19
12. Portable automatic tide gage (left side)____________________.-____- 21
13. Portable automatic tide gage (detail of stylus and float drum) Siete 22
MPa ett kev lea ee apt ae pees Bea sa Re ake ON ue a rd Ve i 26
2 IIr

CONTENTS

Intake coupling for float well_____-------- Pag me Mate eg, Ss

. Cleaning tool for float well_____-
. Schematic view of installation of

Standard disk tidal bench mark_

standard automatic tide gage

. Tide curve and comparative note (standard gage)
. Installation portable automatic t
. Installation portable automatic tide gage against rocky cliff
. Tide curve and comparative notes (portable gage)
. Form 455, Comparative readings
. Form 138, High and low waters (front)
. Form 138, High and low waters (back)
. Form 362, Hourly heights______-
. Form 248, Comparison of simultaneous observations
. Graph for obtaining tide reducers directly from marigram
. Form 457, Density and temperature
. Graduations of hydrometer scale_

ide gage on fish trap stake__________

MANUAL OF TIDE OBSERVATIONS

PURPOSES OF TIDE OBERVATIONS

1. The tidal work of the Coast and Geodetic Survey is carried on
for the purpose of serving the needs of the mariner, the engineer, the
scientist, and the public generally. The work had its origin in the
- necessity for reducing to a common level or datum plane, soundings
taken at different stages of the tide, and this still constitutes one of the
important purposes. Among other outstanding purposes are (a) the
determination of tidal datum planes for general engineering work,
(6) the derivation of data for the prediction of tides, (¢) the securing
of information pertaining to the mean and extreme rise and fall of the
tide which may be necessary in the construction of piers, bridges, and
other structures for which the tidal condition-is an important factor,
(d) the securing of data for the study of crustal movements in the
earth. A continuous series of tide observations in any region also
provides data for the reduction of shorter series of observations in
nearby areas and furnishes information often required in connection
with legal cases involving maritime interests.

2. Tide stations may be classified as primary and secondary. Pri-
mary stations are those at which observations are to be continued for
a number of years for the purpose of deriving basic tidal data. for the
locality. A secondary station is one which is operated over a very
limited period of time to obtain tidal information for a particular
purpose and the length of series will depend upon that purpose.

TIDE GAGES

3. A tide gage is an instrument for measuring the height of the tide.
Tide gages may be divided into two groups—nonregistering gages
which require the presence of an observer to take and record the
height of the tide, and self-registering or automatic gages which auto-
matically record the rise and fall of the tide while unattended.

4, The first group includes the tide staff and some types of float and
pressure gages. The second group includes a variety of types, some
of which record the rise and fall of the tide in the form of a graph,
others by printed figures, and others photographically. The two prin-

cipal kinds of automatic tide gages used by the Coast and Geodetic
- Survey record by means of graphs. One of these, known as the
“standard automatic tide gage,” is designed for use at primary tide
stations or where observations are to be continued for a considerable
period of time. The other, known as the “portable automatic tide
gage,” was designed for use at tide stations which are to be continued
A only a short period and where ease of installation is a desired

actor. :

1

2 U. S. COAST AND GEODETIC SURVEY

TIDE STAFF

5. The simplest kind of tide gage is a plain staff, which may consist
of a board 1 to 2 inches thick and 4 to 6 inches wide, graduated in
feet and tenths. The length should be sufficient to extend from the
lowest to the highest tide which may reasonably be expected in the
locality where the staff is to be used. Such a staff is secured in a
vertical position to a pile or other suitable support, with the gradua-
tions increasing upward. When nailed in
place or otherwise secured so as not to be easily
removable, it is called a fixed staff.

6. Vitrified scale-—To overcome difficulties
resulting from the defacement of the gradua-
tions on a wooden tide staff, which may become
illegible after a comparatively short time, the
office has adopted a set of scales graduated i in
feet and tenths, which are made by baking a
vitrified coating on wrought-iron strips. The
strips are in 3-foot sections about 214 inches
wide, the sections being so graduated that
when placed end to end form a single continu-
ous scale. The scalés may be secured to a
wooden staff or suitable piece of timber, brass
screws and lead washers being provided for
the purpose.

7. Glass tube.—For use in rough water a
glass tube about 14 inch in diameter is secured
to the face of the tide staff by spring clips or
other devices. The lower end of the tube is
partially closed by means of a notched cork
to dampen down the motion of the water inside.

8. Portable staff.—After a fixed tide staff
has been in the water for a considerable period
of time, particularly in harbors:where there
is much refuse or fuel oil, the graduations may
become more or less illegible. To avoid this,
there are used at many of the tide stations a
portable tide staff (fig. 1) which may be easily
removed from the water and stored under
shelter when not in actual use. The tide staff
may be constructed in hinged sections for con-
venience in storing. In order that such a staff
shall always have its zero at the same elevation
when placed in the water for use, a tide staff ©
support permanently secured in place is

, whee Pict idaae | eCRSaen:

ely SE aidipunp Orta 5 3. The. tide staff support, with length cor-

responding approximately with that of the
portable staff, may be constructed of 2-inch plank somewhat wider than
the tide staff and covered with copper sheathing as a protection against
teredos and other marine borers. A metal vlate at the top of the sup-
port forms a shoulder on which a metal stop secured to the back of the
tide staff rests when the staff is in position for use, thus assuring a fixed

MANUAL OF TIDE OBSERVATIONS 3

elevation for the staff zero. At intervals along the support, metal
guides are arranged in pairs to hold the staff in a vertical position.
10. Multiple staff.—Along shore where shoal water extends some

distance offshore and
the range of tide is too
large to be measured by
a single staff, a succes-
sion of staffs may be
used. The different
staffs should be so grad-
ated and installed that
the graduations will be
continuous from one
staff to the next with
the readings on all of
the staffs referred to
the same zero.

TAPE GAGE

11. The tape gage
(fig. 2) is designed as a
substitute for the tide
staff in exposed locali-
ties where the water is
too rough for staff read-
ings. It is operated by
a float in a vertical box
or pipe, known as a
float well, which serves
to dampen out the
larger wind waves. (See
pars. 65-72 for a dis-
cussion of float wells.)
Connected with the
float is a tape which
passes over a pulley in
the ceiling of the tide
house and is kept taut
by means of a counter-
poise. In general, it is
best to have the coun-
terpoise supported by a
movable pulley with
the_end of the tape at-
tached to the ceiling of

Tape Gauge

Reading Mark

Floor of Tide House

Water Surface

FIGURE 2.—Tape gage.

the tide house in order to increase the limits of operation of the appa-
ratus. With a very small range of tide or a very high ceiling, how-
ever, the counterpoise may be attached directly to the free end of the

tape.

12.-There are several kinds of tape gages. In some, an index or

pointer attached to the tape moves over a fixed scale. In others, as

&

4 U. S. COAST AND GEODETIC SURVEY

represented in figure 2, there is a fixed reading mark on a board and the

height of the tide is read on the tape graduations as they pass the

fixed mark. In the latter case, the tape graduations, expressed in feet

and hundredths, should increase towards the float so that the readings

will increase with a rising tide. The tape itself may be either stainless’
steel or a bronze alloy—the advantages of the former being flexibility

and legibility, and of the latter durability. Some of the advantages

of both may be secured by using the bronze alloy for the lower section

of the tape which is normally in the float well and the stainless steel

for the upper section from which the readings must be taken.

13. The fioat for the tape gage must be cylindrical in shape in order
that small changes in the plane of fiotation will not affect the cross-
section area in this plane. The top and bottom, however, may be
tapered to permit easy passage over any roughness which may be
accidentally contacted inside the float well. Either the 814-inch
float of the standard automatic gage or the 31-inch float of the portable
gage may be used for the purpose, but the larger float is to be preferred
as its plane of flotation is more stable and less sensitive to change from
frictional resistance in the operation of the gage.

14. When a tape gage is used, a distinction must be made between
a visible scale zero and the true datum of the gage to which the ob-
served heights are referred. The tape gage datum may be defined
as the level of the water surface at the time the gage reading is zero.
This will obviously be a number of feet lower than any visible scale
zero inside the tide house. For the installation of a tape gage and
the determination of its datum see page 29. -

PIPE GAGE

15. The pipe gage is a type of float gage which has been used on
offshore zieale in a depth of 30 feet of water. This gage consists of
a white-pine rod staff, cross section 1 by 1 inch, with rounded edges,
graduated on each of the four sides in feet and two-tenths with the
zero (0) at the top, and set in a hollow cylindrical white-pine float
11% inches outside diameter and 7%-inch inside diameter. The float
should be thoroughly covered with shellac and liquid paraffin. The
length of the rod will depend upon the range of tide in the locality
where it is to be used and the length of the float should be about
four-tenths that of the rod. The float well consists of a 2-inch iron
pipe, the bottom of which is set in a 1,000-pound concrete block to
serve as an anchor. The pipe should be long enough to reach above
the ordinary waves at high tide and a 14-inch hole-should be drilled
in the pipe several feet above the concrete anchor. A cap with a
square hole through which the staff passes, is screwed on top of the
pipe after the, float staff has been placed inside. Just below the cap
- a 2-inch flange for the attachment of guy wires is screwed on the pipe
and four small sheaves, one for each guy wire, secured to this flange
by wire loops. The top of the float pipe is secured in a vertical posi-
tion by four guy wires of No. 6 wire with leads making an angle of 60°
or more with the vertical. The end of each guy wire is anchored to

MANUAL OF TIDE OBSERVATIONS ~— 5

concrete blocks, giving a total weight of about 2,000 pounds to each
anchor. For convenience in handling each concrete block may be
cast with wire rope loops projecting. After the anchors have been
set the guy wires are led through the sheaves at the top of the float
pipe and sean taut, a fence-wire stretcher being convenient for this
purpose. ;

PRESSURE GAGE

16. The pressure gage operates by measuring the variation in pres-
sure at the bottom of a body of water due to the rise and fall of the
tide. Although not as satisfactory as the types of gages usually
employed for tide observations close to shore, pressure gages have
been used with some success for observations taken on shoals at some
distance from land where it has been impracticable to install the usual

type of gage.
FATHOMETER

17. The fathometer is primarily an instrument for measuring the
depth of water but, since this depth varies with the rise and fall of the
tide, the instrument also has been used with some success for observing
tides from a ship at anchor offshore. The measurements depend
upon the interval of time required for a sound wave to travel from ~
the ship to the bottom of the ocean and for its echo to return. A
full description of the instrument will be found in the Coast and
Geodetic Survey Hydrographic Manual. When using the fathometer
as a tide gage, the ship should be anchored where the ocean bottom
is near level as possible, so that there will be no material change in
depth due to the swinging of the vessel. The anchorage should be
buoyed and the observations repeated on different days to detect irreg-
ularities due to causes other than the tide. The instrument might also
be operated with the transceiver anchored to the bottom of the ocean,
in which case the sound wave first travels upward and is then reflected
from the water surface back to the transceiver. By this method the
tide record would not be affected by irregularities in the ocean bottom
at the ship’s anchorage.

STANDARD AUTOMATIC TIDE GAGE

18. The present standard automatic tide gage used by the Coast
and Geodetic Survey is a development of the Stierle gage adopted by
this Survey many years ago. A float operates in a vertical box or
pipe to which the slow moving tide has free access while the more
rapid moving waves resulting from winds are largely damped out by
the relatively small size of the inlet to the box. The rising and falling
of the float operates a worm screw on the gage which moves a pencil
to and fro across a wide strip of paper which is moved forward by
clockwork. The combined motion of pencil and paper gives a con-
tinuous graph showing the rise and fall of the tide.

6 U. S. COAST AND GEODETIC SURVEY

19. Names of parts.—For convenience of reference there are given
below the names applied to different parts of the standard automatic
tide gage. The numbers correspond to those given in figures 4 to 9.

1. Time clock. 24. Datum pencil. .
2. Motor clock. 25. Datum pencil clamping screw.
3. Clock case. 26. Hour-tripping rod.
4, Supply roller. 27. Clamping screws, tripping rod as-
5. Tension guide springs (2). sembly (2).
6. Pencil screw. 28. Tripping hook stop.
7. Pencil arm return springs (2). 29. Striker weight.
8. Drum shaft ball bearings (2). 30. Striker weight clamping screw.
9. Drum shaft bearing caps (2). 31. Striker weight spring.
10. Counterpoise drum or reel. 32. Striker lifter binding screw.
11. Float drum or reel. 33. Striker lifter.
12. Capstan lock nut, counterpoise | 34. Carrier arm.
drum. 35. Carrier wheel.
13. Capstan lock nut float drum. 36’ Hour tripping hook.
14. Main roller. 387. Recording pencil.
15. Bracing rod. 88. Recording pencil holder.
16. Tension weight drum or reel. 389. Recording pencil clamping screw.
17. Receiving roller. 40. Pencil holder adjusting screw.
18. Receiving roller release buttons | 41. Pencil arm.
(2): 42. Pencil weight.
19. Winged nuts securing clock unit | 43. Pencil arm bearing serew.
(4). 44. Pivot screws for pencil holder (2).
20. Datum pencil rod. 45. Capstan. bearing pin for pencil
21. Datum pencil clamp. serew.
22. Datum pencil holder clamping nut. | 46. Lock screw.
23. Datum pencil holder. 47. Lock nut.

20. Clock unit.—The clock unit consists of two clocks mounted on
a frame. The one on the right (1, fig. 4) is known as the time clock
and the one on the left (2) without hands as the motor clock. For
convenience, the corresponding sides of the tide gage may be called
the time side and the motor side. The time clock operates the device
which makes the hour marks on the record, while the motor clock
serves to regulate the forward movement of the paper. By having a
separate clock to move the paper, the time clock is relieved of unnec-
essary work and may therefore be more accurately regulated for
recording the correct time. Moreover, the use of two clocks aids in
securing a continuous record, because if either one of the clocks stops
for a short period, it is sometimes possible to interpret the record
during this period through the functioning of the remaining clock.

21. The clock unit is secured by four winged nuts (19, fig. 8) on
the back of the clock case and is interchangable with other units when
replacements are necessary. .In the older types of the instrument, the
two clocks were mounted independently in the clock case, the time
clock on the left and the motor clock on the right.

22. Each clock has an 8-day movement and may be regulated and
corrected as similar clocks in ordinary use. To avoid injury to the
hour-marking device of the time clock, however, the minute hand must
not be turned backward when between 10 minutes before and 5 min-
utes after the hour “12.” The hour hand may be turned in either
direction, and if it is necessary to turn the clock backward within the
limits noted above, it may be accomplished by turning the hour hand
back a full hour and the minute hand forward to the correct time.

MANUAL OF TIDE OBSERVATIONS

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MANUAL OF TIDE OBSERVATIONS

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MANUAL OF TIDE OBSERVATIONS

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MANUAL OF TIDE OBSERVATIONS ts be

Figure 9.—Standard automatic tide gage (left forward end).

-

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i Vee Saat U. S. COAST AND GEODETIC SURVEY

FIGURE 10.—Standard automatic tide gage (paper installation).

23. If the time clock runs consistently fast day after day, the regu-
lating lever should be moved slightly toward the letter “S”; and if
consistently slow, the lever should be moved toward the letter “F,”
care being taken not to move the lever so far as to introduce an error
in the opposite direction. A movement of the regulating lever of 1
division usually changes the rate about 3 minutes per day. If the
loss or gain in any 1 day is less than 3 minutes, it is, in general, inad-
visable to move the regulator unless there has been a similar loss or
gain on a number of consecutive days. No refined regulation is nec-
essary for the motor clock. Normally this runs at a rate which moves
the paper forward 1 inch an hour, but any small variation from this
rate is unimportant.

24. A spindle operated by the motor clock extends through the back
of the clock case and has secured to its extremity a toothed carrier ©
wheel (35, fig. 6), also known as the clutch wheel, which actuates
the main roller of the gage when engagement is made through a
hinged carrier or clutch arm (34) attached to the roller. Projecting

MANUAL OF TIDE OBSERVATIONS 15

from the time clock through the back of the clock case is a spindle
carrying a short arm (383, fig. 9) called the striker lifter. This is
actuated by a cam in the clock and operates the hour-marking device.

25. Rollers.—There are three rollers on the gage, which are desig-
nated as the supply roller, the main roller, and the receiving roller.
The supply roller (4, fig. 4) is a solid rod on which the blank roll of
paper is placed. When installing a new roll of paper, this rod may
be readily removed from the gage and passed through the hole in
the center of the blank roll. When on the gage it is held in place by
guide springs (5) at each end. These springs also press against the
ends of the roll of paper to keep it from unwinding too fast and -
thus hold the paper taut as it is fed over the main roller. The pressure
exerted by these springs may be regulated by slightly bending them.

26. The main roller (14, fig. 5) is a hollow cylinder 1314 inches
long and 12 inches in circumference. Near each end of the cylinder
sharp steel pins are set at 1-inch intervals to keep the record paper
from slipping. Attached to the axis of the cylinder at one end there
is a hinged carrier arm (34, fig. 6) which engages the carrier wheel .
geared to the motor clock. Through this connection the main roller
is rotated at the approximate rate of one turn in 12 hours, thus feeding
the paper forward at the rate of 1 inch per hour.

27. The receiving roller (17, fig. 5), which is designed to receive
the completed record, consists of a solid core with one side flattened
and an outer shell in which a slit runs the entire length. With the
slit opposite the flattened side of the core, the end of the paper is
inserted and then secured in place by a slight turn of the shell. The
roller is held in place in its bearings by two pins which may be
released by pressure on buttons (18) near the ends of the roller. At
one end of the roller there is a small drum (16) known as the fenszon
weight drum, upon which is wound a cord attached to a weight which
serves to wind up the record paper on the receiving roller. The drum
is arranged so that it can be turned independently of the receiving
roller when winding up the weight. In some of the instruments this
is accomplished by a pawl and ratchet, while in other instruments the
loosening of a lock nut enables the drum to be moved aside, thus
disengaging it from the roller itself.

28. Record paper.—The paper for the standard automatic tide
gage is furnished in rolls about 13 inches wide and containing ap-
proximately 66 feet in length, which is sufficient for 1 month of
record. ‘The paper is plain without any ruling. After the tide curve
has been traced upon the tide roll the record is called a marigram.

29. Pencil screw.—The pencil screw (6, fig. 4) is a rod about
5~ inch in diameter with a square screw thread with a 1-inch pitch,
except that for a very large range of tide a screw with a 14-inch pitch
is used. The pencil screw is rotated through the action of the float
as the tide rises and falls, and in turn actuates the pencil arm causing
a pencil to trace the record. The threads at each end of the pencil
screw are turned down to prevent the pencil arm from jamming on
reaching the extreme limits of the rod, and springs (7, figs. 7 and 9)
are provided at each end to return the arm to the threaded portion of
the screw as soon as the tide reverses. In the latest type of instru-
ment the pencil screw may be removed from the gage for cleaning
without disturbing the wiring to float or counterpoise. At one end

16 U. S. COAST AND GEODETIC SURVEY

it is connected with the drum shaft by a slotted joint and at the
other end is held in place by a capstan bearing pin (45, fig. 9). By
backing off this bearing pin the pencil screw: is easily released so that
it may be lifted out.

30. Recording pencil.—The recording pencil (37, fig. 7) traces the
tide curve. This pencil is secured in its holder (38) by a clamping
screw (39). The gage is provided with a special automatic pencil
but any ordinary pencil with a medium soft lead can be used for
the purpose. The holder is secured to the pencil arm (41) by two
pivot screws (44) which permit a small lateral movement for striking
the hour marks. One pivot screw is clamped by a lock screw (46).
In the bearing of the pencil arm there is a pin screw (48) which fits
into the thread of the pencil screw, and as the latter is rotated
through the action of the tide, the pencil arm is moved toward the.
clock unit or away from the same according to whether the tide is
rising or falling.

31. Datum pencil.—The datum pencil (24, fig. 7), which draws the
datum line from which the record is scaled, is similar to the recording
pencil. Its holder is secured to the rod (20) by aclamp (21). This .
clamp consists of a split block held together by two screws. One of
the screws is covered by the spring attached to the holder and must
be tightened before the spring is placed in position. The other screw
is secured by the clamping nut (22) after the holder has been ad
justed to the position desired.

32. Hour-marking device.—The hour-marking device is actuated
by the time clock. -A cam attached to the main shaft of the clock
turns with the minute hand and operates a lever which is connected
with a small arm (83, fig. 9) projecting from the back of the clock
case. This arm presses against a spring (31) attached to the striker
weight (29, fig. 8). The latter is secured by a binding screw (30)
to a rod that actuates the tripping rod (26). Beginning 30 minutes
before the time for striking the hour, the cam in the clock gradually
swings the small arm upward, raising the striker weight and moving
the tripping rod in toward the recording pencil. On the exact hour
the cam suddenly releases this arm, thus causing the weight to fall
and the rod to move suddenly outward. The latter strikes the end of
the hour-tripping hook (36) attached to the pencil holder, causing
the pencil to make a short hour mark parallel to the edge of the
paper. Through the action of the pencil weight (42) the pencil is
then immediately returned to its original position.

33. Float and counterpoise drums.—The float drum (11, fig. 5),
which operates the pencil screw, is threaded to accommodate the
wire to which the float is attached. The counterpoise drum (10) is
similarly threaded for the wire to the counterpoise. A small hole
or a clamp near one edge of each drum affords a means for attaching
the wire. The drums now in use are either 11% or 134 inches wide.
As the threads are cut 16 turns to the inch, the narrower drums
will accommodate 18 turns of wire and the larger ones 28 turns.
To provide suitable recording scales for different tidal ranges, inter-
changeable float drums of different sizes may be used. Those now
available have circumferences of 6, 9, 12, 16, and 24 inches. The
counterpoise drum is 6 inches in circumference for all scales. The
oa eat the two drums is so arranged that one winds as the other
unwinds.

MANUAL OF TIDE OBSERVATIONS 17

34, In the old type of gage, the counterpoise drum was mounted
directly on one end of the pencil screw and the float drum was then
clamped to the counterpoise drum. In the newer instruments with
removable pencil screw, the two drums are mounted independently
on a short rod, known as the drum shaft, which turns in ball bearings
at each end and is connected with the pencil screw by a slotted
joint. Around the middle of the drum shaft is a flange which sep-
arates the two drums and contains a pin projecting from both sides
which is designed to engage one of a series of holes in the end of
each drum. The drums are held against this flange by capstan
lock nuts (12, 18, fig. 5), which may be loosened independently to
release either drum so that it may be turned when installing a new
wire. In the normal operation of the gage these lock nuts are not
to be disturbed since any change in the position of the float drum on
its shaft will affect the adjustment of the gage.

35. Wire.—The float and counterpoise drums are designed for use
with No. 23 American gage wire which is 0.024 inch in diameter, and
wire of this size must be used to preserve the correct scale in the opera-
tion of the gage. Formerly a single-strand phosphor-bronze or
nickel-chromium wire was used for suspending the float and counter-
‘poise weight, but there is now available a seven-strand stainless steel
wire containing 18 percent chromium and 8 percent nickel, which is
more satisfactory for the purpose.

36. Float.—The standard float now used for the automatic tide gage
has a cylindrical section 81% inches in diameter and 2 inches high with
tapering top and bottom sections. Its weight is 414 pounds. Assum-
ing the weight of sea water, fresh water, and kerosene to be, respec-
tively, 64 pounds, 62.4 pounds, and 55 pounds per cubic foot, the
corresponding buoyancy per inch of immersion of the cylindrical sec-
tion of the float is 2.10 pounds, 2.05 pounds, and 1.81 pounds, respec-
tively. In the normal operation of the gage with a strain of from 12
to 16 ounces on the float wire, the float will be approximately one-half
yas hate As a free float the immersion will be about one-half inch

eeper.

37. Counterpoise.—The counterpoise acts upon the counterpoise
drum to take up the slack in the float wire as the tide rises. Although
it may be attached directly to the end of the counterpoise wire, it is
preferable to have it act through a movable pulley with the end of the
wire fastened to the ceiling of the tide house as this arrangement
increases the operating limits of the gage when the height of the ceil-
ing is otherwise insufficient to provide for the full range of tide. The
weight of the counterpoise together with the operating scale of the
gage determines the amount of tension on the float wire. Asa tension
of 12 to 16 ounces has been found to be generally the most satisfac-
tory, the weight should be selected accordingly. The weights recom-
mended for use with different size float drums will be found in
paragraph 89.

38. Tension weight.—This weight, acting upon the drum at one
end of the receiving roller, serves to wind up the paper on which the
record is traced, and, by keeping a tension on the paper, also assists the
~ motor clock in turning the main cylinder. A silk fish line is generally
used to connect the weight with the drum. This is called the tension
cord. ‘The weight may be either attached directly to the end of the

cord or supported from a movable pulley with the end of the cord

18 '. JU. §. COAST AND GEODETIC SURVEY

attached to the ceiling of the tide house. The latter method is to be pre-
ferred as it increases the period during which the gage will operate
with a single winding up of the weight A weight of 1 pound is
recommended regardless of whether it is used with or without a mova-
ble pulley. When suspended from a movable pulley, the extra weight
of the latter offsets the reduced strain on the cord resulting from this
arrangement,

39. Scale of gage.—The height scale of the gage depends upon the
circumference of the float pulley and the pitch of the pencil screw.
The different scale combinations are shown in the table below:

Scale combinations

Float Range limit
drum cir- Pencil Seale of
cumfer- | screw pitch| gage
ence Curve Extreme
Inches Inch * Feet Feet Feet
6 1 1:6 6 9 14
9 1 1:9 9 13 21
12 1 i Ea P iy 18 28
16 1 1:16 16 24 Biff
24 1 1:24 24 36 56
16 ¥% 1:32 32 24 37
24 % 1:48 48 36 56
| \

In the above table the range limit of the curve shows the maximum
range that can be recorded by a continuous curve with the scale in-
dicated. Beyond this limit the pencil arm becomes disengaged from
the threads of the pencil screw and further rise and fall is registered
by a series of jogs near the margin of the paper. The extreme range
which can be recorded by these jogs depends upon the size of the
float drum. In the columns for the extreme range limit two values
are given for each scale. The first value is the limit when float and
counterpoise drums have a width of 114 inches, and the second value
when the width is 134 inches.

PORTABLE AUTOMATIC TIDE GAGE

40. The portable automatic tide gage was designed by this Bureau
primarily for use in obtaining short series of observations for the
reduction of soundings in hydrographic surveys. The aim was to
provide a gage more portable and more easily installed than the
standard gage. Besides being smaller, it differs in a number of ways
from the larger gage. It is equipped with a single roller and a single
clock movement, the latter being concealed inside the roller. A
counterpoise spring is substituted for counterpoise weight. It is
provided with a smaller float which operated in a convenient size
iron pipe which serves also as a support for the gage. An iron cover
protects the gage from the weather or molestation and eliminates the
necessity for any specially built shelter. The record is drawn on
sheets of cross-section paper which may be conveniently removed -
from the gage when desired. Although some degree of the precision
of the standard gage has been sacrificed to gain the conveniences of
the smaller gage, it is sufficiently accurate for the purpose for which

MANUAL OF TIDE OBSERVATIONS

19

it was designed, but its use es a long series Saat observations is not
meeemendcd

1.
2.
. Record cylinder.

. Clamping nut for

HE OO

Portable automatic tide gage (front).

Figure 11.

41. Names of parts.—Three views of the portable automatic tide
gage with parts numbered are shown in figures 11 to 18. Following
are the names corresponding to the numbered parts:

Figure 11
Base.
Float pipe section.

record
cylinder.

5. Keyhole for winding clock.
6. Stylus screw gear.
7. Stylus screw gear nut.
8. Idle gear.
9. Idle gear screw.
10. Idle gear lever nut.
a Float drum gear.
13.
14
15,
16

Float drum gear nut.

. Float.

. Cover locks.

. Cleaning tool.

. Intake coupling.

Figure 12

. Float drum. .

. Float drum axle screw.

. Float drum axle.

. Oil container.

. Stylus screw.

. Stylus resting bar.

. Stylus arm return springs.
. Paper holding clip.

. Height adjustment screw.

Figure 13

. Fair leader clamping screw.
. Float pipe anchor hooks.

Counterpoise spring pawl.

. Counterpoise spring ratchet.
. Counterpoise spring axle.

. Float drum cover plate.

. Serew covering oil hole.

. Stylus arm.

. Time adjustment screw.

. Stylus holder.

. Stylus.

. Stylus weight.

. Stylus arm bearing.

20 ‘U. S. COAST AND GEODETIC SURVEY

42. Record cylinder.—The record cylinder (3, fig. 11) on which
the paper for the record-is wound is 7 inches in length ‘and 19.2
inches in circumference. The cylinder is geared to a clock movement
carried within itself which causes it to rotate on an axle through its
center. The axle is clamped in its supports by a capstan nut (4, fig.
11) and the cylinder should be so placed that this nut is on the same
side of the instrument as the train of gear wheels. In this position
the cylinder rotates in such a direction that the top moves towards
the stylus screw. The cylinder is provided with a clip (24, fig. 12)
for holding the record paper in place.

43. Clock movement.—An 8-day clock movement is mounted inside
the record cylinder, its function being to rotate the cylinder at a
uniform rate, which is once in 48 hours. The circumference of the
cylinder moves forward 0.4 inch per hour, the time scale of the
record. Keyholes for winding and regulating are in the end of the
cylinder containing the clamping nut.

44, Stylus screw.—The stylus screw (21, fig. 12) is actuated by a
train of gears connecting with the float drum and operates the arm
that carries the recording stylus, moving this arm backward and
forward as the tide rises and falls. The screw is made of phosphor
bronze and has a square screw thread with a pitch of 0.4 inch. The
screw thread terminates in circular grooves at each end to prevent
the stylus arm from jamming when the limit of its movement is
reached, and springs (28, fig. 12) are provided to force the arm back
again on the thread when the tide reverses. A gear wheel (6, fig. 11)
is clamped to one end of the screw by a milled-head nut (7, ‘fig. 11)
and may be released when it is desired to reset the stylus.

45. Stylus arm .—The stylus arm (38, fig. 18), which carries the
recording stylus, has in its bearing a pin that fits into the thread of
the stylus screw and when the latter is turned with the rising and
falling of the tide the arm moves backward and forward along the
screw. The stylus arm carried a small weight (87, fig. 13) to over-
come the tendency to be thrown back on a rising tide. A stylus
holder (35, fig. 13) is pivoted to the arm. Two slow-motion screws
act upon this holder—one (34, fig. 13) is designed for a refined time
setting of the stylus, and the other (25, fig. 12) is designed to set the
stylus to an exact height reading.

46. Recording stylus.——The recording stylus (36, fig. 13) consists
of a pointed blade, designed to trace the record on a specially prepared
wax-coated paper. The stylus is so shaped that it will ride smoothly
over the clip that holds the paper in place. In some of the older
types of the gage, ordinary pencils and special chronograph pens have
been used, neither of which proved entirely satisfactory. Special
difficulties arose in the use of ink in the chronograph pen because of
the excessive dampness to which the record paper is often exposed at
the tide stations. _

47, Float drum.—The float drum (17, fig. 12) is 12 inches in cir-
cumference and about 1 inch wide with the face threaded to accom-
modate 30 turns of the float wire. The drum, together with its
oil-tight cover (31, fig. 18), forms a housing for the counterpoise
spring. The drum is rigidly fastened by means of a screw (18, fig.
12) to that part of its axle (19, fig. 12) connected with the gear wheel.

MANUAL OF TIDE OBSERVATIONS 21

The subsidiary axle (30, fig. 13) on the opposite side of the drum is
attached to one end of the counterpoise spring and does not turn with
the drum.

FIGURE 12.—Portable automatic tide gage (left side). i

48. Counterpoise spring.—The counterpoise spring enclosed in the
float wire drum operates against the weight of the float and takes up
the slack in the float wire as the tide rises. One end of the spring is
fastened to the inside of the drum and the other end is attached to
the subsidiary axle (80, fig. 13). The latter is held fixed by a ratchet

22 U. S. COAST AND GEODETIC SURVEY

and pawl (29, 28, fig. 13) during the ordinary operation of the gage
but may be turned by a clock key when it is desired to increase the
tension of the spring. The spring is about 18 feet long and is com-

te.
*

-

ee a ee le ee em ton te tee

Mh
~d

FIGURE 13.—Portable automatic tide gage (detail of stylus and float drum).

pletely wound by about 40 turns of the drum. The spring operates
in a bath of watch oil which is introduced through one of the screw -
holes in the cover plate (32, fig. 13).

49. To replace a broken spring proceed as follows: Loosen screw
(18, fig. 12) which holds the shaft connecting the float-wire drum with

MANUAL OF TIDE OBSERVATIONS 23

its driving gear, then take out the six screws holding the cover plate
(81, fig. 13) and remove it from the face of the drum. Slide the drum
away from the standard toward the gear train and remove the screw
which fastens the inner end of the spring to the fixed shaft. (This
screw is slotted in its shank instead of in its head.) Take out the
screw holding the other end of the spring in the drum and remove
broken spring. Now, put in the screw, which is slotted in its shank,
through the round hole in the inner end of the new spring and fasten
this end of the spring in place to the fixed shaft, wind the spring so
that it will fit into its recess in the drum and attach its outer end in
the drum case by means of the screw provided for that purpose, and
reassemble. About a teaspoonful of fine watch oil should be put in-
side the case through the charging hole, which is closed by means of
one of the six small screws (82, fig. 13) holding the cover plate in
place.

50. Fair leader.—This is a pulley mounted on a brass arm attached
to the bottom of the base of the instrument and extending down
inside the short section of float pipe. It is secured in place by a
clamping screw (26, fig. 13). Its purpose is to guide the float wire
from its drum to the center of the float well. .

51. Gears.—The float wire drum actuates the stylus screw through
a train of three gears (11, 8, 6, fig. 11). The two gears (11, 6) are
interchangeable with other gears furnished with the instrument to
obtain different scale ratios. The middle gear (8) is an idler used
for all scale ratios and provision is made for an adjustment of its
position to properly mesh with the other gears in use. Each gear has
the number of teeth stamped in the metal and the combinations to be
used are shown in the accompanying table:

Number of teeth

Maximum

Seale range of Gear Gear
tide attached tolattached to

float-drum| stylus

axle screw
1:11% 6 96 36
1:16% 9% 96 54
1:2214 12% 96 72
1:30 1 72 72
1:45 25 64 96

52. Seale of gage.—By changing the combination of gears as indi-
cated in the table above, five different height scales may be obtained
ranging from 1:1114 to 1:45. The maximum range of tide which
can be recorded as a continuous curve with the different scales is also
indicated in the table. “However, if an unexpected extreme high or
low water does carry the stylus to one end of the stylus screw beyond
the limit, evidence is usually left which will enable an experienced
tabulator to determine the approximate height reached by the tide.
The absolute limit of range which can be recorded by the present
gage, operating under usual conditions, is fixed by the length of wire

\

24 U. S. COAST AND GEODETIC SURVEY

which can be wound upon the float-wire drum, which is approxi-
mately 30 feet.

53. Record paper.—The record paper for the portable automatic
tide gage (fig. 22) consists of sheets with special cross-section ruling.
These sheets are 7 inches wide and 19.7 inches long allowing for a.
14-inch overlap, the ruled portion being 19.2 inches long to corre-
spond to the circumference of the record cylinder. The coordinate
lines ruled parallel to the short edge of the paper provide for the time
scale, and those parallel to the long edge provide for the height scale.

54. The time scale is uniformly 0.4 inch to the hour and the hour
lines are so spaced. The hour spaces are subdivided by lighter lines
into six equal parts to represent 10-minute intervals. The length of
each sheet is sufficient to include 48 hours which are numbered in two
sets from 0 (midnight) to 23 (11 p. m.). The height scale ruling
varies according to the scale with which the gage is to be operated as
indicated by the table on page 23, provisions being made for five
different scales. For the smallest scale, 1:45, the sheets are ruled for
feet and half-feet, but for all other scales the foot spaces are sub-
divided into five parts, each representing 0.2 foot. Printed on the
margin of each sheet is a note indicating the height scale of the paper
and the correct gears to be used with the same.

55. Originally the paper provided with the gage was finished with
ordinary sizing for use with pencil or ink, neither of which was
entirely satisfactory. The difficulty with the use of ink was due to
the large changes in humidity to which a tide station is\exposed.
Because of the effect of excessive moisture on ordinary record paper
it was found impossible to obtain an ink which would give satisfac-
tory results under all conditions. To overcome this difficulty there is
now used a wax-coated paper on which the record is traced by a
stylus which removes the wax coating leaving exposed a colored
paper beneath.

56. Float wire.—The wire used for this gage may be either phos-
phor-bronze or nickel-chromium. Size No. 28, American wire gage,
is required to fit the grooved thread on the float drum. This is a
little finer than that used for the standard tide gage.

57. Float.—The float (13, fig. 11) designed for use with the port-
able tide gage is a hollow brass cylinder 314 inches in diameter and 15
inches long. It is weighted with shot to float with the upper end
about 314 inches above the surface in sea water or about 14 inch above
the surface in kerosene. The float was especially designed for use
in a 314- or 4-inch float well. The gage may also be operated with a
larger diameter float in a larger well.

58. Float pipe.—There is furnished with the gage a short section
of 4-inch pipe 7 inches long (2, fig. 11). The upper end is machined
to fit into the socket in the base of gage and is secured to the latter
by two screw hooks (27, fig. 13) which engage in small rectangular
holes near the top end of the pipe. The lower end is threaded to
form a union with additional lengths of pipe required for the float
well. The gages were formerly designed for use with a 314-inch
pipe, and in order to adapt the earlier gages for use with a 4-inch
float well reducing couplings are necessary. ,

59. Intake coupling.—A conical intake coupling (16, fig. 11) is
furnished as one of the regular accessories to the portable tide gage.
This is installed with the apex of the cone downward. The cylindri-

MANUAL OF TIDE OBSERVATIONS 25
cal part is threaded inside to serve as a coupling between the float
well proper and a supporting section of pipe (fig. 14). In the apex of
the cone there is a threaded 1-inch hole in which may be fitted a bush-
ing with a smooth bore intake opening of the size desired. Bushings
with openings ¥% inch, % inch, and 34 inch are provided with the

gage. :

60. Cleaning tool.—A special tool. (15, fig.11) is furnished with
the portable gage for use in cleaning the intake to the float well.
This consists of a-cylindrical weight about 114 inches in diameter
and 5 inches long containing in the bottom a pointed shank about
1 inch in diameter and 2 inches long. At the top is an eye for the
attachment of a line.

PRIMARY TIDE STATION

61. A primary tide station is one that is maintained over a period
of several years to obtain a continuous record of the tide in any
locality. As the records from such a station constitute basic tidal
data for present and future use, it is very important that the instal-
lation and maintenance of the station should be with the aim of
obtaining the highest degree of reliability and precision that is prac-
ticable. The essential equipment of a primary tide station includes
an automatic tide gage, float well, shelter, tide staff or equivalent
float-gage, and a system of bench marks.

LOCATION

62. Special care should be taken in the selection of a site for a
primary tide station. If possible, there should be a depth of not less
than 5 feet below the probable lowest tide. This is especially desir-
able in cold climates where kerosene is used in the float well to pre-
vent freezing, and also in exposed locations where storm waves of
large amplitude are common. When the determination of the height
of mean sea level is an important aim, the station should be located
on the open coast or in a bay with ample access of sea water. A
river or bay connected with the sea by a relatively small inlet is not a
suitable location for this purpose because of the probable difference
in the mean water level inside of the inlet and on the outer coast.
A place separated by a bar from the main body of water should in
general be avoided less the records obtained will not be representa-
tive of the tidal conditions in the general area.

63. In selecting the site for a primary tide station it is very desir-
able that attention be given to the cleanliness of the surroundings.
Highly congested areas with a large amount of shipping should in
general be avoided. Because of the commercial need for space in
such areas, quarters obtainable for a tide station are usually cramped
and poorly lighted and ventilated. These conditions, together with
the presence of more or less refuse in the adjacent waters with accom-
panying offensive odors, tend to discourage the proper care of the
gage by the observer. Moreover, in such areas both the automatic
gage and tide staff are subject to dangers of disturbance from the
docking and loading of vessels. Especially, the tide staff may be
accidentally knocked out of place and resecured at a different eleva-
tion without any report of the incident. Heavy vehicular traffic
in such an area also renders the leveling between tide staff and bench
marks especially difficult.

26 U. S. COAST AND GEODETIC SURVEY

64. On the other hand, a tide station located some distance from a
congested area, where the tide is equally representative of the general
region, will be freer from these disadvantages and be no less accessible
to the tide observer. In general, a Federal, State, or municipal wharf
is preferable to a private one; but a pier -
maintained for public amusement and recre-
ation often affords an ideal location for a
primary tide station.

FLOAT WELL

65. The float well is a vertical tube or box
with an opening in the bottom designed to
admit the tide to a float operating the gage
while dampening out the larger waves
caused by winds. The float wells now gen-
erally used by this survey are constructed
either of iron or of wood. For exposed loca-
tions on the outer coast the iron float well
is used on account of its strength, but for
more quiet waters a float well con-
structed of wood and covered on the out-
side with sheet copper is more economical
in construction and installation and will
last many years. In cold climates which
require the use of kerosene in the well to

prevent freezing, the iron well has the ad-
oar pice vantage of retaining the kerosene without
leakage over a long period of time.
66. Iron float well.—The iron float well
(fig. 14) is usually made up of sections of
is sts stock pipe. The usual installation when the
sumeorringpiee Gepth is not too great consists of a support-
ing section below the intake and extending
several feet into the ground and containing
large openings in the sides to admit the tide
to the intake. Above the intake a sufficient
number of pipe sections are used to extend
the top several inches above the floor of the
tide house. A special intake coupling (fig.
15) forms a union between the supporting
section and the pipe above. If possible, the
: section immediately above the intake should
Figure 14.—Float well. be sufficiently long to extend above high
water in order that there may be no rough
joints for the float to catch on and to provide an oil-tight section for
kerosene when this is necessary to prevent freezing. Flange cou-
plings may be used for connecting pipe sections above high water.
While such joints are not as durable as sleeve couplings because of
the tendency for the connecting bolts to corrode, they add greatly to
the convenience of installation and the bolts when above high water
may be easily renewed from time to time.

67. For a primary tide station 12-inch pipe is recommended. When
a supporting section is used, this should contain six large openings,
each about 3 inches wide by 9 inches long and arranged in pairs on

Water surface

Sea bottom

MANUAL OF TIDE OBSERVATIONS 27.

opposite sides of the pipe and facing in different directions. The
center of the upper pair should be about 1 foot below the top of this
section of pipe and the others arranged with centers approximating
8 and 5 feet below the top of the section.

68. Wooden float well.—A wooden float well for use at a primary ~
tide station should usually be constructed of 2-inch by 14-inch planks
so assembled to form a box 12 inches square on the inside and long
enough to reach from the
floor of the wharf to sev-
eral feet below the lowest
tides. A sloping bottom
with intake in one of the
lower corners should be
provided, the correspond-
ing corner at the top being.
marked for identification,
preferably by beveling off
the corner. When the

depth of water is not too
great, two opposite sides of
the box may be extended
below the intake to rest as
a support upon the sea bot-
tom. The outside of the Ficurp 15.—Intake coupling for float well.
float well should be covered

with 16-ounce sheet copper from the bottom to a height above mean
high water as a protection against teredos and other marine animals,
copper nails being used in construction of the well as a precaution
against electrolytic action between nails and copper sheathing.

69. Intake.—The intake to the float well should be of sufficient size
to permit the free access of the tide while damping down the effect
of heavy seas. The opening must, however, be large enough so that
rough water on the outside will always leave an unmistakable trace

upon the tide record. In

aa protected localities an in-
take 114 inches in diameter

is recommended for a 12-

inch float well, but for

Fiegur® 16.—Cleaning tool for float well. places exposed to heavy seas
: fy es : an intake from 34 to 1 inch -
is sufficient. A single large intake is to be preferred to several small
ones, as the former is less likely to become clogged and is more easily
cleaned when clogged. An intake should be in the bottom of the float
well rather than in the side to facilitate cleaning. For an iron float
well a special conical intake coupling is provided, and when installed
this is placed with the apex downward. For a wooden float well
a square hole may be sawed in one corner of the bottom before as- -
sembling the parts. At the time of the installation of a primary
tide station facilities should be provided for the periodic cleaning
of the intake by the tide observer. (See pars.-132-135.)
70. Installation of float well.—Special care must be taken to secure
the float well in a vertical position in order that the float may rise
and fall freely without scraping the sides of the well. Where suffi-
cient depth is available, the well should be installed with the intake

28 U. S. COAST AND GEODETIC SURVEY

from 5 to 6 feet below the lowest probable tide. It is desirable, how-
ever, that there be sufficient depth below the intake to make it un-
likely that it will become covered by a shoaling of the water in the
vicinity. It is desirable that the top of the well extend several inches
above the floor of the tide house for convenience of access and also to
Jessen the chance of objects on the floor accidentally falling into the
well. For an iron float well, a flanged collar at the top may be ar-
ranged to rest upon iron plates secured to the floor of the wharf. The
well should be strongly braced in position. .

71. Precaution against freezing.—In cold climates kerosene is
generally used in float wells to prevent freezing. The column of
kerosene should be from 2 to 5 feet in height, depending upon the
severity of the winters. A 12-inch cylindrical pipe will require about
6 gallons of kerosene for each foot of height and a 12-inch square
float well about 714 gallons for each foot of height. The amount of
kerosene which can be retained in the float well will be limited by the
depth of the intake below the lowest tide. When an iron float well
is used and the intake is sufficiently low, the kerosene introduced at
the time of installation may remain for many years, the loss from
evaporation being small. In a wooden float well there is more or less
leakage and the supply of kerosene must_be renewed at frequent
intervals.

72. As the specific gravity of kerosene is less than that of water,
the surface of the kerosene in the float well will stand above the
water level outside, the difference in level being approximately equal
to one-eighth the entire height of the column of kerosene. Note
should therefore be entered in the record whenever kerosene is intro-
duced into the well and a comparative staff reading taken before and
after this is done. In the tabulation of the automatic tide gage
record, the effect of this height difference is eliminated in the com-
putations involving the comparative staff readings. Jt zs very im-
portant, however, that no kerosene shall be used in a float well
designed. for a tape gage or other nonregistering float gage the read-
ings from which are to be taken directly, as errors of indeterminate
amounts may thus be introduced.

TIDE HOUSE

73. When sufficient space is available, the tide house should be ap-
proximately 6 feet square and from 7 to 8 feet high. A house of this
size provides room for the tide observer to move around the gage and
give it such attention as may be required. The house should be well
lighted by windows, neat in appearance, and painted to conform
with other buildings in the immediate vicinity. A standard sign
with the following inscription will be provided upon requisition :

U. S. DEPARTMENT OF COMMERCE
COAST AND GEODETIC SURVEY
TIDE STATION

, 74. The tide house will in general be placed over the top of the float
well so that the wire from the gage may be led directly to the auto-
matic tide gage. When this arrangement is impracticable, the float
wire may be led from the float well over a system of pulleys and
through a suitable conduit to the gage in the tide house. When this

‘

MANUAL OF TIDE OBSERVATIONS 29

is necessary, facilities should be provided for the convenient replace-
ment of a broken float wire by the tide observer. The top of the
float well must be provided with a suitable cover as a precaution
against the accidental dropping of anything into it.

75. A table or shelf must be provided for the support of the tide
gage. A shelf can be conveniently constructed by setting the ship-
ping box of the tide gage on one end and then laying boards from
this to a cleat nailed to the wall of the tide house. The box and
boards are secured by screws which may be easily removed when it
is necessary to dismantle the gage. By hinging the cover of the
shipping box a convenient cupboard is formed for the storage of extra

tide rolls and other small articles.

INSTALLATION OF TIDE STAFF

_ %6. Each primary tide station must be equipped with a tide staff
or equivalent tape gage to provide a temporary datum and reference
scale for the automatic gage record. The tide staff, which is de-
scribed on page 2, is to be used in preference to a tape gage when
practicable. In selecting the location for the tide staff consideration
should be given to convenience in taking accurate readings and also
in leveling between staff and bench marks. The staff must be reason-
ably near the automatic gage to avoid any long delay between the
reading of the staff and the recording of the reading on the auto-
matic gage record, and should be placed so that the graduations are
clearly visible to the tide observer from an easily accessible position.

77. Special attention should be given to placing the tide staff so
that its elevation may be conveniently checked by levels to bench
marks. There should be sufficient clearance above the staff to hold
the leveling rod in a vertical position. Sometimes a fixed tide staff
can be so located that it can be sighted upon directly from a leveling
instrument set up on the shore; but when this is possible only during
low water, other arrangements should be made in order that a level-
ing party may not be unduly delayed by an unfavorable stage of the
tide.

78. When installing a tide staff care must be taken to secure it in
a vertical position. The piling of a wharf often offers « convenient
support for a tide staff, but if an inclined pile is used for the purpose
offsets must be provided to keep the staff vertical. At the time of the
original installation, the exact elevation of the staff zero is usually
unimportant and it is the general aim to place the zero sufficiently
low to avoid frequent negative readings. However, if it is desired to
set the zero at some previously determined datum, such as mean low
water, the staff graduations must extend negatively below the zero.
After installation it is very important that the zero be maintained at
a fixed elevation, and any known or suspected change should be
reported to the office.

INSTALLATION OF TAPE GAGE

79. In exposed localities where rough water renders the use of a
tide staff impracticable, a nonregistering float gage may be substi-
tuted for the staff. The most satisfactory gage of this type now
in use by this office is a tape gage (fig. 2) with a graduated steel

735445 O - 47-3

30 U. S. COAST AND GEODETIC SURVEY

tape attached to an 814-inch float operating in a 12-inch float well.
The reading mark should be a well-defined line on a board just
back of the tape and from 41% to 5 feet above the floor for convenient
reading by the observer. This reading line may be engraved on a
metal plate screwed fast to a board or it may be defined by the
slot in the head of a brass screw placed in the board close to the
tape. The counterpoise of the tape gage should be attached to a
movable pulley to increase the range of operation otherwise limited
by the height of the tide house ceiling. ;

80. To facilitate the determination of the plane of flotation and
also to provide greater durability, it is recommended that the bottom
portion of the tape which remains below the wharf floor during all
stages of the tide be a detachable piece of phosphor-bronze tape.
The work of determining the plane of flotation will be further
facilitated by installing a secondary reading board about 18 inches
above the level of the floor with a horizontal line from the tape to
a distance a little greater than the radius of the float.

81. The float well for the tape gage may be similar to the one used
for the automatic gage (page 26). ‘The size of the intake should not
be less than that indicated for the automatic gage and must be large
enough to show a perceptible motion in the tape when the outside
water is moderately rough. When installing the float special care
must be taken to arrange the supporting pulley so that the float will
swing clear of the sides of the well at all stages of the tide.

82. Determination of plane of flotation.—To establish directly the
relation of the datum of the tape gage to bench marks it is necessary
to determine the plane of flotation of the tape gage float under nor-
mal operating conditions. This is accomplished by placing the float
in a pan or bucket of water of the same density as that in the float
well and measuring the distance from the water surface to some
graduation of the tape. The pan is placed over the top of the float
well and the float must be connected with its counterpoise. . In order
that the counterpoise may swing clear of the floor, surplus tape
may be folded into several loops, avoiding sharp bends, and tied
together with a cord. If the tape contains a detachable portion,
this may be removed and placed in a coil on top of the float in order
that its weight may be included in the determination of the plane
of flotation. The length of the removed section must afterwards be
taken into account in obtaining the relation of the plane to the
scale graduations.

83. The float must be moved slowly up and down and allowed to
come to rest from both the upward and downward movement to
ascertain any difference in tape reading due to friction in the sup-
porting pulleys. If any difference is noted the float should be
placed with the mean of the two tape readings at the reading mark.
Careful measurements should now be taken of the vertical distance
from the water surface in the pan to some graduation on the tape,
which requires that the line of the tape graduation be extended
horizontally over the float to a point above the water surface. This
may be conveniently done:by having a horizontal line drawn on the
reading board or on a similar board arranged closer to the floor of

MANUAL OF TIDE OBSERVATIONS 3l

the tide house for the purpose. The measured distance added to the
tape graduation to which the measurement was made, together with
an allowance for any detached section of tape, will give the plane
of flotation as referred to the tape graduations extended. It is also
the distance which the tape gage datum is below the reading mark
of the gage. The result, however, is subject to a small correction
described in the following paragraph.

84. Correction to plane of flotation.—The necessity for this cor-
rection arises from the fact that the shifting of the tape from one
side of the supporting pulley to the other may make a slight differ-
ence in the plane of flotation. Although the variation is too small
to be of material importance in taking readings during the normal
operation of the gage, it is desirable that in a careful determination
of the plane of flotation under conditions differing from the normal,
a-correction shall be applied to reduce the measurement to a mean
sea level reading. The correction will depend upon the weight of
the tape, the diameter of the float, and whether the counterpoise is
attached directly to the end of the tape or supported by a movable
pulley.

85. In order to make this reduction, the tape reading taken at the
reading mark at the time the measurements are made should be
noted. In taking this reading it is assumed that the looped portion
of the tape is below the reading mark.

Let &’=tape reading at time of measurement.
& =tape reading corresponding to mean sea level.
D =diameter of float in inches.
t =weight of tape per linear foot.
W=weight of a cubic foot of water at tide station.
dD
The cross section of the float is 376 square feet and the buoy-
ancy due to an immersion of 1 foot of a cylinder of the same diameter
2
is SET pounds. Therefore, when the float is operating under

normal conditions, an application of a force of 1 pound would change

576
the plane of flotation by 7777 feet. When the apparatus is ar-

ranged as in figure 2 with the counterpoise supported by a movable
pulley, the shifting of 1 linear foot of tape from one side of the
supporting pulley to the other side will cause a change of 114 ¢
pounds in the pull on the float and the plane of flotation will be

864t
changed by 7 y7p2 foot. The total correction to be applied to the
plane of flotation as obtained by direct measurement, in order to

864 ¢ -
reduce to the mean sea level value, is Wp: (&—#’) foot.

86. Taking the average weight of sea water as 64 pounds per cubic
foot, the weight of steel and phosphor-bronze tape, respectively, as
0.0071 pound and 0.011 pound per linear foot, and assuming the
diameter of the float to be either 814 or 314 inches, there have been

74 U. S. COAST AND GEODETIC SURVEY

computed the following factors which when applied to the difference
(#—f’) will give the necessary correction to the plane of flotation:

8%-inch | 3%-inch
fl

oat float
Steel tape with counterpoise on movable pulley_____--_-- 0. 00042 0. 0029
Steel tape with counterpoise on end of tape________-_--- . 00056 . 0039
Phosphor-bronze tape with counterpoise on movable
Oey ire Se, Sh a ee 2 a oes oe a . 00065 . 0045
Phosphor-bronze tape with counterpoise on end of tape_- . 00087 . 0060

As the correction varies directly as the weight of the tape and
inversely as the square of the diameter of the float, corresponding
corrections for floats of other sizes or for tapes of different weight
may be readily derived from the above values. For example, the
correcting factor for a 914-inch float with steel tape and movable
pulley is

0.00042 x (814)2/(914)2 = 0.00084.

INSTALLATION OF STANDARD AUTOMATIC TIDE GAGE

87. Preparatory to the installation of the tide gage, the instrument
should be carefully checked to see that all parts are in satisfactory
working order. The scale of the gage and the corresponding float
drum will depend upon the extreme range of tide at the locality and
will be selected in accordance with the table in paragraph 39. Spe-
cial attention must be given to the pencil screw to see that the pencil
arm moves freely along its entire length and that the arm is properly
released on reaching each end of the screw thread and automatically
returned to the thread through the action of the return springs when
the direction of rotation is reversed. In the latest type of gage, the
pencil screw may be removed from the gage and rotated git hand
while the pencil arm is hanging freely by its own weight. Any
tendency for the arm to swing upward with the rotation of the screw
should be investigated and steps taken to remove the cause of any
sticking until all perceptible resistance to a free movement of the
arm along the screw has been eliminated. Special attention should
also be given to the hour-marking device to see that it is functioning
satisfactorily and such adjustments are made as may be necessary.
(See pars. 127-128.)

88. Setting up automatic tide gage.—The standard installation,
in which both counterpoise and tension weights are supported by
movable pulleys, is illustrated in figure 17. When practicable, the
gage is to be placed on a table or shelf over the float well so that the
float may be suspended directly from its drum without any interven-
ing pulleys. By means of a plumb line or the temporary installation
of the float, the position of the gage should then be adjusted to
center the float in the well. In doing this, consideration must be
given to the possibility that the well may not be exactly plumb and
ey ae should be done at the water level rather than at the top
of the well.

MANUAL OF TIDE OBSERVATIONS

f \
MO DLLALLLLLLLLLLALALA ALLA ALLELE LAALELLLLLLLS EL =
> P a

nO Gy)

a

A
N vv"

Wicur® 17.—Schematic view of installation of standard automatic tide gage.

_33

34 U. S. COAST AND GEODETIC SURVEY

89. Installation of pulleys.—The standard pulley now used for
the counterpoise wire and tension weight cord are 4 inches in diam-
eter and weigh 1 pound each. For the arrangement illustrated in
figure 17, six such pulleys are required, two being used as movable
pulleys attached to the weights and the other four as fixed pulleys
which are secured to the ceiling of the tide house by screw eyes.
Under operating conditions the fixed pulleys do not hang vertically
but are deflected by an amount depending upon the weight of the
pulley itself and the strain on the wire or cord. In order that the
wire may pass vertically upward from the gage and also to provide
for parallelism in the strands supporting the movable pulley and
weight, it is necessary to make allowances for the deflection of
the fixed pulleys when securing them in position. Computations
depending upon the counterpoise weights used with different float
drums have been made and the results are given in the table below.
In these computations the distance from the center of the pulley sheave
to the end of the supporting hook is taken as 4 inches and a small
allowance has been made for the depth of the groove in the sheave.

Float drum 6-inch 9-inch 12-inch 16-inch 24-inch
Counterpoise weight__________------_- 1 oon, Ss 2 Dee S1b2 c=) ee “1b Ae 6 lb.
Counterpoise with pulley ____--_------ Zip ice eee 3 tb: 2 yoeses})4 Ibs =~ =|) b bse ee
Counterpoise wire tension_______-___- Vol UO terete anata. 1} Ib_____- PN | ee, ae 2% lb. ___.-| 3% lb.
Mloat wire tension 2 es SP 1 | oa | 9 a Uo ee Pa a0) ee = 14 oz.
Offset pulley<A 22 Be a ee +1¢ inch__-| —}4inch_.-| —%4 inch___| —3 inch___| —¥% inch.
Offset pulley-B 2a ees eee 4 inches____| 334 inches__| 344 inches -_| 336 inches__| 344 inches.

The offset of pulley-A for the counterpoise wire indicates the position
of the screw eye for the support of the fixed pulley immediately above
the gage as measured from a point in the ceiling in a vertical tangent
to the counterpoise drum on the side from which the wire is led. The
measurement is to be made in the direction toward which the wire
leads if preceded by the plus sign and in the reverse direction if
preceded by a minus sign. The offset of pulley-B indicates the
position of the screw eye for the second pulley as measured from the
point of attachment of the fixed end of the wire, the latter being
in line with the two fixed pulleys. Measurements for the installation
of the pulleys for the tension weight cord are made in a similar manner,
the offsets being those corresponding to the 6-inch float drum, provided
that a 1-pound tension weight is used. In this case the first offset
is measured from a point vertically above the tension weight drum.

90. In order that the gage may e operated to its full capacity in
recording extreme ranges of tide, it should be the aim to have the
adjustment such that at approximate mid-extreme tide level the float
drum and counterpoise drum will each be half filled with wire, the
counterpoise half way between the limits of its motion, and the re-
cording pencil near the middle of the main roller. As the larger of.
the counterpoise drums contains 14 feet of wire when filled, a fall
of 7 feet for the counterpoise weight as supported by a movable pulley
will be sufficient for the maximum limits of the gage operation. These
limits would require a ceiling height of approximately 8 feet including
an allowance of 1 foot for the combined length of the counterpoise

MANUAL OF TIDE OBSERVATIONS 35

weight and supporting pulley. The extreme limit, however, will sel-
dom be required, and a ceiling height of 614 to 7 feet will in general
be ample for the operation of the gage. On the open coast the mid-
extreme tide may be taken as approximately the same as mean sea
level; but in rivers the height above mean river level reached by the
extreme high waters is usually much greater than the depth,to which
the extreme low waters fall. However, in estimating the mid-extreme
tide for the adjustment of the gage, no consideration need be given to
a high-water height that is above the bottom of the tide gage or to a
low water that is below the bottom of the float well, since beyond these
limits the gage would cease to function regardless of adjustment.
(See par. 119.)

91. Attaching counterpoise.—The weight of the counterpoise de-
pends upon the size of the float drum and is to be selected in accordance
with the table in paragraph 89. In the standard installation the
counterpoise is used with a movable pulley, but if it is attached directly
to the end of the counterpoise wire, its weight should be one-half the
tabular value given for the combined counterpoise and pulley. Before
installing the counterpoise, the pencil screw should be removed from
the gage (par. 29) and the float drum released (par. 34) so that this
drum and the counterpoise drum may be turned independently of each
other. For the older type of instrument which does not permit the
easy removal of the pencil screw, this screw should be rotated until
the pencil arm has cleared the screw thread at the end nearest the
clock case. Following the scheme of installation illustrated in figure
17, the necessary fixed pulleys being secured to the ceiling of the tide
house and the counterpoise fastened to a movable pulley, the wire
(par. 35) is passed through the fixed and movable pulleys and one end
attached to a screw eye provided for the purpose in the ceiling of the
tide house. The wire is then cut of such length that the counterpoise
will hang just clear of the floor as the free end of the wire is attached
to the counterpoise drum. A small hole or clamp near the edge of
the drum is provided for this attachment.

92. Attaching float.—The proper float drum for the range of tide
having been selected from the table in paragraph 39, the length of
wire to be wound upon this drum at the time of installat.on. will de-
pend upon the stage of the tide. Let Z equal length of wire to be
wound upon the drum at time of installation, C the length of wire
necessary to half fill the drum, and ZH the height of tide at time of
installation referred to the mid-extreme tide level, H being negative
4 ae surface is below this level, then L=C+A for the required
ength.

The value for C for drums 114 and 134 inches wide may be taken
from the following table:

Cireumference of float drum in
Santee : 6 9 12 16 24
Width of drum: Feet Feet Feet Feet Feet
Eo er SEE EE ip acs eer 4% 634 9 12 |} 18

A Se SO ee oo am sania 7 10% 14 18%] ° 28

36 U. S. COAST AND GEODETIC SURVEY

Unless previous tide observations have been taken in the locality the
value for H must be estimated. From best information available
ascertain the approximate heights reached by the highest and lowest
tides and then note the height of the water surface at the time of
installation as compared with, a plane midway between the estimated
highest and lowest, taking H as positive if the surface is above mid-
extreme level and negative if below this level.

93. Before installing the float it is a good plan to pass several loose
loops of wire through the supporting ring to aid in its recovery from
the float well if the float wire becomes accidentally broken. Next,
taking a spool of wire, attach one end of wire to the float and lower
it into the well until it comes to rest on the surface of water or kero-
sene, Lead the wire to the float drum of the gage and, after measur-
ing off an additional distance equal to Z as determined from the
formula in the preceding paragraph, cut the wire from the spool and
attach the end of the wire to the float drum by passing it through a
small hole near the outer edge and knotting it.

94, If the gage is of the latest type, with removable pencil screw,
wind up the float wire by turning the float drum on its shaft until all
threads on the drum are filled or until the float has reached its upper
limit of motion. When doing this the counterpoise drum is left un-
disturbed with the counterpoise hanging close to the floor. Both drums
are then secured rigidly to the drum shaft by tightening the capstan
lock nuts on the two sides. The float is then allowed to descend
slowly until it rests upon the surface of the water in the well. As the
float descends it automatically winds up‘the counterpoise wire on its
drum. The pencil screw may now be replaced on the gage with the
pencil arm set to bring the record curve near the middle of the paper,
consideration being given to the present stage of the tide in its relation
to the mid-extreme tide.

94a. The following instructions apply only to the old type gage
without removable pencil screw. Having determined the proper
length of wire in accordance with paragraphs 92-93, remove the float
drum entirely from the gage and wind the float wire upon it, raising
the float as necessary, until all the threads on the surface of the drum
are filled or until the float has reached its upper limit of motion. Re-
place the drum on the gage and secure it with the two clamping nuts
provided for the purpose, but holding it carefully against the sudden
falling of the float. Next, holding the pencil arm to one side to keep
it from immediately engaging the threads of the pencil screw, allow
the float to descend slowly. With a gage equipped with a pencil
screw of 1-inch pitch, release the pencil arm after 214 turns if the
drum is 11% inches wide, or after 714 turns if the drum is 134 inches
-wide. With the pencil screw of 14-inch pitch, release the pencil arm
immediately for the drum 11% inches wide or after 114 turns for the
drum 134 inches wide. After the release of the pencil arm let the
float continue to descend slowly until it rests upon the water or kero-
sene in the well. The lowering of the float automatically winds up
the counterpoise wire and moves the pencil arm approximately to its
correct position. If any further adjustment of the position of the
pencil arm is necessary, it may be accomplished by loosening the
clamping nuts holding the float drum and turning the pencil screw

MANUAL OF TIDE OBSERVATIONS 37

as desired. In doing this, care must be taken to hold the counterpoise
drum against the pull of the counterpoise weight and also to press the
float wire against its drum to keep it from springing off as the wire
slackens.

95. Attaching tension weight.—In general it is best to have the
tension weight operate from a movable pulley (fig. 17) to reduce the
rate of fall. When thus suspended, the weight will descended on an
average about 1 foot a day but the rate will vary with the amount of
paper on the receiving roller. Place the weight with pulley attached
on the floor at the lowest limit of its motion. After installing the
necessary fixed pulleys in the ceiling of the tide house, take a spool of
cord and pass one end of the cord through the fixed pulleys down
through the movable pulley attached to the weight, and then secure
the end to a screw eye in the ceiling. Next unwind enough addi-
tional cord to reach to the tension drum on the gage and cut from the
spool. Pass the end of the cord through the small hole provided for
the purpose and secure by knotting. The cord is now ready to wind
upon the drum as soon as the record paper is installed.

96. Starting the gage.—The gage is now ready for the installation
of the paper (pars. 115-116), adjustment of pencils (pars. 118-119)
and entry of comparative note (pars. 120-126), these processes being
described in detail under “Operation of tide station.” Both springs
on each clock should be wound. If the care of the gage is to be turned
over to a new observer, he should be given such preliminary instruc-
tions as will aid him in interpreting the printed instructions in this
volume.

TIDAL BENCH MARKS

97. An essential part of the establishment of a tide station is the
installation of a system of bench marks to which the observed tides
may be ultimately referred. A bench mark may be defined as a defi-
nite point on a more or less permanent object used as a reference for
elevations. Bench marks established-in the vicinity of a tide station
for the purpose of preserving tidal planes determined from the ob-
servations are known as “tidal bench marks.” These bench marks
serve as the basis for elevations which are carried by leve.s to numer-
ous other bench marks established in various parts of the country.

98. Qualities.—The two principal qualities desired in bench marks
are permanency and certainty in identification, and these qualities
should be kept in mind when establishing new bench marks. In a
settled community substantial buildings afford excellent locations for
the establishment of bench marks. In an undeveloped country a
ledge of rocks or a mass of concrete partly buried will serve as a
suitable foundation. Except for temporary use only, bench marks
should not be located on hydrants, curbstones, trees, or any structure
especially liable to destruction or change in elevation.

99. Standard disks.—Certainty in identification can be best ob-
tained by the standard Coast and Geodetic Survey disk bench marks
(fig. 18) and these should always be used when possible. The disk,
which is made of copper alloy, is 314 inches in diameter and contains
the inscription “U. S. Coast and Geodetic Survey Bench Mark”
together with other information. It contains a shank for cementing

38 U. S. COAST AND GEODETIC SURVEY

in place. The number of the mark and the year of establishment
should be stamped on the disk. Duplications of numbers should be
avoided and a number previously assigned to another bench mark in
the same locality, whether destroyed or extant, should not be used for
a new mark. Sets of dies for stamping letters and figures can be ob-
tained upon requisition.

100. Bench marks on buildings.—In a town or city, the post office,
customhouse, city hall, schools, railroad stations, banks, and other

Ficurm 18.—Standard disk tidal bench mark.

substantial public and business buildings generally afford the best
- locations for bench marks. Buildings with foundations of question-
able stability should be avoided. Permission will usually be granted
for the installation of the standard disk bench mark on such build-
ings as may be selected when the use of the same is explained to the
authorities in control.

101. General permission for the establishment of bench marks on
Federal buildings is contained in the following authorization:

TREASURY DEPARTMENT,
Washington, November 11, 1914.
The honorable the SECRETARY OF COMMERCE,
Washington, D. C.

Siz: By direction of the Secretary, I have the honor to acknowledge the re-
ceipt of your communication of the 7th instant, requesting that permission be
granted to the officers of the Coast and Geodetic Survey to place on the Federal
buildings under the control of this department small inscribed metal tablets,
which are to be used as bench marks in connection with the system of leveling,
the custodians of the buildings to designate where the tablets are to be placed.

In reply you are advised that no objection will be interposed by this depart-
ment to the placing of the tablets on the various public buildings, as desired,
and this letter, or a copy thereof, upen its presentation to the custodian of a
Federal building, is to be considered by him as his authority for permitting the
placing of one of the tablets on the building in his custody.

Respectfully,
(Signed) B. R. Newton,
Assistant Secretary.

MANUAL OF TIDE OBSERVATIONS 39

102. When a suitable place can be found, it is in general desirable

to set the face of the bench mark disk horizontal for convenience in
holding the leveling rod, but there should be sufficient clearance
above the bench mark for the rod used in the first-order leveling,
which is approximately 11 feet long. Sometimes greater permanency
can be secured by placing the bench mark in the vertical wall of a
building. In this position the short horizontal line through the
center of disk becomes the bench mark and a graduated tape may
be substituted for the rod in making the leveling connections. <A
bench mark in a vertical wall should be several feet above the ground
for convenience in holding the tape used by the leveling party. Care
should be taken to set the disk with the reference line horizontal.
The bench mark is to be countersunk with its face flush with the
wall, and when set in a conspicuous place special care must be taken
to have the work neatly done so that the building will not be
defaced.
_ 103. Bench marks in rock.—Rocky outcrops and boulders in areas
not likely to be disturbed for many years make satisfactory locations
for bench marks. If a boulder is used the bottom should extend far
enough below the surface of the ground so that it will not be affected
by action of frost. The standard disk should be cemented in place
with its face horizontal-and countersunk.

104. Bench marks in concrete.—Where a suitable building or
natural rock is not available, the bench mark disk may be set in a
concrete monument built in the place desired. The monument should,
extend not less than 3 feet below the surface of the ground, and
in localities having severe winters the depth should be sufficient
to withstand frost action. In general the top will extend several
inches above the ground in order that it may be readily found when
needed. The base should have a cross section greater than the top

_and there must be no projecting edges which might provide leverage
points for frost attion. If the monument is in the form of a frustum
of a cone or pyramid, the sides should have a batter of not less than
1 inch to the foot. A convenient shape for making the concrete
monument is a series of square blocks from 8 to 12 inches thick, the
bottom block being about 18 inches square and successive blocks of
diminishing cross section with the top block about 12 inches square.

105. It is important in making the concrete that all materials be
clean and thoroughly mixed before adding water. It should not be
too wet and should be well tamped into the mold. When using
rough aggregate the proportion should be about 1:3:5, the upper
part of the mass to be of a richer mixture. When only sand and_
cement are obtainable the proportion of 1 part of cement to 3 parts
of sand should be used for the lower part of the mass and 1 part of
cement to 2 parts of sand for the upper part. To prevent rapid
drying of the concrete, its surface should be covered with dampened
paper or cloth or wet seaweed held in place with earth.

106. Number of bench marks.—To insure against loss of datum,
there should be a number of bench marks widely scattered but in
general within a radius of 1 mile from the tide station. For all
series of tide observations, no matter how short, there should be a
reference to at least three substantial bench marks. The minimum
requirement for a series of observations covering a year or more is

40 U. S. COAST AND GEODETIC SURVEY

five bench marks. For a primary tide station which is to be main-
tained over a long period of years ten substantial bench marks may
be considered as adequate.

107. Primary bench mark.—At each principal tide station there
is usually one bench mark, known as the “primary bench mark,”
which is selected for its stability and convenience of location and is
used in checking the elevation of the zero of the tide staff from time
to time, so that if any change takes place the proper allowance may
be made in the tabulations. The stability of the primary bench
mark is in turn checked by levels to a number of other bench marks
which are so located that their elevations will not be likely to be
changed by a common cause.

108. Basic bench mark.—The basic bench mark used by the Coast
and Geodetic Survey consists of one of the standard ie set in
top of a concrete monument which extends about 2 feet above the
ground. The monument contains the inscription “United States
Coast and Geodetic Survey Basic Bench Mark” on one side and
rests upon a reinforced concrete foundation with an enlarged base
extending a number of feet below the ground.

109. Descriptions.—F ull descriptions of all bench marks must be

carefully prepared and forwarded to the office with form 258, leveling
record—tide station. The descriptions should be clear and distinct
and sufficiently complete to enable the bench marks to be readily
recovered and identified. If a standard disk is used, the description
must include a definite statement whether the number of the mark,
the year of establishment, and its elevation have been stamped in
the metal. When a bench mark is located on a building, the street
and number should be given when possible, or the name of thé owner.
When not on a prominent structure a bench mark should be refer-
enced by distance and direction to several prominent objects. A
sketch which will aid in locating or identifying the bench mark is
desirable. 2 ! :
' 110. Leveling.—At the time of the establishment of a tide station
and at intervals thereafter the tide staff must be connected with the
bench marks by levels. If possible, all bench marks within a radius
of 1 mile of the tide station should be connected with the tide staff
at intervals not greater than 5 years, but it is desirable that the
staff be connected with not less than three bench marks including
the primary bench mark each year or at more frequent intervals, in
order that any change in the elevation of the tide staff may be de-
tected without unnecessary delay. It is also desirable that bench
marks of other organizations which may be in the vicinity should be
connected from time to time with the bench marks of this Survey.

111. The graduating lines on the tide staff now in use have a
width of about 0.01 foot. The middle of each line is to be taken as
the reference for heights on the staff. If a portable tide staff is used
at the station, the leveling rod should be held on the flat top of
the brass plate on top of the tide staff support, and the reading on
the tide staff to which this plate._corresponds must be entered in the
record. This staff reading is determined by the position of the
metal stop attached to the back of the staff. It is important, how-
ever, that the staff be placed in its support to see that this stop comes
in actual contact with the plate on the support without interference

MANUAL OF TIDE OBSERVATIONS 41

from any obstruction. When a tape gage is used at the station,
levels must be run from the reading mark of the gage to the bench
marks, the relation of this reading mark to the tape gage datum being
determined from the position of the plane of flotation (pars. 82-86).

112. Before beginning the leveling care must be taken to have the
instrument in adjustment and also to see that the bottom of the
_ leveling rod corresponds exactly with the zero of the graduations.
- The latter is important if some sights are to be taken directly on the
tide staff or on a graduated tape. Foresights and backsights should
be approximately equal. The leveling between the marks must be
checked by a forward and a backward line and the closing error in
feet must not exceed 0.085 HX, in which £ is the distance in statute
miles leveled between adjacent bench marks. For convenience of
use the following table is given:

Distance between bench! Maximum Distance between bench | Maximum

marks (feet) error allowed marks (feet) error allowed
Foot Foot
BOO OrAeSssai2= 2 Ss 2 Sk Os OLE aie 3:0002 oi ee eee 2 0. 027
OOO eee ae Ee a Dt ONG G (24, 000 22 ies so eee . 030

"2100 0) lc uel HO2TAl 5 O00L ore . 034

If the difference between the results from the forward and back-
ward lines between any two bench marks exceeds the allowable error,
both the forward and backward lines between the marks must be
repeated until an acceptable agreement is obtained. No one of the
questioned values is to be used with the new levels to obtain the
agreement. Form 258, leveling record—tide station, is to be used for

this leveling.
OPERATION OF TIDE STATION

113. The satisfactory functioning of a tide gage depends largely
upon the observer in charge, and he should be a person with some
knowledge of mechanics and a sufficient scientific training to under-
stand the importance of careful observations.

_ 114. Summary of observer’s duties.—Exclusive of work incident
to density and temperature observations and the tabulation of
records described elsewhere in this volume, the duties of the tide
observer at a primary tide station may be summarized as follows,
the details of the work being described in separate paragraphs:

Daily.—Inspect tide station, enter comparative time and staff note
(pars. 120-126) on tide roll and in form 660 (par. 136), correct gage
clocks if necessary (par. 123), readjust pencils for wearing away of
lead (par. 128), wind up tension weight, and enter in form 660 any
item of interest relating to the station (par. 136).

Semiweekly.—Wind both clocks. Although these are 8-day clocks,
semiweekly winding insures against stoppage that might result from
an unexpected interruption to the observer’s daily visits.

Weekly.— Mail form 660 to office after completing information
required on back of form.

Monthly.—Change paper on gage (par. 117), clean pencil screw
(par. 131), and forward records to office (pars. 137-139).

42 U. S. COAST AND GEODETIC SURVEY

Oceasionally.—As frequently as necessary clean float well intake
(pars. 132-135).

115. Placing paper on gage.—Before placing the paper on the
gage a note should be made at the beginning of the roll giving the
name of the station, the date, the scale of the gage, the kind of time
used, and the name of the person in charge. The supply roller (4,
fig. 4) is removed from the gage and passed through the central hole
in the blank roll of paper. The roller is then replaced in its supports -
so that the loose end of the paper will pass from below inward toward
the main roller (fig.10). The tension springs (5, fig. 4) at the sides are
moved down so that they will bear against the ends of the roll of paper.
These springs may be bent slightly if necessary to hold the roll firmly
in place and provide the desired tension in the paper as it is drawn
forward.

116. With the main roller disconnected from the motor clock by
throwing out of gear the carrier arm (34, fig. 6), the paper is now
passed over the main roller and the end inserted in the slot in the
receiving roller (17, fig. 5) where it is locked in place by turning the
core in the sleeve of the roller. The end of the paper should be cut
square before attaching it to the receiving roller and care must be
taken to secure it evenly so that the paper will wind up smoothly on
the roller. Several turns of the paper are now wound on by turning
the roller in a direction such that the paper passes from the main
roller over the top of the aed ye? roller. The main roller is then
again connected with the motor clock by throwing the carrier arm
into gear with the carrier wheel. Finally, the tension weight is wound
up and the paper is ready for the record.

117. Changing paper on gage.—On the first day of each calendar —
month, or the following day if the first occurs on Sunday, the roll of
paper on the gage should be changed. A comparative time and staff
note (pars. 120-126) must be entered on the old roll before removal
from the gage and another note entered on-the new roll immediately
after installation. Before removing the old roll, place the tension
weight upon some support to remove the strain on the paper and then
disconnect the main roller from the motor clock by throwing out of
gear the carrier arm (34, fig. 6). A few feet of the paper remaining
on the supply roller is wound upon the receiving roller and the paper
cut. The receiving roller is then removed from the gage after pressing
on the release buttons (18, fig. 5) at the sides. The paper itself is
removed from the roller after being released by turning the core
inside the shell of the roller. For the installation of a new roll of
paper see paragraphs 115-116.

118. Adjusting pencils.—For the original setting of the datum
pencil a position near the middle of the paper is in general recom-
mended. After having been once set, any change in its position should
be avoided without good cause, but when necessary an explanation
should be noted in the record. When securing the datum-pencil
holder in place care must be taken to provide sufficient clearance for
the passing of the recording-pencil arm, special attention being given
to the position of the tripping hook (36, fig. 7) just before the Ves
since at that time this hook is at its lowest position. If difficulty is
experienced in securing the datum-pencil holder in position by its
clamping nut (22, fig. 7), remove this nut together with the datum-

MANUAL OF TIDE OBSERVATIONS 43

pencil holder and tighten a small screw which is then exposed to view
(par. 31), and then reassemble the removed parts.

119. For the original setting of the recording pencil it is the aim
to make the adjustment so that as far as practicable extreme tides of
occasional occurrence will be recorded. For stations on the open
coast this adjustment will bring the curve of the normal tides in the
middle portion of the paper, but for stations on rivers the pencil
will be adjusted to bring the normal tide curve nearer the low-water
side of the paper to leave room above for recording the flood stages of
the river.. After the original set-up of the instrument changes in
adjustment are in general to be avoided as they necessitate certain
allowances in the
tabulations to pre-
serve a uniform
datum. When a
change of adjust-
ment is necessary
for any reason an
explanatory note
should be entered
in the record.

120. Comparative
note—When an
automatic tide gage
is originally in-
stalled, and daily
during its opera-
tion, a comparative
time and staff note
must be entered on figure 19.—Tide curve and comparative note (standard gage).
the tide roll with
the corresponding point on the tide curve clearly indicated (fig. 19).
This is absolutely essential in order to obtain the correct time rela-
tions and to establish tidal datum planes. :

121. The comparative note must contain the date, the correct time
as obtained from a reliable source, the corresponding time as indicated
by the time clock of the tide gage, and the reading taken directly
from an outside tide staff or from a nonregistering float gage which
may have been installed as a substitute for the tide staff. A statement
relative to the wind and weather and the name or initials of the ob-
server making the note should be added. The exact point-of the tide
curve to which the note applies may be conveniently indicated by
first tilting the recording pencil to make a short horizontal line similar

_to an hour mark and then drawing a tracer from the note to this line.

The point may also be indicated by a short vertical line made by
rocking the float drum, care being taken to hold the float wire so that
it will not spring off the drum.

122. A rubber stamp with suitable inscription is provided for
convenience in making the note. In entering the date it is desirable
to include the day of the week as a check on the day of the month.
Standard time should be used for the record consistently throughout
the year regardless of the fact that daylight saving time may have
been adopted temporarily for other purposes during certain months.

E4.m.

Tube 5:40 :5.25 ft.

5./-

“Correct time 7:/7@.mGauge time 9//5

Date Wad. May 30,1727

Staff 5.5 :

44 U. S. COAST AND GEODETIC SURVEY

If desired the times may ‘be expressed according to the 24-hour
system in which the hours are numbered consecutively throughout
the entire day thus avoiding the necessity of using the designations
“a.m.” and “p.m.” If the usual 12-hour system is used care must be
taken to indicate the forenoon and afternoon hours by their custom-
ary designations. .

123. At the beginning of the record on a tide roll the correct time
and the gage time should be in agreement. In subsequent notes the
difference between the correct time and the gage time will indicate
how much the time clock of the gage has gained or lost since the
previous note was entered. After each entry has been made the
time clock should be set to agree with the correct time. The minute
hand of the clock, however, must not be turned backward when be-
tween 10 minutes before and 5 minutes after the hour. (See pars.
22-23 for setting and regulating clock.)

124. Reading tide staff.—In taking the tide staff reading, both the
highest and lowest points reached by the waves are to be recorded,
the two readings being separated by a dash or in some other distinc-
tive manner. If a glass tube is used the height of the water in the
tube should also be recorded.

125. The observer will note that the tide staff is graduated in tenths
of feet and not inches. The heights in general are to be read to
the nearest tenth or half-tenth of a foot, the half-tenth being recorded
as 5 in the second decimal place. When the staff is equipped with
a glass tube (par. 7) the staff reading corresponding to the water
surface inside the tube should be read, but the observer must assure
himself that the opening in the bottom of the tube is not clogged,
and if there is any wave movement on the outside the water in the
tube should show a perceptible oscillation. _

126. While individual staff readings may at times seem rough and
inaccurate, the final results, depending upon-~an average of a great
number of such readings over a considerable period of time, reach a
very satisfactory degree of precision provided they are taken in an
unbiased manner. It is of great importance, therefore, that the tide
observer when taking staff readings should be uninfluenced by any
other consideration, and such readings should be entirely independent
of any scale on the automatic tide gage.

127. Adjustment of hour-marking device.—Small variations in
the hour-marking device on different instruments preclude detailed
instructions applicable to all gages now in use. In all cases, however,
care must be taken to avoid any binding between the tripping hook
(36, fig. 7) and the tripping rod (26, fig. 7). In the latest type gage
the tripping hook must just clear the rod during the first half of each
hour. During the last half of the hour the tripping rod gradually
recedes from the hook preparatory to striking the hour. When ad-
justments are necessary, they should, therefore, be made with the
minute hand in the first half of the hour.

128. Adjustments may be made at four different points and several
trials may be necessary before securing a satisfactory operation of
the device. One adjustment consists of changing the position of the
recording pencil in its holder, the pencil being secured in place by a
binding screw (39, fig. 7). A very slight change in the elevation
of the pencil and even a change resulting from the wearing away of

MANUAL OF TIDE OBSERVATIONS 45

the lead in the pencil may have a sensible effect on the hour-marking
device. A second adjustment consists of changing the angle which
the recording pencil makes with the paper by means of an adjusting
screw (40, fig. 7). A third adjustment changes the angle between the
striker weight (29, fig. 8) and the upright supporting the tripping
rod. In general this angle will not differ greatly from 90°. The
weight is held in position by a binding screw (80, fig. 8), but because
of the strain produced by the hourly dropping of the weight there
may be a slipping at times. The fourth adjustment relates to the
position of the striker lifter (33, fig. 9) attached to a spindle from the
time clock. This is secured to the spindle by a binding screw (22,
fig. 9) and during the first half of each hour its position should
be vertical. During the last half of each hour its position changes
through the action of a cam in the clock.

129. Cleaning pencil screw.—One of the most common sources of
trouble, which cannot be overemphasized, in the operation of the
automatic gage, is the sticking of the pencil arm on the pencil
screw. Even when the pencil screw itself appears clean, there may
be an accumulation of dirt in the bearing of the pencil arm. Also,
while the arm may operate freely in covering the normal range of
tide, there may’ be a sticking as it advances toward either end of
the screw for an unusually high or low water. The following oper-
ating difficulties frequently. have their origin in a sticking pencil
screw: (a) recording pencil thrown back off the paper on a falling
tide, (b) curve recorded on falling tide less legible than that of rising
tide, (¢) paper torn by pencil digging in on a rising tide, (d) dis-
tortion in tide curve, especially a flattening at time of high or low
water and steps on a rising or falling tide, (e) float wire broken
or off drum.

130. The pencil screw should be cleaned at least. once each month
and at other times when there is any evidence of sticking. The
latest type gage is provided with a removable pencil screw which
can be taken out of the gage without disturbing any of the wiring.
It is released by backing off the capstan bearing pin (45, fig. 9)
at the forward end of the screw. The other end of the screw is
connected with the drum shaft by a slotted joint. When replacing
the pencil screw in the gage, the pencil arm should be placed in the
same position as before with allowances for any change in the height
of the tide in the meantime. While out of the gage, the pencil
screw and the inside of bearing of the pencil arm are to be thoroughly
cleaned with kerosene or nonleaded gasoline and any roughness due
to corrosion removed by a fine file. The use of emery or crocus
cloth is to be avoided. Before replacing the pencil screw in the
gage, the movement of the pencil arm along the entire length of the
screw as well as the functioning of the return springs at the ends
should be checked in the manner described in paragraph 87. Lubri-
cating oil must not be used on the pencil screw or in the bearing of
the pencil arm, as the oil tends to collect dust from the atmosphere
and when dry forms a sticky film on the metal surfaces. The move-
ment of these parts during the operation of the gage is so slow that
the friction between the clean dry metal surfaces is practically
negligible.

735445 O- 47-4

46 U. S. COAST AND GEODETIC SURVEY

131. With the older type gage, a thorough cleaning of the pencil
screw cannot be accomplished without disconnecting the wiring, and
this is generally undesirable. However, at regular intervals the
screw must be wiped clean with a rag moistened with kerosene or
nonleaded gasoline, and the best time to do this is when changing
the paper after the close of each month. With care to avoid letting
the float wire spring off of the drum, the screw may be turned a
Jimited amount to move the pencil arm forward and backward dur- °
ing the cleaning process. A liberal application of kerosene or non-
leaded gasoline with a squirt can will be helpful in removing dirt
accumulated inside the pencil arm bearing.

132. Clearing float well intake.—Particular care must.be taken to
keep open the intake to the float well, as even a partial clogging may
destroy the value of the record by creating a lag in both time and
height of the tide. Clogging may be caused by an accumulation of
sediment inside the well, by marine. growth at the intake, or by the
shoaling of the water in the vicinity. In an iron float well clogging
may result from an accumulation of iron rust scales inside the pipe.
A clogging of the intake is indicated by a smooth tide curve when
the water outside the float well is somewhat rough. Although the
float well is designed to dampen down the outside waves, there should
always be an unmistakable oscillation of the recording pencil when-
ever the water is somewhat rough in the vicinity. A smooth, regu-
larly traced curve is to be regarded with suspicion and should occur
only when the water outside the float well is smooth.

133. In localities where there is any tendency toward clogging, the
observer should establish the practice of clearing the float well at
regular intervals. When clearing an iron float well with intake in
the center of the standard conical intake coupling, it will generally
be necessary first to remove the float from the well and, to avoid any
tangling of the float wire when raising the float, the wire should be
secured by a clamp or loop around some convenient object to take up
the slack. When the intake is not badly clogged it is most conven-
iently cleared by use of the cleaning tool shown in figure 16. This
tool is lowered into the well by a line and is used to tap out any
foreign matter which may have collected in the intake.

134. If there is a considerable accumulation of sand in the bottom
of the well it may be necessary to use a jointed rod made up of sec-
tions of iron pipe which may’be fastened together as it is lowered into
the well. One-half-inch pipe with outside diameter a little less than
% inch will generally be found satisfactory for the purpose when the
diameter of the intake is an inch or more. If the obstruction in the
intake is found to be especially difficult to remove, a drill of suitable
size soldered in the end of the lowest section of pipe will usually be
found effective when other means have failed.

135. For a wooden float well when the intake is in a lower corner
of a sloping bottom, the use of the jointed rod without removing the
float from the well will. probably be found both convenient and
effective in ordinary cases. When the intake is in the side of the
well rather than the bottom, its clearing will generally be more diffi-
cult and necessitate the use of a boat at low tide. When the clogging
occurs as a‘result of the shoaling of the water around the float well,
the clearing of the intake becomes a problem which cannot be han-

MANUAL OF TIDE OBSERVATIONS 4T

dled by the usual facilities of the observer and the circumstances
must be reported to the office.

136. Weekly report (form 660).—Tide observers are required to
forward to the office at the end of each week a report on form 660.
The daily notes should correspond to those entered on the marigram
itself, but the tube readings may be omitted in this form. [If staff
readings are taken more than once during the same calendar day,
a single set will be sufficient for the weekly report, but all such read-
ings are to be noted on the marigram. In the column of “Remarks”
there should be noted such items as “Float well cleaned,” “Float wire
broken,” “Counterpoise wire off drum,” “Time clock stopped at
11:15,” etc. The operating troubles are to be explained in detail on
the back of the form, which must also include all other information
requested. :

137. Forwarding records to office.—Unless the tide observer tabu-
lates the records, the tide roll is to be forwarded to the office imme-
diately after removal from the gage after the close of each calendar
month. If the observer does tabulate the records, the work must be
expedited and the records forwarded promptly as possible, as the office
has frequent calls for information based upon them. It is expected
that the tabulations will be completed and the records forwarded
within 1 week after the marigram has been removed from the gage.

138. Before forwarding the tide roll, it should be rewound to bring
the record on the inside with the first of the month at the beginning.
A label, form 489, is to be filled out as completely as possible, except
that at a primary tide station it will be unnecessary to repeat the
latitude and longitude each month. The marigrams are numbered
consecutively from the beginning of the series regardless of calendar
years. ‘The label is to be pasted on the outside of the marigram in
such a manner as not toseal the roll. As a protection against tearing,
a few inches of the paper at the beginning of the roll should be folded
inward, making a smooth edge of double thickness. The label is then
pasted parallel to this edge and about 1 inch from it, with the bottom
of the label towards the edge.

139. The roll should now be well wrapped for mailing and an)
addressed franking slip (form 110) pasted on the outside. Two cop-
ies of form 413 listing the record must be sent under separate cover.
This form is used only for the formal transmission of records, and
any matters requiring special attention should be sent as separate
communications.

140. Shipment of Government property.—Shipments of instru-
ments or other Government property when too bulky to be sent by
mail are sent by express or freight on Government bill of lading.
When it is necessary for the tide observer to make any such shipment
to the office he will be provided with a bill of lading for the purpose,
and no payment of charges, other than drayage which will usually be
arranged for in advance, should be made by the tide observer for
shipments sent or received by him. Form 412, in duplicate, is used as
a transmitting letter when shipping instruments to the office.

141. Requisition for supplies.—A requisition for stationery for use
at a tide station may be made by ordinary letter or by a short form
requisition letter mimeograph copies of which may be obtained from
the Chief, Division of Tides and Currents, upon request.- Requisitions

48 U. S. COAST AND GEODETIC SURVEY

for instruments and general property must be made on form 12. Req-
uisitions for stationery and instruments should always be made on
separate forms, as they are handled by different sections of the office.

142. Requisitions for supplies may be made from time to time as
needed, but it is recommended that at a primary tide station the
quantity of any article requested in a single order be sufficient for a
period of about 1 year. The following list is suggestive of the quan-
tity of any article to be included in a single requisition:

Tide rolis"for standard tide gagze_2 2": On a ee ee 12

Form 489 (label for tide gage record) --___________________-_-_-_ 20 —
Form 660 (weekly report of tide station) _______-_-______________________ 50
Form 457 (density and temperature observations)_______________________ 25
Form 413 (letter transmitting field records)________________________ 50
Form 110 (frank, mailing addressed to Director)_____-___________________ 20
Form 12 (requisition for instruments and general property)_______________ 10
Requests for stationery (short form) 22224 =) oho 20
Letter paper, for field, use, medium::.-2 —. == 23 58 eS tablet_. 1
Envelopes addressed to Director, 3% by 8% inches______________ packages__ 2
dinvelopes; -manila,.956 by 12 inches? oe 5 en See Se ee eee package__ 1
Peneilleads;’-Seripto: -bididk, SB Bes = 32222 5 a ee ee dole Ath
Cheesecloth for cleaning tide gage______________.._________________ yards__ 3

Tabulation forms (if observer tabulates his marigrams) :
Form. 138 (high andslow, waters) 22" + ee a ee
Rorm’ 362: (hourly heights) e222 os ee ee
Form.455 ‘(comparative readings) =-—---.2_ 2 2-2 eee

143. Emergency expenses.—Tide observers are not expected to
incur any expenses in the operation of the tide station unless espe-
cially authorized by the Director of the United States Coast and
Geodetic Survey. Unless an emergency exists, the tide observer
should inform the office of any needed repairs and then wait for in-
structions. Time will be saved if the tide observer obtains an esti-
mate of the cost of making the needed repairs and submits this when
informing the office of the need for the repairs. If the cost is more
than nominal, at least three estimates are to be obtained when.
possible.

144, In case of an emergency in which there would be a considerable
loss of record if the observer waited until receiving instructions from
the office, he may make immediate arrangements for having the work
done, provided the cost is reasonable and does not exceed $5. In
such cases arrangements will be made to have payment made directly
by: the office to the party doing the work, or the observer will be pro-
vided with suitable forms for obtaining necessary receipts.

145. When emergency work of considerable magnitude is necessary,
the tide observer should inform the office by telegram, which is to
be sent collect and not prepaid by the observer.

146. Furnishing information to public.—All employees of the
United States Coast and Geodetic Survey are expected to be cour-
teous to the public when inquiries are made concerning their work,
but the regulations prohibit the furnishing of copies of the records
without authority of the Director. When a tide station is so situated
that there may be more or less frequent calls by local authorities for
data from the tide-gage record, and this is brought to the attention
of the office, permission will usually be granted to the tide observer
to supply such information upon request, but when this is done the
party to whom the information is given should be informed that the
results are preliminary and subject to revision by the office.

RSK

MANUAL OF TIDE OBSERVATIONS 49

147. Operating difficulties.—It would be impossible to anticipate
all difficulties which might arise in the operation of the automatic tide
gage but the principal ones are listed below. The tide observer should
become familiar with different parts of his gage by reading the de-
scription of the instrument contained in this manual. All the gages
in use are not of the same identical pattern but the observer will note
any differences which may exist in the gage at his station.

148. Broken or tangled float wire——Observer visits station and
finds float wire off drum, tangled, and perhaps broken. Although
this may result from several causes, the most frequent one is a dirty
pencil screw which has jammed the gage and prevented the counter-
poise from taking up the slack in the float wire as the tide rose. The
wiring should be entirely removed from the gage and the pencil screw
examined and cleaned in accordance with paragraph 130. New wir-
ing may then be installed as described in paragraphs 91-94. It will
not be necessary to attempt to adjust the gage exactly as before, but
‘a note should be entered on the roll itself and also in the weekly re-
port stating that new wiring was installed, and a comparative staff
reading must be taken and recorded.

149. Other causes which might lead to a broken or tangled float
wire are improper instdllation of wiring, an obstruction in the path
of the counterpoise weight, jamming of a pulley through which one of
the wires passes, and an interference between the recording pencil and
the datum pencil. If the instructions for attaching counterpoise
and float are not strictly followed, it is possible that an excess of
wire attached to the float may run off the edge of the float drum at
the time of a specially high tide, thus causing a sudden jerk which
could break or tangle the wire. An obstruction in the path of the
counterpoise weight or the jamming of a pulley through which the
wire passes might prevent the weight from functioning and thus per-
mit the slackening of the float wire. A similar effect may be pro-
duced if the recording pencil fails to clear the datum pencil in pass-
ing because of an improper adjustment of the pencils. See paragraph
118.

150. Clock failure.—If either clock stops frequently or runs per-
sistently fast or slow regardless of all efforts to regulace it by the
customary method, a requisition should be made for a new clock unit,
and an effort should be made to keep the old clocks functioning while
waiting for the new. unit. The stopping of either clock may be caused
by winding too tight or by a collection of dirt in the works. The
latter condition may be temporarily remedied by the use of kerosene.
In some of the older types of gages, clock failure may result from a
flexure of the frame caused by drawing too tight the screws securing
the clock to its case. Any jamming of the supply roll of paper might
affect the running of the motor clock, and any unusual resistance in
the operation of the hour-marking device would have its effect on the
time clock.

151. Failure of hour-marking devices.—Assuming that this is not
the result of the stopping of the time clock, any failure will probably
be found to be due to an improper adjustment of the device. Instruc-
tions given in paragraphs 127-128 should be carefully followed.

152. Torn paper.—If the paper is found torn along the record
‘curve during a rising tide, it is probably the result of a dirty pencil

50 U. S. COAST AND GEODETIC SURVEY

screw which has forced the pencil arm downward and caused the
pencil point to dig into the paper. The softening of the paper by
damp weather may be a contributing factor. See paragraphs 129-131
for cleaning pencil screw. An improper adjustment of the hour-
marking device whereby the tripping rod presses too tightly against
the tripping hook may also result ina tear. A tear along the datum
line suggests too great a pressure, too hard a pencil, or a broken pen-
cil point. A tear along the line of prick points at the margin of the
paper may result from too heavy a tension weight or too strong a
resistance from the tension springs acting upon the supply roll of
paper. A torn margin may result from an improper alinement when
the tide roll is installed on the gage.

153. Failure to trace curve.—Sometimes sections of the tide curve
will be missing without any apparent cause. Assuming the pencil
point to be unbroken, if the missing section occurs on a falling tide, it
is probably the result of a dirty pencil screw which has lifted the
pencil off of the paper temporarily. In some cases the pencil arm
may be thrown entirely back away from the paper and remain there
until returned to its proper position by the observer. The obvious
remedy is the cleaning of the pencil screw in accordance with para-
graphs 129-131. If the entire tide curve is tery faint, a pencil with
softer lead should be used. —

154. Distortion of tide curve.—A series of steps in the record or a
flattening at the times of high or low water is most frequently caused
by « dirty pencil screw and this should be given first attention when
the trouble arises. An irregular distortion in the tide curve may also
result from the same cause. In very cold weather, a distortion of
the curve may be caused by the presence of ice in the float well. Ice
collected on the inside wall of the well may impede the movement of
the float without stopping it completely, thus causing an irregular
movement. See paragraphs 71-72 for precautions against freezing.
If ice has already formed in the well a liberal use of salt or hot water
is helpful in freeing the float. If an insufficient quantity of kerosene
has been placed in the well as’a precaution against freezing it is possi-
ble for ice to form below the kerosene. See also the following
paragraph.

155. Clogged float well.—aA clogged float well is evidenced by a
consistently smooth tide curve regardless of any roughness in the
water outside. As a clogged well causes a lag in the record which
greatly impairs its value, it is important that the observer be especially
watchful for any indications of this trouble in order that it may be
remedied without delay. Unless the surface of the outside water is
quite smooth, there should always be a perceptible short period wave
oscillation in the record with an amplitude increasing in proportion
to the roughness of the outside water. Whenever there is any evi-
dence of the float well being clogged, the observer should follow the
instructions in paragraphs 132-135 for clearing the intake. In
localities where the clogging occurs frequently, the observer should
adopt the practice of clearing the intake at regular intervals.

INSPECTION OF TIDE STATION

156. In making an inspection of a primary tide station note should
be made of the following matters and a report of the conditions found
forwarded to the office in form 681, report—tide station. Informa-

MANUAL OF TIDE OBSERVATIONS 51

tion relating to the station supplied by the office should be verified
by the inspector as far as practicable.

157. Location.—If changed conditions in the vicinity of a tide sta:
tion make it desirable that the location be changed, the inspector
should ascertain whether there is a more suitable site and report
accordingly with his recommendations. _

158. Tide staff.—A careful inspection should be made of the tide
staff, the legibility of its scale, and to see whether it is firmly secured
in a vertical position. Ifa portable tide staff is in use, the staff sup-
port should be examined to see that it is substantially secured and
that the metal plate at the top is firmly fastened in place. The metal
stop on the back of the portable staff must be checked in regard to its
position relative to the tide staff scale (see par. 111) and to see that it
is firmly secured. The staff should be lowered into its support to see
whether any obstructions exist to prevent the stop on the staff from
resting flatly upon the metal plate at the top of the support.

159. The tide observer should be questioned in regard to any changes
in the position of the tide staff of which he may have knowledge, and
should be advised to always report immediately to the office any
occurrence that might affect the elevation of the tide staff. Ifa tide
staff has been accidentally knocked out of place or loosened and after-
ward resecured by the observer, it is important that the office have a
record of the date on which this occurred.

160. Tape gage.—lIf there is a tape gage or other substitute for a
tide staff at the station, this should be described and any known
- changes since the previous inspection, reported. For a tape gage, the
relation of the plane of flotation to the tape scale should be checked by
the method described in paragraphs 82 to 86. If there is a tide staff
in addition to some other form of nonregistering tide gage at the
station, some simultaneous readings from both gages should be taken,
preferably when the water is reasonably calm, but if the water is
rough the amplitude should be noted as an indication of the reliability
_ of the comparison.

161. Automatic tide gage.—The automatic tide gage is to be ex-
amined to ascertain if it is operating satisfactorily, special attention
being given to the clocks, the hour-marking device, the pencil screw,
and the wiring, which have been described in detail in the preceding
pages. Note whether the gage is so placed over the float well that the
float clears the sides of the well without scraping. Inquiry should
be made as to whether the observer has experienced any of the operat-
ing difficulties listed in paragraphs 147 to 155, and such adjustments
should be made and advice given as necessary.

162. Float well.—Note the general condition of the float well and
whether it is securely fastened in place. Special attention must be
given to the intake to the well and a rod or cleaning tool should be
passed through the opening. (See paragraphs 132-135.) If there is
a separate well for a tape gage, this must be given the same attention
as the one for the automatic gage.

163. Observations.—The methods and procedure used by the ob-
server in attending to his duties are to be noted and his attention
called ‘to any matter requiring change. Any unusual method or pro-
cedure should be noted in the report. The inspecting officer should
make an independent reading of the tide staff and enter it on the tide

52 U. S. COAST AND GEODETIC SURVEY

record in accordance with instructions for comparative note in para-
graphs 120-126.

164. Measurements.—The depth of the harbor bottom at the tide
staff and at the float well as referred to the floor of the wharf or other
specified fixed point should be taken and reported at each inspection
in order that there may be a record of any change in the depth which
may occur from time to time. These depths need be given only to
the nearest foot or half-foot. Previous measurements of the relation
of the tide staff and float well to the wharf floor should also be
verified. 7

165. Bench marks.—At the time of inspection search should be
made for all bench marks within a radius of 1 mile from the tide
station, their condition noted, and the old description revised if
necessary. (See par. 109.) If an old bench mark cannot be found,
a statement to that effect should be included. in the record, together
with an explanation whether the failure to find the mark results
from an inadequate description, inaccessibility of location, or posi-
tive information of its destruction. If the number of bench marks
recovered is less than the minimum of five marks for a primary tide
station, the inspecting officer should arrange for the establishment of
such additional marks as may be necessary. Descriptions and reports
concerning bench marks are to be included in the leveling record,
form 258.

166. Leveling.—On the occasion of each inspection the tide staff
must be connected by a double line of levels with not less than three
substantial bench marks including the primary bench mark, and the
results must come within the limits of accuracy indicated in para-
graph 112. For procedure in regard to leveling to a portable tide
staff or to a tape gage, see paragraph 111. The height of a bench
mark above the datum of a tape gage of the type illustrated by figure
2 is equal to the height of the bench mark above the reading mark
of the gage plus the reading of the plane of flotation on the tape scale
extended. If the bench mark is below the reading mark, the differ-
ence in elevation is subtracted from the reading of the plane of
flotation. Form 258 is to be used for the leveling record.

167. Recommendations.—A report of inspection should include
recommendations for such repairs or changes as may appear desirable,
with an estimate of the cost when possible. When repairs are
urgently required, the inspecting officer should give them immediate
attention, requesting by telegraph approval for expenditure of such
extra funds as may be needed.

SECONDARY TIDE STATION

168. Secondary tide stations include those which are operated over
a very limited period of time, the observations in general extend over
less than a year but in some cases covering more than a year.
Secondary tide stations are established for the purpose of obtaining
general tidal information for a locality and also to obtain specific
data for the reduction of soundings in connection with the hydro:
graphic surveys. Observations at a secondary tide station are not
usually sufficient for a precise independent determination of tidal
planes, but when reduced by comparison with simultaneous obser-
vations at a suitable primary tide station very satisfactory results
may be obtained.

MANUAL OF TIDE OBSERVATIONS 53
LOCATION

169. The selection of a location for a secondary tide station will
depend upon the purpose for which it is to be established. For the
_ hydrographic survey of any area there should be a principal station

somewhat centrally located which will be maintained through the
time the survey is in progress. If available, a standard automatic
tide gage should be installed at the principal station. If there is
already a primary tide station located within the area to be surveyed
this may serve as the principal station.

170. Gages are to be established at other points in the immediate
vicinity of the soundings as the work progresses, the distribution
depending upon the change in the tide from point to point. In some
localities the tide may occur practically simultaneously with nearly
equal range over a large area. In such a case a single tide station
will serve for the entire area. In other localities the tide may change
rapidly in passing from one point to another and it may be necessary
to establish gages at close intervals. For the subsidiary stations, the
portable automatic tide gage will generally be found most convenient
to install and the records sufficiently accurate for the purpose. For
a very short series of observations, a plain tide staff to be read at
certain fixed intervals may be found sufficient. .

171. For sounding on outer coast.—For use in reduction of sound-
ings on offshore sounding work the tide record from a gage located in,
a harbor, such as at a primary tide station, can be used with cor-
rections applied for difference in time and height. It is advisable,
however, to avoid a location well inside a river mouth or shallow
estuary or in a body of water having a narrow connection with the
sea. For use in connection with inshore hydrography along the
outer coast subsidiary staffs or portable gages should be established
where possible in the immediate vicinity of the work and connected
by means of comparison of simultaneous observations with nearby
stations where long series have been obtained, if available.

172. Exposed channel approaches.—For surveys of exposed chan-
nel approaches, where the depths are near the draft of vessels and
where especially accurate soundings will therefore be required, for the
reduction of which the record from an inshore tide staff is not suf-
ficiently accurate, a temporary station should be established by pump-
ing down a pile or stake and several hours’ tide observations obtained
on a portable gage or fixed staff simultaneous with the inshore sta-
tion. The observations at the temporary station should cover at
least a high and a low water. ; :

173. Abnormal tides due to configuration of shore.—In straits
connecting two bodies of water having tides of different ranges and
epochs of occurrence it usually happens that in portions of the straits
the tide varies rapidly from place to place. Hell Gate, East River,
N. Y., and the channel north of Vancouver Island, British Columbia,
are examples. When sounding in such straits, tide stations should be

established at frequent intervals. At times also appreciable differ-
ences both in time and height of tide occur on the different sides of the
same island in an archipelago.

174. Upper reaches of tidal rivers.—In the narrow upper reaches
of tidal rivers a tide station is not representative of any considerable
area, and stations should be spaced sufficiently close together that

54 U. S. COAST AND GEODETIC SURVEY

sounding will not be done at a considerable distance from a tide
station. ;

175. Effect of wind.—In large bays of comparatively shallow depth
and with small range of tide, and in broad stretches of rivers or
along shores where the water is shoal, the wind has considerable
effect on the time and height of the tide. In such places under
unfavorable weather conditions the state of the tide at moderate
distances from a tide gage may be quite different from that at the gage,
particularly if differently exposed to the wind direction. Sounding
at some distance from a tide gage under such conditions will result
in failure of sounding lines to cross, at times by several feet.

176. Where such conditions prevail, a tide staff or portable gage
should be established in the immediate vicinity of the work for the
reduction of soundings and connected with the central or control
gage by simultaneous observations made during normal weather
conditions. If a portable gage is established at the auxiliary station,
2 or 3 days of simultaneous observations should be obtained. If a
plain staff'is established at. the auxiliary station, the observations for
connecting with the central gage need cover only those hours during
which sounding dependent upon the auxiliary gage is being done,
except that several high and several low waters should be included,
eeeebly a complete range of tide on each day that the staff is
used.

177. When surveying shallow bodies of water, such as the sounds of
North Carolina, where the range of tide is small and fluctuations
due to meterological conditions considerable, it will be necessary
to confine the sounding work of any day during periods of heavy
winds to an area in the vicinity of a tide gage or, if sounding on
long lines, to have sufficient auxiliary tide stations in actual oper-
ation scattered over the area to furnish correct reducers for the
soundings at different positions along the sounding lines.

ESTABLISHMENT OF SECONDARY TIDE STATION

178. Certain precautions which are required when establishing a
primary tide station because of its more or less permanent character
are unnecessary when installing a secondary tide station. Without
sacrificing anything which might impair the reliability of the record,
the installation of a tide station to be occupied for only a short period
of time may be somewhat simplified.

179. The tide staff may be a plain board with graduated scale
nailed to a convenient support. The staff must, however, be con-
nected with a system of bench marks as described in paragraphs 97—
112. The smaller and more conveniently installed portable auto-
matic tide gage may usually be substituted for the larger standard
gage. The float well may be a simple construction of plain boards;
or, if the portable automatic gage is used, made up of convenient
lengths of standard 4-inch iron pipe. If the observations are taken
during the summer time only, no tide house is necessary, a waterproof
box affording sufficient shelter for the standard gage, and the iron
cover provided with the portable gage serving as ample protection
for the latter. In the winter time a tide house is more or less necessary

MANUAL OF TIDE OBSERVATIONS 55

as a protection against the elements when changing the record on the
gage. |
_ 180. In selecting the site for a secondary tide station existing facili-
ties and the accessibility of the location to an observer must generally
be taken into ac-
count. For the
standard tide gage’
a convenient wharf |
is especially de- -
_sired. Otherwise ~
some special plat-
form must. be con-
structed. The port-’
able gage, partially
supported by its
own 4-inch iron
pipe float well, may
be secured to a sin-
gle pile, to a fish-net
stake, or against a
cliff (figs. 20-21).
Care should be
taken to install the
gage at a point
where the depth is
sufficient to operate
the gage at low tide.
181. The installa-
tion of the standard
automatic tide gage
has already been
described on pages
32-37 and its opera-
tion discussed on
pages 41-50. -How-
ever, all the in-
structions given for
the guidance of the _
tide observer at a
primary tide station
will not necessarily
be applicable to a
tide observer at a
secondary tide sta-

: : FicurRE 20.—Installation portable automatic tide gage on
tion, who is work- fish trap stake. Spi
ing under the direc-

tion of the chief of party. The installation and operation of the
portable automatic tide gage are described on‘the following pages.

INSTALLATION AND OPERATION OF PORTABLE AUTOMATIC TIDE GAGE

182. Since the portable tide gage is designed to be set with stylus
reading in agreement with the reading on the tide staff, the latter when
installed should be so placed that its graduation corresponding to the
middle of the height scale of the record paper shall be at approximate

56 U. S. COAST AND GEODETIC SURVEY

half-tide level or midway between the extreme tides to be expected |

during the period of observations. With such a setting the curve
traced by the portable gage will be approximately centered on the
record paper.
183. Float well.—The base of the portable automatic tide gage is
provided with a
sleeve to fit on the
top of a float well.
In the later gages
this provides di-
rectly for a 4-inch
float pipe, but in the
earlier gages it pro-
vided only for a
31-inch pipe and a
reducing coupling
is necessary to
adapt it to the
larger pipe. The
pipe in addition to
serving as a float
well acts also as a
support for the in-
strument. When
the gage is installed
on a wharf a flange
coupling witha
short section of pipe
above the deck af-
fords a ready means
of supporting the
float pipe. The
conical inlet cou-

on the bottom of

reaching below ex-
treme low water.
To provide a sup-
port for the instru-
ment in a location
at which no wharf
or platform is
available, an addi-
etre _. tional section of

Ficurb 21,—Installation ee tide gage against pipe is screwed into
this conical inlet

coupling on its bottom end and perforated with several large holes
to allow free access of the water to the inlet in the coupling. (These
holes should be as large as can be conveniently made, so as not to
become clogged.) This lower section rests on the bottom, and the
float pipe may be lashed to a single pile or net stake or lashed securely
against an overhanging cliff where depth of water permits (figs. 20 and
21). In localities where little penetration can be obtained for a single

pling is screwed -

the longer section,.

MANUAL OF TIDE OBSERVATIONS 57

stake three sharpened poles of suitable length may be driven about
6 or 8 feet apart and the upper ends brought together and lashed in
the form of a tripod, the float pipe being lashed in a vertical position
to the apex of the tripod.

184, Attaching float——When installing the instrument on the float
pipe the first step is the attaching of the float. The length of wire
required may be determined as follows:

Let A=distance in feet of float drum to water surface. (Actual
measurement not necessary.)
at B=height in feet of tide above or below approximate mean tide

evel.

In the formula below use plus (+) B if tide at the time is above
mean tide level and minus (—) B if below mean tide level.

Length of wire required=A+15 feet+B.

Take a spool of wire and attach one end of the wire to the float
and lower it into the well. Allowing a few inches for the distance
from the top of the well to the float-wire drum, measure off an addi-
. tional length equal to 15 feet+B, the value of B being estimated from
the stage of the tide, and then cut the wire. Pass the loose end of the ©
wire over the fixed fair leader and through the opening in the
base of the instrument, and then before attaching the wire to the
drum wind up the spring either by turning the drum counterclock-
wise, as viewed from the side shown in figure 13, or by use of the
clock key on the drum axle (30, fig. 18). The spring should be
wound up completely and then slackened off about four turns. Hold-
ing the drum in this position, the float wire is now passed through
the small drill hole near the edge of the drum and knotted. The
drum should now be permitted to turn slowly through the action
of the spring, winding up the float wire, care being taken that the
wire follows the threads in the face of the drum. When all the
slack in the wire has been taken up the gage is placed in position
on top of the float pipe and locked by means of the two anchor
hooks (27, fig. 13). |

185. Counterpoise spring.—If the float wire has been cut off at the
proper length and the instructions described in the preceding para-
graph followed the counterpoise spring will be wound to the proper
tension ; but if further adjustment to the tension is necessary this ma
be made by using the clock key on the subsidiary axle (380, fig. 13) of
the float drum. ;

186. Gear train.—Having determined the scale of the record to be
employed from a knowledge of the approximate range of tide at the
station, the proper gears for the float-drum axle and the stylus screw
corresponding to that scale are given in the table on page 23. The
gears to be used are also printed on the cross-section record paper for
each scale. The number of teeth is stamped in each gear and care
must be taken in selecting and installing the correct gears corre-
sponding to the scale used.

187. Although the same idle gear (8, fig. 11) is used for all scales,
its position varies with the different gear combinations. When chang-
ing the gears the idler must be removed, and after the other gears
have been installed, it is replaced, being secured by the gear screw
(9, fig. 11) in the particular hole provided for the combination. The
lever nut (10, fig. 11) provides a convenient means for loosening or
tightening the gear screw securing the idle gear. The gears attached

58 U. S. COAST AND GEODETIC SURVEY

to the stylus screw and the float-drum axle are easily removed after
taking off the nuts (7, 12, fig. 11) holding them in place.

188. Record cylinder.—The record cylinder with the clock move-
ment inside is now installed in its supports. The clockwork rotates
the cylinder in such a direction that its top moves toward the stylus
screw, and when installing the cylinder it should be placed in its sup-
ports with the capstan nut (4, fig. 11) on the same side of the instru-
ment as the train of gear wheels, this being designated as the front
of the gage.

189. Record paper.—F ive different scales of record paper are pro-
vided and care must be taken to select the scale desired and to see
that the gears printed on the sheet are the same as the corresponding
gears on the gage. With the capstan nut (4, fig. 11) loosened, the
paper is placed on the cylinder with the zero of the height scale
toward the front of the instrument; that is, at the same end of the
cylinder as the capstan nut. The paper is held in place by a metal
clip (24, fig. 12), this clip being released at one end when installing
the paper. After the paper is in place, the cylinder is turned until
the stylus reading on the paper scale corresponds approximately to
the correct time and the nut at the end is then tightened to secure
the cylinder in this position.

190. Two days of record are traced on the sheet of paper for each
revolution of the cylinder. Since the times of high and low water
occur about 50 minutes later each day, the tide curve traced on the
same sheet during several revolutions of the cylinder separates suffi-
ciently to be distinctive unless the range of tide is very small. Al-
though a singe sheet of record paper might serve for an entire week,
it recommended that a new sheet be placed on the gage every 3 or
4 days.

191. Setting stylus.—To set the stylus to the approximate height of
the tide as read on the tide staff, the nut (7, fig. 11) holding the upper
gear to the stylus screw is unclamped, and the stylus screw may then
be turned freely to bring the stylus to the reading desired. The gear
is then again clamped in position. A finer adjustment for height may
now be obtained by means of the slow-motion screw (25, fig. 12) pro-
vided for the purpose. The stylus should be reset to agree with the
staff reading each time the record paper is renewed.

192. While the approximate time setting of the stylus is obtained
by the turning of the record cylinder as described in paragraph
189, a finer adjustment is secured by means of the slow-motion screw
(34, fig. 13). When setting the stylus for time, whatever slack there
may be in the record cylinder due to lost motion in the gears should
first be taken out by lightly placing the hand on top of the cylinder
and drawing it in a direction away from the stylus.

193. Care of clock.—Although the clock enclosed in the record
cylinder has an 8-day movement, it is recommended that it be wound
semiweekly. The clock is regulated by means of the small end of the ~
clock key inserted in the small keyhole in the front end of the cylin-
der. When regulating the clock, however, the record cylinder should
be allowed a full 48-hour revolution and the clock movement adjusted
according to the amount that it fails or overruns a complete revolu-
tion of the cylinder and not for any particular hour on the cross-
section paper.

ichinn tape | eer 4

Sa tks $ “ pina nnn , aesacicings

opr aint

n

SRA pact ok oN eon pe

‘a

A HERETIEE
ee eo
“HEN HIE EE
eet

tH

Lap other end of sheet. over this blank space

Scale 1:16 (Maximum Tide 95 ff.) Use Pulley Gear96, Screw Gear d4

ge).

22.—Tide curve and comparative notes (portable ga

FiGure

- 47 (Face p. 59)

735445 O

MANUAL OF TIDE OBSERVATIONS 59

194. Cleaning stylus screw.—For the efficient functioning of the
gage it is important that the stylus screw be cleaned at frequent
intervals. This should be done in the same manner as with the pencil
screw of the standard gage. (See pars. 129-1381.)

195. Cleaning float pipe—If the gage is to be operated for a con-
siderable period of time the float pipe should be cleaned occasionally
to prevent the opening in the conical inlet coupling becoming clogged,
It should be cleaned by removing the float from the pipe, inserting
the cleaning tool to which a line has been attached, and raising and
lowering it several times in the pipe. (See pars. 132-134.)

196. Comparative note.—Each day the tide gage is visited, a com-
parative note giving the date, the correct time, and staff reading
should be entered on the marigram (fig. 22). The stylus is to be
adjusted when necessary to agree with the correct time and staff
reading. As unbiased staff readings are necessary for the proper
reference of the various tide planes to bench marks, care must be
taken by the observer to avoid being influenced by a previous setting
of the stylus on the cross-section paper when taking a new staff
reading. However, when an unusual rough state of the water renders
a staff reading somewhat unreliable, a previous setting of the stylus
under more favorable conditions should be retained without change.

197. To indicate on the tide curve the exact point to which the
comparative note refers, first rock the stylus holder (35, fig. 13) slightly
to make a short line parallel to the edge of the paper. Next place
the hand lightly on the float drum (17, fig. 12) and raise the float a
little way out of the water and then lower it, making a short vertical
line on the marigram. The point indicated by the intersection of
a lines is then connected with the comparative note by a light
ine.

FIELD REDUCTIONS

198. Preliminary computations of tidal data to obtain tide reducers
for soundings are often carried on in the field. The processes are
covered in the following chapter on Tabulation and Reduction, but
for field purposes, parts of the instructions not directly applicable to
the work in hand may be omitted. Unless there is special need to
expedite the reductions in the field the planes of reference for the
reduction of soundings will be furnished by the office upon the receipt
of the original tide and leveling record. |

TABULATION AND REDUCTION

PRELIMINARY WORK

199. The original tide records to be tabulated will vary in form
according to the kind of tide gage used in taking the observations.
The records generally consist of tide rolls from the standard automatic
tide gage or tide sheets from the portable automatic tide gage. The
following general instructions are applicable to the several forms used
for the tabulations and reductions. The work is to be done neatly
and in ink. Interpolated or inferred values are to be indicated by the
use of brackets. The heading on each sheet will in general be filled
out as completely as possible in order that it may be fully identified,
but when a tabulation covers several sheets a repetition of the latitude

60 U. S. COAST AND GEODETIC SURVEY

and longitude of the place is unnecessary, and for a continuing series
the spaces for the beginning and ending of the observations may be
left blank. The height datum in the heading refers to the datum
actually used for the tabulation, which is generally the tide staff zero.
Standard time is to be used consistently throughout the year regard-
less of the fact that daylight-saving time may have been temporarily.
adopted in some localities. ‘The hours of the day are to be numbered
consecutively from 0° (midnight) to 23" (11 p. m.) to avoid the
necessity of using the terms “a. m.” and “p. m.”

200. Checking time.—In the portable gage record, the hours are
indicated by the numbered vertical lines of the cross-section paper,
the hours being subdivided into 10-minute spaces by finer vertical
lines. The time as indicated by these vertical lines is to be compared
with the correct time as given in the comparative notes entered by the
observer. Assuming that the observer has corrected any clock error
at each visit to the tide station, any loss or gain may be prorated over
the period intervening between the visits unless there is evidence to
indicate that the loss or gain was not uniform.

201. In the standard gage record the hours are indicated by short
horizontal lines made automatically by the time clock of the gage.
The hour begins at the instant the mark leaves the curve, the length of
the stroke having no significance. The time notes entered on the
record by the observer enid be examined, and if it is found that
the difference between the correct time and the gage time does
not exceed 3 minutes at any time, the hour marks as ante
made by the gage may be accepted as correct and marked accord-
ingly. In cases where the hour marks are appreciably in error due to
failure of the time clock to keep correct time, the total error indicated
by the time clock may be prorated among the hour marks effected on
the assumption that the time clock has lost or gained uniformly
between consecutive comparative notes. The marks are to be num-
bered consecutively from 0 (midnight) to 23 (11 p. m.), and the
numbering checked at each time note on the marigram. In order to
expedite the work, the numbering of the odd hours may be omitted if
desired. The beginning of each day at the 0 hour should be marked
with the appropriate date.

202. In cases where the hour-marking device has failed to work
the following method may be used: First, from the comparative time
notes ascertain the position on the curve of the nearest exact hour for
each note made during the period when the hour-marking device was
not functioning. ‘This is done by laying off 1 inch on a piece of paper
and dividing it into 12 equal parts, the inch measured parallel to the
datum line representing 1 hour on the tide curve and each of the
divisions 5 minutes. The correct time of the point on the curve bein
known, as indicated by a time note, the nearest. exact hour is laid o
by means of this “time scale.” Second, through the points thus found,
indicating the exact hours, draw lines eae to the datum line
and extending across the paper. Third, prepare a “dividing scale”
from a strip of paper somewhat longer than the greatest distance
between the time notes on the marigram. On the edge lay off equal
divisions about 1145 inches long. These divisions should be numbered
consecutively from 0° to 23" and repeated if necessary. This scale is
then adjusted obliquely between two consecutive cross lines passing
through the correct hour points so that the numbers on the scale will

MANUAL OF TIDE OBSERVATIONS 61

agree with the hours represented by the cross lines. With the scale
in this position, each division is marked on the marigram by a dot.
Fourth, these hour dots are referred to the tide curve by lines drawn
through the dots and perpendicular to the datum line. These hour
lines are numbered in the same manner as the hour marks automatically
made by the time clock.

203. Checking height datum.—In the portable gage record, the
heights are indicated by the horizontal ruling of the profile paper, the
zero of which is assumed to correspond to the tide staff zero unless
otherwise stated. The height as indicated by the horizontal ruling
is to be compared with the actual staff reading as recorded in the
comparative notes and allowance made for any difference when tabu-
lating the record. The portable gage is designed to be reset whenever
necessary to keep the scale and staff readings in agreement.

204. The determination of the height datum for the standard gage
record is radically different from the method used for the portable
gage. In the standard gage, no attempt is made to establish in
advance any particular relation between gage and staff datum. The
aim is to keep the gage datum uniform throughout the entire month,
and then, after the record has been removed from the gage, to deter-
mine the relation of its datum to the staff zero by the average of the
daily comparisons covering the entire month. A special form is
provided for the computation and its use is described in the following
paragraphs.

205. Comparative readings (form 455).—This form (fig. 23) is
used to obtain the relation between the datum line of a standard
automatic tide-gage record and the datum adopted for the tabula-
tions. The latter datum is-either the zero of the tide staff or tape
gage in actual use, or has a definite relation to the same. At primary
tide stations it is the aim to maintain a’ fixed datum for the tabula-
tions throughout the entire series of observations, and constants are
introduced to take account of any changes in the elevation of the tide
staff. The corrected setting as calculated in form 455 represents the
scale reading of the datum line as referred to the datum adopted for
the tabulations. A movable scale, usually of glass, is used in making
these tabulations. In using this scale the side on which the gradua-
tions are cut should always be kept down next to the paper to avoid
errors due to parallax in reading. The numbering of the divisions
may be written on the upper surface.

206. In the first three columns of form 455, the tabulator notes,
respectively, the date, the time of staff reading, and the water level
as read on the staff or tape, these items being taken directly from the
observers notes on the tide roll. The staff reading entered in the
form is the mean between the highest and lowest readings recorded,
but if a glass tube is used on the staff, the reading entered in the form
should be the mean of the tube readings. If a tape gage is used, the
word “staff” at the head of the third column may be changed to
“tape” and the reading entered should be the mean of the highest and
lowest tape readings. :

207. The preliminary scale setting of datum line, to be entered in
the heading of the form, may be arbitrarily chosen at any convenient
value. This preliminary setting should preferably be of such a value.

735445 O - 47-5

62 U. S. COAST AND GEODETIC SURVEY

that the scale readings from the tide curve will be from 14 to 114 feet
less than the corresponding staff readings. The scale reading selected

Form 455
DEPARTMENT OF COMMERCE
COAST AND GEODETIC SURVEY

Ed. July, 1928
TIDES: COMPARATIVE READINGS

Station: .. A eatthe, Maobiong frei: ce! 1G A eee Dee 47° 37.
Party of .. Ww. (A £44 Time meridian .. 120° WW. me cael o'
Obs. ee pe ae Obs. end... Oe abuloled bg: Ges 22 Alaae. Data Fick. By he a 1928.
Tide ulin No: Secs Gets Scale .. ys: 2 y. +s. Preliminary scale selting of datum line..........4€@+O00... ....... Jeet.
Sr pkre' ee oF STAFF an SCALE BIE “PHASE py

a | READING cate A B TSS, TIDE* RES

"In Eas : headed “ of Tide™ write the eppropeate one of the fur flowing mba: H. iakeh ean i ielew suet Rie ommeens tide: and F, for falling tide.
Use Form nts em ened Pag Ti = saves iosvom eee ia

FIGURE 23.—Form 455, comparative readings.

for the comparison should be ruled across the glass scale on the
underside.

208. The values in the fourth column of the form are obtained by
placing the scale on the record with the preliminary setting in coin-
cidence with the datum line and reading the height of the curve at
the point to which the corresponding staff reading refers. These
readings are to be taken to the nearest 0.05 foot. The difference
between staff and scale reading is to be entered in the fifth column,

MANUAL OF TIDE OBSERVATIONS 63

and the phase of the tide at the time the staff reading was taken in the
next column. Any change in the adjustment of the gage should be
explained in the column of “remarks.”

209. If there has been no change in the adjustment of the gage, the
differences (A—B) should be approximately equal. If an individual
value differs materially from the apparent average of all, it must be
rejected and excluded from the computation of the mean. The rejec-
tion is indicated by encircling the value in question. The differences
are now summed and a mean obtained, the result being carried to
two decimal places. To this mean difference there is added the pre-
liminary scale setting and also any constant that may be necessary to
‘refer to any datum other than staff zero. When a constant is neces-
sary itis furnished by the office. The algebraic sum of these quanti-
ties will give the corrected scale setting to be used in the tabulations
of the hourly heights and the high and low waters.

210. If there has been any change in the adjustment of the gage,
such as would be caused by replacing a broken float wire, the intro-
duction of kerosene in the float well, a change in the position of the
datum line, etc., the differences will form distinct groups. In such
cases separate means and corrected scale settings must be computed

for each adjustment of the gage.

_ 211. Sometimes an extreme high or low tide may move the pencil
arm to the extreme limit of its motion so that it becomes disengaged
from the threaded portion of the pencil screw. If the change in the
height of the tide beyond this limit is small, the pencil arm will auto-
matically reengage the screw thread after the tide reverses without
any change in the adjustment of the gage, but if the tide continues
to rise or fall after the disengagement of the pencil arm by an amount
equal to or greater than the circumference of the float-wire drum,
the adjustment of the gage will be automatically changed. Each com-
plete turn of the drum at such times will be found registered by a jog
in the record near the edge of the paper, and each such turn will sig-
nify a change in the adjustment equal to the circumference of the
float drum in use. If such change takes place at the time of high
water, the curve will be lowered and the scale readings diminished, but
if it takes place at the time of low water, the curve will be raised and
the scale readings increased. :

212. In using form 455 the tide roll should be taken as the unit,
regardless of the beginning or end of the calendar month, and each
comparative note made by the observer on the tide roll should be
included in the form.

HIGH AND LOW WATER TABULATION

213. Form 188 (figs. 24, 25) is designed for the tabulation of high
and low waters, but for a very short series the high and low waters
may be tabulated directly in form 248 for the comparison of simulta-
neous observations (fig. 27). When form 188 is used the tabulations
will in general be arranged by calendar months, but if the entire series
of observations does not exceed 31 days the tabulations may be in-
cluded on a single sheet of the form. Tide observers who tabulate
their own records should, after completing the tabulations for any
calendar month, retain a memorandum of any high and low waters at

64 U. S. COAST AND GEODETIC SURVEY

the end of the tide roll which pertain to the following month in order
that they may be included in the tabulations for that month.
214. Times.—The times of the high and low waters are to be ex-

Form 138
OR CONST AN geoornic sunver TIDES: HIGH AND LOW WATERS
: d. Feb., 1928
Station: _Heattle, Waahiogtion. Lat. MA oy boo Sy I
Observations begin end Long. 22°20 a
Time meridian __/2 O° W SSS ee datum 7. S. 2 Fr O(1F 04) which is 2 which is ~24- -56 ft. below B. Mi SPS

DATE || Moon's TIME OF— “LUNITIDAL — | LEIOGHT OF—
TRANSITS crc
REMARKS
een ich
gas | Si os

Jan. 1

eat

are eae wre eae Gn eee,

TT a-al iz -ol 14th c4-5) (1 -eil vas

| tice 2-4] 7-ol e|-H -oll 17-ol i4-b

| | aoa 12-4] 2o-oll c+ 5} 1 - pl 9b or

[| «ica. 3-5] 2-2] e-ol all 17-9] 15-0]
| | 2a ell 13-2] coal c+) Ci sy 19-27 5-91
| siao-ay 4-3] 9-3 4-5 si 19-2] 15-4)
[a2 -ell pees] 2y-7i] c+ 2) ca -#il 1st sof
[sti wl 5-2] po el oe] yall 79-71 ys-ol
| st aa-4f 5-3] 22-4l (a 9) ci-col 19-0] 4-14
[7] —-TT ¢-ol al ei] nail 79-91
[laa ye-ol 23-ol (3 -b) Uo-by 12-9
[si o-al 6-7] y2-0f se] il 19-97 15-0]
[| las-ay| 17-ol 23-41 73-7 o-bi yee] 4-9
ia rear! Sere ee Te

>

O
o

- . a) *
om K ‘ BO DO

Sums; carried forward

FicurRn 24.—Form 138, high and low waters (front).

pressed in hours and tenths, the hours being numbered consecutively
from 0 (midnight) to 23 (11 p.m.). A high or low water occurring
at midnight (0") is taken as belonging to the day just beginning.
Standard time is to be used consistently throughout the tabulations
regardless of any temporary local use of daylight-saving time. In
form 138 two lines are provided for each calendar day, and in general

MANUAL OF TIDE OBSERVATIONS

ce
vu

6

the morning tides will be entered on the first line and the afternoon
tides on the second line for the day. Blank spaces should be
left for tides missing because of lost record in order that inferred
values may later be interpolated if desired; but when only a single

Form 138
DEPARTMENT COMMERCE
COAST AND Groner c SURVEY
Ed. Feb., 1923

TIDES: HIGH AND LOW WATERS

Station: _Leattle, Washington.
Highest tide: Daie Sire Neves Height 2O-2 fi. Lowest tide: Date Gth Height #- 1 ft.
(K,+0,)=M or 2(0DHQ+ DLQ+Mn=_ 1.2 F(Mn)=1.004 F=.0. 85
I ~fi i i cas I] |
[| pare jj arose | meor | espa. |] ometonror— |]
H| Year i + ——— | REMARKS
g (Grem Rick a teu “ 1O\eeet | a len ny iH Ru 1GH ees ir
19 eZ mean civil) 4 ATLK ATER | ATER ATER | ATER ATER
= + = = a a
mo. dl hr dee. hr. dec, ir, dee. |] Pr. dec. | hr. dec. feet feet |
: : |
|

Brought forward
ee ee a ee

are
agele ade es pl

zac

Ecos MepAl Wiig 5 tamieegl Oden eel LES

iT

| 11-9] 6-0! J7- Tee 5) (11 -6) 49 - Ia IiGes

Pees ih

C6 0)

sonia aI stan
=p) Ia

2 GSR Pe Ol anno ES eC SD

LES £1035. 1g: |

|| Unrecuced intervals
Greenwich intervels

Local intervals 4b Sy ; Lo -b& :
A) Or

—O56 670110 57

J

Mn

MTL

|| Factor

Tabulated by P.L. 8B.
Reduced by EAGT:

Date Feb 4S, 1928 Checked by E.c.M.

Dae Feb. 16,1928 Checked by W.H.M.

Corrected | 7-76 | 9-89! 2- 9g

Uw, covuanmenr Paintose orrica: ins L1—671

FIGURE 25.—Form 138, high and low waters (back).

high or low water actually occurs on any calendar day, the unused

space may be filled with a

dashed line.

215. In selecting the time of high or low water from the tide curve
attention should be given to the general trend of the curve rather

than the individual “peaks arising from various causes.

The aim

66 U. S. COAST AND GEODETIC SURVEY

should be to take the middle of a smooth arc covering an hour or
more during the high or low water period. It is not necessary to
actually draw such an arc, but the point at which a smoothed curve
would have reached its maximum or minimum should: be estimated
as closely as possible. In determing the times of the high and low
waters to the nearest tenth of an hour it may be found convenient
to construct a small scale 1 inch long and divided into 10 equal parts
for use between the hour marks on the curve. An experienced tabu-
lator, however, will usually be able to estimate the tenth accurately
without the use of such a scale. 7

216. Heights.—The high and low water heights are to be tabulated
in feet and tenths and should refer to a uniform datum throughout
the entire series of observations. In general the datum adopted for
the tabulation is the zero of the tide staff as originally installed at
the station, allowances being made for any subsequent change in
elevation. However, any other datum may be adopted for the tabu-
lations, but it is desirable that the datum be sufficiently low to avoid
many negative readings.

217. When tabulating from a portable gage record, the heights may _
be taken directly from the scale of the profile paper provided the
gage has been consistently kept adjusted to agree with the tide staff
readings, but allowances must be made for any material variations
from such an adjustment.

218. When tabulating from the standard gage record, a glass scale,
graduated to conform to the scale of the record is used. A line is
ruled across the underside of this scale to correspond to the corrected
scale setting as computed on form 455 (pars. 205-212). The scale is
then moved along the record with this line in coincidence with the
datum line on the record, the heights as read on the scale being
referred directly to the desired datum. At any change in the adjust-
ment of the automatic gage, the scale setting must be changed to
accord with the new setting as computed in form 455,

HOURLY HEIGHT TABULATION

219. The hourly heights are tabulated in form 362 (fig. 26). Unless
otherwise directed these heights will be tabulated in yearly series,
beginning with January 1 as the first day of each series. The days
are to be entered consecutively, 7 days to the page and using both
sides of the form, without regard to change in calendar months or to
the time of changing the tide roll. The side of the form having the
wider margin on the left side should be used as the first page of each
sheet. The “day of series” will be numbered consecutively through
the year, beginning with “1” for the first day of January. If any
part of the record is lost, blank spaces should be left for later interpo-
lation of the missing tides. As stencils are to be used in connection

MANUAL OF TIDE OBSERVATIONS 67

with this form, it is important that the tabulated heights be written
within the spaces provided, these spaces being indicated by the
printed decimal points. ~

Derantwentorcommence TIDES: HOURLY HEIGHTS

Station: —teattle., Uaabioglnas __ Vearj— /7 25 os
iil e272 SE Tignes aoe

| Observer: L
' Time Meridian: _/ZO°U/ _ Tide Gauge No. 24 Scale 1:24 Reduced to Staff of /904 _

j @OYARAMENT FRDCTOXS oFTicE 11—793
!
G Z zontal

Month || mo. d. a. a d, d. d. d. d.
and | Hori-
Day [facet PRE Dg Re SEA 7 A WR
| : 1 Sum
ee 2 EES} oe Gis MIS eae a a
[pees | eee a

| SESS Fase ee ERE GT es Es oe ea
[EE TE a FS Le (a (RS (ESL ea
3

Sum [ 10.5 2, 347. 6.6 12 452: O

Sum for = Divisor=(28d) 672; (29d) 696; (30d) 720; (31d) 744. Mean for month=
Tabulated by SV eh) RAD om Peo LoD ae Date FebJs5,/922 Summed by YAS 0 bide DOOD ESS, Date Mar. 14,1928

FIGURH 26.—Form 362, hourly heights.

220. As a check on the arrangement of the days in the form, the
following table gives the page, column of page, and day of series, for

68 U. S. COAST AND GEODETIC SURVEY

the first of each calendar month, when the series commences with
January 1:

Common year . Leap year

Col- | Day of Col- | Day of

Month Page umn | series Month _ Page umn | scries
eis anes 1 1 1 Jans al 1 if 1
Heb: 5 4 3 Feb. 1 5 4. 5 32
Mar. 1 9 4 60 Mar. 1 9 5 61
Apr. 1 13 7 91 35) 01 wel | 14 “yall 92
May 1 18 2 121 ayer 18 3 122
June 1 22 5 152 June 1 22 6 153
July oo 26 lis 182 Jiuth ae 27 1 183
Aug. 1 31 3 213 Aug. 1 31 + 214
Sept. 1 35 6 244 Sept. 1 35 fy 245
Oct. 1 40 1 274 Oct: a 40 2 275
Nov. 1 44 4 305 Nov. 1 44 5 306
Dace 48 6 335 Dec. 1 48 7 336
Dec. 31 53 I 365 Dec. 31 53 2 366

|

221. The tabulated hourly heights are to be expressed in feet and
tenths and will be referred to the same datum as adopted for the
high and low waters, the heights being scaled from the record in the
same manner as described for the high and low water tabulations.
The hourly heights are to be taken on the exact hour and allowance
must be made for any time error in the record. This is especially
necessary if the heights are to be used in an harmonic analysis of the
observations.

222. Tide observers who tabulate their own records should retain
at the end of each month the last incomplete sheet of the hourly
heights in order that it may be completed when the record for the
following month is available,

INTERPOLATIONS

223. Before beginning the reductions, if any portion of the record
is lost, it is desirable that the missing tides be supplied by interpola-
tion. Interpolated tides should be enclosed in brackets to distinguish
them from observed tides.

224. If the heights of alternate high waters are plotted on cross-
section paper it will be found that fairly smooth curves may be drawn
through the plotted points. Such a graph affords a means of inter-
polating for missing high waters where only a few tides have been
lost. When only a few tides are missing the alternate heights for
several days before and several days after the break may be plotted
and spaces left for the missing values. The smooth curve connecting
the observed values will generally determine the missing values with
sufficient accuracy. Similar curves may be drawn for alternate low
waters. The times as well as the heights will be found to plot in

MANUAL OF TIDE OBSERVATIONS 69

fairly regular curves, and times of missing high and low waters may
also be determined by this graphic method. :

225. Another method of supplying missing values is by direct com-
parison with simultaneous observations at some other station in the
vicinity. This method is especially useful in extrapolating values at
the beginning and end of a series of observations.

LUNITIDAL. INTERVALS

226. A Greenwich lunitidal interval is the absolute difference in
time between the transit of the moon over the meridian of Greenwich
and the time of the occurrence of the following high or low water at
any place, separate intervals being obtained for the high and low
waters. <A local lunitidal interval is the difference in time between
the transit of the moon over the local meridian and the time of the
_ following high or low water at a place on the same meridian. The
mean local high-water intervals for many places are included in the
annual tide tables of this Bureau. The Greenwich intervals have
the advantage that they afford a direct means for obtaining the differ-
ence in the time of tide at different places.

227. Form 138 (figs. 24, 25) in which the high and low waters are
tabulated provides for the computation of both high and low water
lunitidal intervals. The Greenwich transits in hours and tenths,
which will be furnished upon request or which may be derived from
the American Ephemeris and Nautical Almanac where they are given
in hours and minutes, are to be entered in the column of “Moon’s
transits,” the lower transits being distinguished from the upper
transits by being enclosed in parentheses.

228. From the time of each high and low water subtract the time
of the first preceding moon’s transit and enter the difference in the
appropriate column of the form and on the same line as the tide from
which it was obtained. In case the time of high or low water is
nearly the same as that of the moon’s transit, take the transit which
precedes the tide by about 12 hours, but in no case must the same
transit be used for two consecutive high waters or for two consecutive
low waters. The lower transit of the moon applies to both high and
low water, just the same as the upper transit does. Wnen the time
of the moon’s transit is on one day and the following high or low
water is on the next day the time of this tide must be increased by
adding 24 hours before attempting to subtract the time of the transit.
The high-water intervals will usually be approximately 6 hours
greater or less than the low-water intervals, but the intervals for each
phase of tide will usually agree among themselves within an hour or
two. Intervals from the lower transits of the moon are to be indi-
cated by parentheses. The high and low water intervals for the
calendar month are summed separately and the means obtained to two
decimal places.

_ 229. In these computations the moon’s transits are given in Green-
wich time while the high and low waters are given in the standard
time of the place of observations. To obtain the Greenwich lunitidal
intervals it 1s therefore necessary to apply a correction corresponding

70 U. S. COAST AND GEODETIC SURVEY

to the time meridian of the place of observation, the correction to be
added for west longitude and subtracted for east longitude. In
figure 25, the mean unreduced high and low water intervals for Seattle
for the month represented are 4.98 hours and 11.12 hours, respectively.
As the time meridian for Seattle is 120° or 8 hours west, a correction
of +8 hours must be applied to each of these intervals giving 12.98
hours and 19.12 hours, respectively, from which the lunar semidiurnal
period of 12.42 hours may be rejected, leaving 0.56 hours and 6.70
hours, respectively, as the Greenwich high and low water lunitidal
intervals from this particular series of observations.

230. Local lunitidal intervals—To change from Greenwich to
local intervals, it is necessary to apply a correction equal to the time
required for the moon to pass from the meridian of Greenwich to
the meridian of the place of observations. <A table (p. 73) has been
prepared giving the correction for each degree of longitude from
1° to 180°, and the value for each minute of are from 1’ to 60’ for
interpolating between the whole degrees. In changing from Green-
wich to local intervals, the tabular value is to be subtracted if the
place of observation is in west longitude and added if in east longitude.
In order that the local intervals may be positive and less than the
half lunar day, the lunar semidiurnal period of 12.42 hours may
be added or subtracted as desired. The longitude of Seattle, which
is used as an example in figure 25, in 122°20’. In the table on page 73
we find the corrections corresponding to 122° and 20’ to be 8.418 hours
and 0.023 hours, respectively. Combining these we have 8.44 hours
as the difference between Greenwich and local intervals at Seattle,
and as Seattle is in west longitude this difference is to be subtracted
from the Greenwich intervals. To avoid negative results, the Green-
wich intervals are first increased by adding the semidiurnal period of
12.42 hours, after which the difference 8.44 hours is subtracted leaving
4.54 hours and 10.68 hours, respectively, as the local high and low
water lunitidal intervals for these observations.

231. Method of checking intervals.—The mean unreduced inter-
Bente obtained above may be conveniently checked by the following
method :

(a) Sum times of moon’s transits occurring during month. The
number of transits will usually be two less than twice the number of
days in the month; that is, 54 for a 28-day month, 56 for a 29-day
month, 58 for a 30-day month, and 60 for a 31-day month. For the
sake of uniformity, if the number of transits should be one greater
than this, the last transit must be omitted from the sum and the last
day be considered as having only the single value. On the other
hand, if the number of transits should be one less than the usual num-
ber, the deficiency must be supplied by including the last transit of
the preceding month. From the sum of the transits subtract the
product of 24 hours by the sum of the numerals indicating the days
of the month on which only a single transit occurs. For example, if
single transits occur on the 3d and 17th of a month, the sum 20 is
multiplied by 24, and the product 480 hours is then subtracted from
the sum of the transits. Designate the remainder by T, which may
be either positive or negative.

MANUAL OF TIDE OBSERVATIONS 71

(5) Sum separately the times of high and low waters, limiting the
number of items in each case to two less than twice the number of
days in the month, following the method described for the summa-
- tion of the moon’s transits. From the sum of the time of the high
waters subtract the product of 24 hours by the sum of the numerals
indicating the days of the month on which only a single high water
occurred, and from the sum of the times of the low water subtract

the product of 24 hours by the sum of the numerals indicating the
days of the month on which only a single low water occurred. Des-
ignate these results by H and L, respectively.

_ (ec) Find the differences (H—T) and (L—T) and divide by the
number of items included in each summation, which should be twice
the number of days in the month less two. Such multiples of 12.42
hours may be applied or rejected from the means as may be necessary
to reduce them to positive values of less than 12.42 hours. The
results obtained should check very closely with unreduced means as
obtained directly from the individual intervals, although at times
there may be a small difference in the second decimal place.

232. Example.—This method for checking the mean intervals is
illustrated below for Seattle, Wash., for the month of January 1928,
these intervals having been computed in the regular manner in fig-
ures 24 and 25. This being a 31-day month, the number of items
included in each summation must be 60. 5

Sum of times of 60 transits, January 1928__________--_________________ 727.6
24X (7-+22), single transits occuring Jan. 7 and 22__________________ 696
DB) T PETE TA Ce (Dy) jo pe EA ON CC TLE Tc Set lea Rae ee NS een 31.6
Sum of times of 59 high waters, January 1928__2_____________________ 665. 7
Time of high water Dec. 31, 1927, for deficiency__-__________-_________ 23. 2
SUMUOLcCiImMes OL GO nish wwaterske= se se ee eee 688. 9
24 X (1+15+30), single high waters occurring Jan. 1, 15, and 30________ 1,104 ~
TELE esiveca 17S )) Sasa Rs RY oi as es —415.1
Sum of times of 60 low waters, January 1928_____________-_ eed 745.6
24x (9-++24), single low waters occurring Jan. 9 and 24__________.____- 792
Write rem cess (Ms) es wis ie ira ee eee a 2 eas Sa —46. 4

(H—T)=63=— 446. 7+60=— 7.44 hours for high-water interval.
(L—T)+60=—78.0+-60=—1.30 hours for low-water interval.

Applying the tidal period 12.42 hours to each of the above negative
intervals, we have 4.98 and 11.12 hours, respectively, for the unre-
duced high and low water intervals. It will be noted that these
results agree exactly with those obtained in figure 25 by the regular
computations. An exact agreement, however, is not always to be ex-
Late but the results will usually agree within 1 or 2 hundredths of
an hour.

233. Lunitidal intervals from Form 248.—For a short series of
observations the lunitidal intervals can be computed in Form 248

72 U. S. COAST AND GEODETIC SURVEY

(fig. 27) from a comparison with simultaneous observations at a
standard station suitably situated. The form provides for the com-
putation of the local intervals and contains an explanation of its use

DEPARTMENT OF COMMERCE
U. 3. COAST AND GEODETIC SURVEY

Form 248

pi 2 ia TIDES : Comparison of Simultaneous Observations
‘A) Subordinate station... ABAcortes, Weshis 6 nese. i rene Eats co cmaeee Long. 222° 36" We
(B) Standard etation.......5 h. i TESS re Se a
Cine! of partys >> 52s ee ee ee ee Time Meridian: (A) -.-

(B) STATION. (Ay-a) sds (A) STATION. (A)=(8)
ie aoe Ete Pe W's Sania aifference: al Height of— Height of— Height difference,
a Fo "Hw.. | bw. Hw. LW. Hw. LW. Hw. Lw.

Hours.

Hours, | Hours. Hours. Hours. Fect. Fect. Feet. Feet, Feet, Feet.

Wea =O56= ee Ose! | =0.5_ egrets fae he 24° _ 18.3) eed ana een ei: a BE
18.8 | 13.0 || -0.5 | -0.7 |_25.24 18.7 |. 19.2731.0 4.071 7.7
8.0 | 22.6] 15.0") 18.2! 7.0” 4.4] 8.0

Haste as.| O21
18.5 | 12.3

4
22.S | 15.2”! 18.1! 7.5”%l 4.8 | 79

22.94% 20.7 | 18.4713.5 4.5"_7.2

20.5 _|_16.0| 21.5 22.1| 20.9 | 17.1/13.6 5.C |_7.

‘6.47
Rte i|12.5 14.01 12.0 ).4.3 1.0.51 -0.2 122.77 .15.61 180° 8.271 4.71704,
18.0] 22.9 | 17.6 Cc 16.1/13.6 6
Ss.

HHW. 7|_ HIW 7| _HHW. 7,
oa ESE Se Deter a cee Ce a Eni ee es) 160.6/140.2 | 128.8) 89.9 | 31.8) 50e4

22.94 20.C4 16.40) 12.84||" 4.54) 7.20
| io ji) Sole LHW. 6) LLw. 7 L

| \ oS esr Y3< pe
aaa was 0.3/0.4 | 23.2") _ 19.6 18.3) 12-7 4.6 Tel

LLw. | LHW.

LEW, 6 | va|| Law. @,_LLw.
-1.2 |-5.0 ||152.0/109.C | 103.6) 55.0 || 28.4 | 54,0 |

=Mean difference in time of high and low water respectively.

&... =Correction for difference in longitude. (Table on back of form.)

(3)= -O.11 70.38 (1) 4.(2)=Mean difference in high and low water intervals, respectively.
Feet. Feet.
(4)=..2e294..=Mean HHW height at (A). (5) = 20-04. —Mean HLW height at (A).

(6)=..@0+00..=Mean LHW height at (A).
(8)=....0.694.=(4)—(6)=2DHQ at (A).

=Mean LLW height at (A).
=(5)—(7)=2DLQ at (A).
=3{(5)+(7)]=Mean LW height at (A).
=}{(10)-+(11)]=MTL at (A).

=Mcan HLW difference.

=Mean LLW difference.
=(15)—(17)=2DLQ differencé. |
=4{(15)+(17)]=Mean LW differenca.

(12)=....4¢.67..=(10)—(11)=Mn at (A).

(14)=....4024.=Mean HHW difference.

(16) =... 4e73..=Mean LHW difference.
--=(14)—(16)=2DHQ difference.
=4{(14)+(16)]=Mean HW difference.

2+82..=(20)—(21)=Mn difference. (23)=__6-C5_—31(20)+(21)]=MTL difference.
(24)=O- 623 — (12)-+-{(12)—(22)]=Mn ratio. (25)=0-832 _—(8)-+1(8)—(18)]=DHQ ratio.
: (26)=-0+ 898 _—(9)+{(9)—(19)]=DLQ rain, aad ae
Results from comparison of Stations A and B. 5 oe | & DLQ. |

Accepted values for standard station, from obs. for 19 years 4248 405.70. 14.C6l_ 7.64

Mean LW on staff at subordinate station=MTL—4Mn ae feet.
Mean LLW on staff at subordinate station=MTL—}Mn—DLQ=..15.19.._ feet.

Cc. D. A. 5 - 12 - 27 rend ing F.J.H. 5 - 12 - 27
Reema DY ae eed enna eae Nenhied by == oe ee as

FIGURE 27.—Form 248, comparison of simultaneous observations.

on the back. The table on page 73 may be used to obtain the correc-
tion for difference in longitude. The Greenwich lunitidal intervals
at the subordinate station may be obtained by the simple application
of the mean differences in the time of high and low water to the
corresponding Greenwich intervals at the standard station.

MANUAL OF TIDE OBSERVATIONS 73

Table for reducing Greenwich intervals to local intervals

>) =| D = o ga o i=} o i=} o q o q ) q
ve) 3 us} i us) S ao iS ro 3 as} S us) iS} cS S
3 Ss =! Ss =| = = 3S 5} S = 3 =] BS 5} 6
Boi Se cee ee) iesk ee obese imate! peal nena) [Renee msigan bonehead (Sta
= ba a bay FS] 5 | by q bay ] bay a ba a ba
(o} Ss (o} o fo) ic) ° fo) o o i>) o fo} o lo} So
4 1S) =) iS) 4 1S) =) iS) = iS) =) iS) 4 oO 4 1e)

: TIour } ’ | Tour} ° IIour } ° | Hour | ° | Hour 2 TIour 2 TIour ° | Hour
1 | 0.001 } 31 | 0.036 1 0.069 } 31 2.139 f 61 4. 209 91 6. 279 | 121 8.349 | 151 | 10.420
2 002 } 32 037 2 .188 | 32 2.208 | 62 4. 278 92 6.348 | 122 8.418 ] 152 | 10.489
3 .003 } 33 038 3 . 207 | 33 2.277 § 63 4.347 93 6.417 } 123 8.487 9 153 | 10. 558
4 .005 f 34 039 4 . 276 | 34 2.346 | 64 4.416 94 6.486 | 124 8.556 f 154 | 10.627
5 .006 f 35 040 5 345 ff 35 .415 | 65 4, 485 95 6.555 9 125 8.625 | 155 | 10.696
6 .007 | 36 041 6 414 | 36 . 484 | 66 4, 554 96 6.624 # 126 8.694 | 156 | 10.765
7 .008 } 37 043, ili 483 | 37 553 | 67 4. 623 97 6.693 | 127 8.763 | 157 | 10. 834
8 .009 | 38 044 8 552 9 38 . 622 | 68 4.692 98 6. 762 | 128 8.832 | 158 | 10.903
9 -010 } 39 . 045 9 .621 | 39 | 2.691 | 69 4.761 99 6.831 | 129 8.901 | 159 | 10.972

10} .012 4 40! .046 | 10 . 690 | 40 .760 1 70 | 4.830} 100 | 6.900} 130 | 8.970 f 160 | 11.041

.829 ] 71 | 4.899 | 101 | 6.969 | 131 | 9.039 9 161 | 11.110
.898 | 72! 4.968 | 102! 7.038 } 132 | 9.108 | 162 | 11.179
.967 7 73 | 5.037 7 103 | 7.107 | 1383 | 9.177 | 163 | 11.248
.036 } 74 | 5.106 § 104 | 7.176 | 134) 9.246 } 164 | 11.317
-105 } 75 | 5.175 f 105 | 7.245 f 135 | 9.315 | 165 | 11.386

2

2

2

2

2

2

2

2.

2.

3

3
1 3.174 ] 76 | 5.244] 106 | 7.314 | 136) 9.384 } 166 | 11.455
: : ik 3.243 | 77 | 5.313 | 107 | 7.383 7 137 | 9.453 7 167 | 11.524
18} .021 $48 | .055] 18) 1.242] 48| 3.312] 78 | 5.382 7 108 | 7.452 | 138 | 9.522 f 168 | 11.593
1 3
1 3
3
3
3
3
3
3
3
4
4
4

3.381 | 79 | 5.451 J 109} 7.521 | 139 | 9.591 | 169 | 11.662
-450 | 80 | 5.5209 110 | 7.590 | 140 | 9.660 } 170 | 11.731

.519 | 81 | 5.589 J 111 | 7.659 } 141 | 9.729 7 171 | 11.800
.588 | 82) 5.658} 112, 7.728 | 142 | 9.798 | 172 | 11.869
.657 | 83 | 5 727 $113 | 7.797 | 143 | 9.867 § 173 | 11.938
.725 | 84 | 5.796 | 114 | 7.866 | 144 | 9.936 | 174 | 12.007
.795 | 85 | 5.865 J 115 | 7.935 | 145 | 10.005 f 175 | 12.076

864 | 86 | 5.934 } 116 | 8.004 f 146 | 10.074 f 176 | 12.145
.933 | 87 | 6.003 } 117 | 8.073 } 147 | 10.143 } 177 | 12.214
.002 | 88 | 6.072} 118 | 8.142 | 148 | 10.212 } 178 | 12.283
.071 | 89 | 6.141 } 119 | 8.211 7 149 | 10.281 | 179 | 12.352
.140 | 90 | 6.210 | 120 | 8.280 7 150 | 10.351 | 180 | 12.421

HEIGHT REDUCTIONS

234. Form 138, on which are tabulated the high and low waters,
provides for the regular computation each month of certain tidal
planes and ranges. Mean high water (HW) and mean low water
(ZW) for each month are obtained by summing all the high waters
and all the low waters and dividing by the number of observations,
the latter being indicated by small figures just above the sum. The
means, written below the sums, should be carried to two decimal
places. The mean range (d/n) is obtained by subtracting the mean
of the low waters from the mean of the high waters. The mean tide
level (Z7L), which is also known as half-tide level, is obtained by
taking the half sum of the mean of the high waters and the mean of
the low waters.

235. For stations on the Pacific coast, the means of the higher high
waters and of the lower low waters and the diurnal inequalities
should also be obtained. The higher of the two high waters and
the lower of the two low waters of each day of the month are first
indicated by a check mark. If the two high or two low waters on
the same day are equal, either may be selected as the higher high
or lower low water. When only one high or one low water occurs
on a calendar day, by reason of one of the tides having occurred

after midnight and therefore on the next calendar day, the single
tide is to be checked if the tide just above it is unchecked, otherwise

74 U. S. COAST AND GEODETIC SURVEY

it should not be checked. If, however, the tide has become diurnal
and only one high and one low water occur during the tidal day, these
should both be checked. The checked heights are to be summed
separately for the high and low waters, the sums being entered in the
spaces provided, with the number of observations written above in
small figures. The mean of the higher high waters (HH W) and the
mean of the lower low waters (LZ W) are then obtained, each result
being carried to two decimal places.

236. The diurnal high water inequality (DH@) is obtained by sub-
tracting the mean of all high waters from the mean of the higher
high waters, and the diurnal low water inequality (DZL@Q) is obtained
by subtracting the mean of the lower low waters from the mean of
all low waters. , :

237. Correction for longitude of moon’s node.—There is a long-
period variation in the range of tide due to changes in the inclination
of the moon’s orbit to the Equator. When the longitude of the
moon’s node is 0° the inclination of the orbit to the Equator is at a
maximum and the average range of the four tides of the day is less
than usual; and when the longitude of the moon’s node is 180° the
inclination is at a minimum and this range of tide is greater than
usual. The time required for the longitude of the moon’s node to
pass through the cycle of 360° is approximately 19 years. In addition
to the variations caused by changes in the longitude of the moon’s
node, the diurnal inequalities are subject to variations from month to
month caused by changes in the declination of the sun. Because of
these variations certain factors are necessary in order to reduce to
mean values the ranges and inequalities obtained from short series
of observations. At the primary tide stations, the correction need
not be applied to the results for each individual month, it being
sufficient to correct the annual means as needed. If the series
of observations extends over a period of 19 years, the factors: are
unnecessary.

238. Tables containing the factor (Mn) for reducing the observed
range of tide to its mean value and factor F for reducing the diurnal
inequalities DHQ and DL@ to their mean values are given on page
75. These tables cover the years 1941 to 1950, inclusive, and are
based upon tables 6, 14, and 32 of Harris’ Manual of Tides. Similar
tables covering the years 1891 to 1950, inclusive, will be found in
Special Publication No. 135, Tidal Datum Planes. The factor F (Mn)
was computed for the middle of each calendar year, but as the factor
changes very slowly, the tabular value may be used for any month
in the year without material error. The factor #', was computed for
the middle of each calendar month. The table includes also the mean
of the monthly factors for each year which may be taken as the factor
for correcting the yearly inequalities to their mean values.

239. The factor /(M/n) depends not only upon the year of observa-
tion but also upon the relation of the diurnal to the semidiurnal wave
in the locality, this relation being expressed approximately by the

formula ee or if harmonic constants are available by

the ratio £ ee For stations along the Atlantic coast of the United

States from Maine to Florida this relation is generally small, and, if

MANUAL OF TIDE OBSERVATIONS 15

not already computed, may be assumed to be less than 0.2 without
material error. For stations along the Gulf of Mexico from Key
West to the Rio Grande the mean range of tide is very small and the
correcting factor may be omitted. For stations on the Pacific coast
the value of the ratio may be computed either from the inequalities
or from the harmonic constants but need be carried to only one
decimal place. If this ratio is larger than 2.0, no correction need be
applied to the mean range.

Factor F (Mn).—For reducing the observed range of tide to its mean value

Hoo 900

1
|
eRe OO): 2 LQ) | i941 | 1942 | 1943 | 1944 | 1945 | 1946 | 1947 | 1948 | 1949 | 1950
Din Ce eee ae 0.971 | 0.972 | 0.977 | 0.985} 0.995] 1.005] 1.014] 1.023] 1.028] 1.029
Big OM mae INES 9797] 2073 97 .985 | .995| 1.005] 1.014] 1.022] 1.027] 1.028
levi OSGeo eee gene 974] .975 979 | .986 | .995 | 1.005|- 1.013 | 1.021 | 1.025 | 1.026
Hitt OB Ase eo 98 976 | .977 981! .987 | .996| 1.004] 1.012] 1.018] 1.022] 1.023
igi a 979 | 980 984} .990} .996| 1.004] 1.010] 1.016! 1.019 | 1.020
Miteprigg nese tye 983 | . 984 987 | .992| .997] 1.003] 1.008] 1.013 | 1.016 | 1.017
Wetopdea .987| .988| .991| .994] .998] 1.002] 1.006] 1.009] 1.012] 1.013
MattoueGen sess ee 992 | . 993 994} .997| .999| 1.001 | 1.004] 1.006] 1.007] 1.008
Wate Wee sei 2. 999 | .999 999 | .999 | 1.000} 1.000] 1.001 | 1.001} 1.001 | 1.001
Hey DOP ees. eS 1.006 | 1.006 | 1.005! 1.003] 1.001 | 0.999] 0.997| 0.996] 0.995} 0.994
i
i
Factor F;.—For correcting DHQ and DLQ_

Month 1941. | 1942 | 1943 | 1944] 1945.| 1946 | 1947_| 1948 | 1949 | 1950
Vanianyssees en 1.01 | 1.01 | 0.98 | 0.95 | 0.90 | 0.87 | 0.83 | 0.81 | 0.79 0.79
Feburary ..____.__- fOr eteate eteS) es) eeIvOG he lO |: 0: 9741 0894.71 10591 0. 90
Wiech wale ws PAG) 45> 12139) le LS) 1s) en Gy) TAL] | te 07 | 21604 1.03
Aerie 0h AS bil e34 aLSS (dates Mon Qin 1 14ele 0.08 to 103%, 1.0051) 40.98 0.97
Mirage 1.07 | 1.07 | 1.03 | 0.99 | 0.94 | 0.90 | 0.87 | 0.84 | 0.83 0. 82
STU) GAS 0.96 | 0.96 | 0.93 | 0.89 | 0.85 | 0.82 | 0.79 | 0.77 | 0.76 0.76
lye eee 1.00 | 0.99 | 0.96 | 0.92 | 0.88 | 0.84 | 0.82 | 0.80 | 0.78 0. 78
Rticast rege ts TROON 10 dt 5 ol 09uel 04 590: 99 | (0595 1010593) | 0. 9F 0.91
September_._______ 1.47 | 1.43 | 1.36 | 1.29 | 1.20 | 1.14 | 1.09 | 1.06 | 1.04 1.04
Octobersses 2515s 1.33 | 1.30 | 1.24 | Tel7 2 fe Og ae 05: (5 101 1210; 98.4 |) 096 0.96
November_____.-_- 1.07 | 1.04 | 1.00 | 0.96 | 0.91 | 0.88 | 0.85 | 0.33 | 0.82 0.82
December_________- 0.96 | 0.94 | 0.91 F 0.87 }| 0.83 | 0.80 | 0.78 | 0./6 | 0.76 0.76

Mean. ___..- | 11760)“ 1.161, | as178 | 1.065 | 1.007 | 0.962] 0.925| 0.899] 0.882] 0.878

240. Comparison of simultaneous observations.—For a short
series of observations, reduction by comparison of simultaneous ob-
servations is generally the best method provided there is a suitable
standard tide station from which the necessary simultaneous data may
be obtained. For this purpose the standard station should be so situ-
ated that the effects of meteorological conditions may be expected to
be similar to those at the station for which the results are sought. A
reference to the use of this method in computing lunitidal intervals
has already been made in paragraph 233. The process is especially
valuable in the reduction of the heights of the tide. For observations
covering a period of less than 1 month, form 248 is generally used, but
for series extending over longer periods, form 657 for the comparison
of monthly means will be found more convenient. The latter form
is self-explanatory.

76 3 U. 8S. COAST AND GEODETIC SURVEY

241. Form 248 (fig. 27) is designed to bring out the individual dif-
ferences between the tides at the two stations compared, the accuracy
of the resulting means depending somewhat upon the uniformity of
these differences. If any single difference varies greatly from the
apparent average of its group, and an examination of the original
data fails to show an error, the difference should be rejected from the
sum and the fact indicated by encircling the rejected value. For
stations where the diurnal inequalities are desired, the higher high
waters, lower high waters, higher low waters, lower low waters, and
their differences are summed and averaged separately; and all the
spaces in the bottom portion of form 248 are filled in. If the diurnal
inequalities are not. needed, as is generally the case for stations on the
Atlantic coast, all high waters and corresponding differences may be
summed and averaged without distinction and likewise all low waters
and differences, the notations at the bottom of the columns being
corrected to read HW and LW. The mean high and low water
heights for (A) station will then be entered directly as items (10) and
(11) in the form, and the mean differences from the last two columns
as items (20) and (21). The form will then be completed as far as
necessary to obtain the results desired.

TIDAL DATUMS

242. A tidal datum is a plane or surface which may be defined by
the tides and which is used as a reference for heights or depths. The
principal tidal datums now in use by this Survey are (1) mean sea
level, the datum of the first-order level net. and in general use as a
reference for heights; (2) mean low water, the datum of soundings on
charts of the Atlaritic coast of the United States; (3) mean lower low
water, the datum of soundings on charts of the Pacific coast of the
United States, Alaska, Hawaii, and the Philippine Islands; and (4)
mean low water springs, the datum for the Pacific coast of the Canal
Zone and in more or less general use as the datum of charts published
by foreign countries.

243. Mean sea level.—Mean sea level may be defined as the average
height of the sea for all stages of the tide. It is obtained by averag-
ing “the hourly heights as tabulated in form 362. The heights in this
form are summed both vertically and horizontally, and the total page
sum covering 7 days of record is entered in the lower right corner of
the page. For a continuing series of observations, the mean for each

calendar month is obtained by combining all the daily sums for the

month and dividing by the total number of hours as indicated at the
bottom of the form for months of different lengths. The monthly
mean carried to two decimal places is entered at the bottom of the
sheet containing the record for the last day of the month.

244, Form 472a provides for the compilation of the monthly means
and the computation of the yearly means from the same. It also
provides for an accumulative mean combining all yearly means up
to date. The precision of an independent determination of mean sea
level depends‘largely upon the number of years of observations. In
general a series covering not less than 3 years should be obtained for
an independent determination of the datum. For a shorter series of
observations, the sea level as directly obtained should be reduced by
comparison with simultaneous observations provided there is a pri-

Ne

MANUAL OF TIDE OBSERVATIONS 17

mary tide station suitably located from which the necessary data for
the comparison may be obtained. Form 657 for the comparison of
monthly means may be used for this reduction.

245. The name “mean sea level” should be applied only to the
datum derived from observations taken on the open coast or in adja-
cent waters having free access to the sea. The average of the hourly
heights taken in a river is called “mean river level” and is higher than
the:mean sea level because of the river slope. The plane of half-tide
level derived from the high and low waters approximates very closely
to the mean sea level or mean river level determined from the hourly
heights. For any one station the difference remains nearly constant
from month to month and affords a convenient check on the work
when both planes are computed.

246. Mean low water.—Mean low water is generally adopted as a
datum for hydrographic operations along the Atlantic coast of the
United States. It may be defined as the mean of all low waters
over a considerable period of time. The datum may be derived inde-
pendently from a long series of observations, but from a short series
can best be obtained from a comparison of simultaneous observations
ata nearby standard station (pars. 240-241). For the longer series,
the range of tide is corrected for the longitude of the moon’s node
(pars. 237-239) , and one-half of the corrected range is then subtracted
from the half-tide level to obtain the corrected mean low water.
When reduction is made by a comparison of simultaneous observa-
tions both range and: half-tide level are subject to correction, and the
corrected mean low water is obtained by subtracting one-half the cor-
rected range from the corrected half-tide level.

247. Mean lower low water.—This datum is generally adopted for
hydrographic operations along the Pacific coast of the United States
and may be defined as the mean of the lower of the two low waters
of each day over a considerable period of time. When determined
independently from a long series of observations, the mean range
and diurnal low water inequality must be corrected for the longitude
of the moon’s node as explained in paragraphs 237-239. The cor-
rected mean lower low water is then obtained by subtracting from
the half-tide level height the sum of the corrected half range and the
corrected diurnal low water inequality. For a short series of obser-
vations, the mean lower low water datum may be computed by means
of form 248 for the comparison of simultaneous observations (pars.
240-241).

248. Mean low water springs.—While datums approximating this.
plane have been rather generally used by foreign countries, its use by
this Survey is limited to the Pacific coast of the Panama Canal
Zone. The datum may be defined as the mean of the low waters of
the spring tides which occur within a day or two after the moon is
new or full, and may be obtained by subtracting one-half the spring
range of tide from the half-tide level. Because of the limited use of
this datum it is not regularly obtained at all tide stations. The most
satisfactory method of obtaining the spring range of tide is from an
harmonic analysis, an involved process not adapted to field use.
From such analyses it has been found that the ratio of spring range to
mean range is fairly constant over wide areas. For Balboa, Canal

735445 O- 47-6.

78 U. S. COAST AND GEODETIC SURVEY

Zone, the ratio is 1.26, and this factor may be applied without mate-
rial error to the corrected mean range at any station on the Pacific
coast of the Canal Zone to obtain the spring range of tide. There-
fore to obtain the datum of mean low water springs at any station
in this vicinity, first compute the corrected mean range and half-tide
level by methods already described. The datum may then be ex-
pressed by the formula “Mean low water springs=half-tide level—
0.63 X mean range of tide.” The factor may vary in other locations.

TIDE REDUCERS FOR SOUNDINGS

249, After the datum or plane of reference has been derived, the
tide reducers for soundings are obtained by subtracting the reading
corresponding to the datum from the recorded heights of the tide
taken at intervals during the time of the soundings. The differences
will in general be positive except when the tide falls below the datum.
The positive differences are to be subtracted from the soundings, but
when entered in the sounding books, the minus sign is usually omitted
for convenience. When the tide falls below the datum, the differ-
ence is to be added to the soundings and this difference must always
be prefixed by a plus sign when entered in the sounding record. De-
tailed instructions pertaining to the application of the tide reducers
to the soundings will be found in the Hydrographic Manual (Special
Publication No. 143).

250. Graphic method.—When the reduced soundings are to be
given in integral feet, reductions for tide may be made easily and
rapidly from either the standard automatic tide-gage or the portable
automatic tide-gage marigrams by a graphic method described in the

following paragraphs. In using this method care should be taken, -

however, to avoid confusion as to the times or the heights of tides,
especially when reading from a portable automatic tide-gage record
on which the curves representing the tide for different days are often
close together. At times it may be necessary to strengthen a faintly
traced curve of the record so that it may be sufficiently bold to be
readily seen through the transparent graphic scale.

251. This scale is constructed on transparent tracing cloth or trac-
ing paper by ruling a series of horizontal lines spaced at intervals
representing feet in the same height scale as used on the marigram
and a series of vertical lines spaced at intervals representing hours
in the same time scale as used on the marigram. For the portable
tide-gage records the vertical lines may usually be omitted. The
horizontal lines are numbered upward from the bottom of the scale
+3, +2, +1, 0, —1, —2, —3, —4, etc., according to the range of
tide, small figures being used for this numbering. A _ horizontal
line in red ink is now drawn on the tracing at the scale reading corre-
sponding to the value of the formula «—y—0.7 ft., where 2=height
of piane of reference (LW or LZW) above zero of the tide staff,
y=height of datum line of marigram as referred to zero of tide staff,
and the value “0.7 ft.” represents the fraction at which the reduced
sounding changes by an integral foot.

For the standard tide-gage record the value of “y” is the corrected
scale, setting as computed on form 455 (fig. 23). For the portable
tide-gage record the “0” of the marigram record may be taken as
the datum line and “y” becomes equal to zero. The red line will be

>
ee eS

MANUAL OF TIDE OBSERVATIONS 79

above or below the zero of the scale, according to whether the value
from the formula is negative or positive.

252. The graph is laid over the marigram with the red horizontal
line in coincidence with the datum line if a standard gage record, or
with the scale zero of the portable gage record (assuming that this
corresponds to the tide staff zero), and the hour marks of the graphic
scale and of the marigram in coincidence. In case a time allowance
is to be made, the graph is shifted to the right or to the left, according
to the amount of time allowance.

253. The spaces between the horizontal lines are numbered. with
_ figures somewhat larger than those used for the scale lines, beginning

“EE aie
Be i

ST4

=6

-5

-¢g
—35
-3

Re.

-2
~/

ae,

Ficurn 28.—Graph for obtaining tide reducers directly from mariram.

with “0” for the space just above the zero (0) line and numbering
consecutively above and below this space, using the plus (+) sign
before the numerals for the lower spaces. These numbers will be
the tide reducers to integral feet for all portions of the tide curve
falling within the space so marked.

254. An example of a graphic scale is given in figure 28. For the
standard-gage record, assuming that “a,” the height of the plane of
reference above the zero of the tide staff, is 2.3 feet, and that “y,”
the height of the datum line above the zero of tide staff, is 5.1 feet,
the formula »—y—0.7 gives —3.5 feet as the scale reading at which
the red line is drawn, this line to be placed in coincidence with the
datum line of the marigram when the scale is in use. For the porta-
ble gage assuming that “a,” the height of plane of reference on tide
staff, is 2.9 feet, and taking “y” as zero, the formula gives +2.2 feet
as the scale reading for the red line, which is to be placed in coinci-
dence with the zero of the marigram scale when in use. ;

80 U. S. COAST AND GEODETIC SURVEY,

255. Time allowance.— When there is much difference in the time
or height of the tide at the place of sounding and at the tide gage,
allowance should be made in the reduction of the soundings. The
difference may generally be estimated from observations made at
several stations in the vicinity of the work, but when it has been
impossible to establish more than one tide station in the locality, the
following formula may be useful in estimating the velocity of a
progressive tidal wave and enable one to obtain the approximate
difference in the time of the tide: »= /gd=9.67/d feet per second,
when g=382.17 feet per second and d=depth of water for the average
cross section between stations, in feet.

In order to convert feet per second into nautical miles per hour,

3600
multiply by §9g9 =9-.592, and we have

v= 3.367/d nautical miles per hour
The time required for the tide wave is

6080 17.87

eS 60X5.672/d eer minutes per nautical miles.

a 5280 y doe ee
=60X5.672yd oA i minutes per statute mule.

For convenience the following brief table is given:

t

_ Time required for the tide wave to travel

1 nautical | 1 statute 1 nautical | 1 statute
~Depth mile mile Depth mile mile
Fathoms Minutes Minutes Fathoms Minutes Minutes ¢

1 (i 6. 9 4 1
2 Ee 4.5 10 4 2.0
33 42 Baie VS 1. 9 1.6
4 3. 6 a2 20 1. 6 174
5 35 2. & 30 Wee LZ
6 3. 0 2.6 40 IZ 1.0
7 2.8 2. 4 50 1.0 0. 9
8 2aG Ae 60 0. 9 0. 8

\

256. For offshore areas where the continental shelf is broad and
~ the tidal wave approaches parallel to the coast the tide will arrive off-
shore earlier than inshore, and for an accurate reduction of soundings
a time correction will be necessary. This time correction can be
applied from the shore outward by taking sections where the time of
the tide averages 15 minutes, 30 minutes, 45 minutes, 1 hour, etc.,
earlier than at the tide station. These sections can be determined
by means of the above table, giving the time required for the tide
wave to travel at different depths.

257. Height Allowance.—When the tide station used for deriving
the tide reducers is made to cover an area over which the range of

MANUAL OF TIDE OBSERVATIONS 81

tide varies height corrections will be necessary. For the adjustment
of tide reducers between stations along the coast and inside waters
it will be found convenient to divide the area covered into sections.
Each section may cover an area in which the variation in range of
tide does not exceed three-tenths of the unit used for the tide reducers;
that is, 0.3 foot, 0.9 foot, and 1.8 feet when the reducers are entered in
units of a foot, half fathom, and whole fathom, respectively. Take,
for example, an area 15 miles long with no narrow restrictions and
with depths of 3 fathoms or less. At one end is station A, where tide
observations have been taken during the time of the soundings; at
the other end is station B, where the time differences and ratio of
ranges have been determined. The mean range of tide at station A
is 2.6 feet, at station B 3.5 feet, the difference in range being 0.9 foot.
The difference in the time of tide between the two stations will be
assumed to be 45 minutes. The area should, therefore, be divided
into four sections. Assuming the tide to increase uniformly with the
distance, the first section will be 214 miles long and the height and
time of the tide the same as at station A. The second section will
be 5 miles long and the height of high water 0.3 foot greater and time
15 minutes later than at station A. The third section will also be
5 miles long and the height of high water will be 0.6 foot greater and
the time 30 minutes later than at station A. The fourth section will
be 214 miles long and the height of high water 0.9 foot greater and
the time 45 minutes later than at station A.

258. The tide reducers for soundings in each of sections 2, 3, and 4
may be derived directly from the curves for station A by reading
the curves at points which are as many minutes earlier than the times
of the soundings as there are minutes in the time allowance for each
section and multiplying the readings by the ratio of ranges. For
offshore areas the range of tide may generally be taken the same as at
the nearest point along the coast.

TEMPERATURE AND DENSITY OBERVATIONS

259. At some tide stations, temperature and density observations
are taken incidentally to the main purpose of securing the tide record.
These observations are taken daily at the time the tide station is
visited and are recorded in form 457 (fig. 29). The water to be
tested must be dipped from just below the surface. The order of
procedure in making the observations is indicated in detail on the
back of the form. The record is to be forwarded to the office at the
close of each calendar month.

TEMPERATURE

260. In general, the temperatures will be given to the nearest tenth
or nearest half degree in the centigrade scale. The scale of the
centigrade thermometer is usually divided into degrees and half
degrees. If a Fahrenheit thermometer is being used, it should be
indicated by the letter F at the top of the column containing the
temperatures. The record includes the temperature of the outdoor
air, the temperature of the sea water immediately after it has been

Ni oy

82 U. S. COAST AND GEODETIC SURVEY

drawn from the harbor or sea, and the temperature of the water in the
jar in which the hydrometer is floated taken at the time of the hy-
drometer reading. The temperature of the sea water must be taken
in the bucket immediately after the water has been drawn and before

Form 457 i
Pe. Cons Ano GEODETIC SuRvE DENSITY AND TEMPERATURE
Pave Dees ey Bete ADO oe ea eee eee
Station ..............- Atlentic Gity (Steel Pier), Ne Je 2. Long. ... .74°..25°
Month ......... HUgUSE.._.... Year .....4934... Obseroer 4 dee ee etts
Degreas. Date. Date.
Warmest Sea Water -..........- var tee paella des Auga20tb *Heaviest Sea Water. Augas..l lta... ae
Coldest Sea Water .............. Taek ees éUG-_ 11th -£UGa..21sh..:
ean TEMPERATURE : %
TT cach ae ; 5
1 {12 42 | 26.2 | 16.8 | 18.1 | 13035] 1.0
2) | dias | Ve5s2= eer sal Zes0
3 |16 10 | 21.7 | 18.1 | 19.0
4 }18 15 | 26.9 | 17.4 | 19.5
8 (4g 125.1 SreSe8s0" ||" Toss
6 |12 11 | 20.2 | 19.5 | 19.6
7 | 885 |:20.2 | 19.4 | 19.6
s |12 02 | 24.2 | 21.4 | 21.9
hed age Waa pre-e foal pip iets are]
10 |12 04 | 22.3 | 22.5 | 22.6
1/14 00 | 21.6 | 15.6 | 17.5
12 |13 52 | 21.8 | 19.0 | 19.6
13 |13 51 | 21.9 | 18.9 | 19.4
4/12 22 | 23.4 | 22.1 era
15 |12 55 | 25.9 | 20.7 | 212.9
16 |12 23 | 22.8 | 20:9 | 21.4
17 ¢|'12650 | 21.0: 21.65 '|| “2Le6
18 |16 41 | 25.5 | 22.7 | 23.4
19 | 9 27-| 21.9 | 22.4 | 22.5
20 112 13 | 29.0 | 22.4 | 23.3
zt 122 29 | 20.7! 22,0 || 21.7
22 |12 47 | 22.1 | 22.2 | 22.5
23 112 25 | 24.0 | 22.6 | 23.0
24 112 36 | 24.C | 22.8 | 23.3
25 |12 50 | 27.2 | 23-3 | 24.0
26 | 9 50 | 22.8 | 22.2 | 22.5
27 |12 58 | 23.8 | 22.5 | 22.6
28 |12 21 | 20.2 | 22.4 | 22.4
209 111 48 | 20.2 | 21.2 | 21.0
30 |12 10 | 18.6 | 20.5 | 20.4
31 }12 03 | 20.5 | 20.4 | 20.2
Sum 644.6
| Mean 20.79
* Not to be filled out by observer. date
M418 (oveR) Density reduced by. CalaQas..NOVe 30,1954.

-

FIGURE 29.—Form 457, density and temperature. ~

it has had time to be affected by the temperature of the surrounding
air. The temperature of the water in the jar, which is used to reduce
the hydrometer reading to a standard temperature, is not taken until
there has been time for the water, the jar, and the hydrometer to
become adjusted to a uniform temperature. Sometimes the column
of mercury in the thermometer may become separated in sections,

MANUAL OF TIDE OBSERVATIONS 83

causing erroneous readings. When this occurs the broken sections
can usually be jarred together by hand. Readings of different ther-
mometers should be occasionally checked by comparison with
each other.

261. The table below, which gives equivalent readings in centi-
grade and Fahrenheit thermometer scales, affords a convenient means
for converting the readings from one of these scales to those of the
other scale between the limits —20° and 39.5° C. Beyond these limits
the- following formula may be used: Fahrenheit reading=32° +
(centigrade reading X 1.8).

Conversion of centigrade to Fahrenheit scale

°C °F OA Oks Oy oO cents AC dl apo 2a): Shy. 2 (Cs 2a
—20.0 —4 || —10.0 14 0.0 32 10. 0 50 20.0 68 30. 0 86
—19.5 —3 —9.5 15 0.5 33 10.5 51 20.5 69 30.5 87
—19.0 —2 —9.0 16 1.0 34 11.0 52 21.0 70 31.0 88
—18.5 —1 —8.5 17 1.5 35 11.5 53 21.5 71 31.5 89
*—18.0 0 || *—8.0 18 *2.0 36 *12.0 54 *22. 0 72 *32. 0 90
—17.5 0 —7.5 18 2.5 36 12.5 54 22. 5 72 32.5 90
—17.0 1 =e) 19 3.0 37 13.0 55 23.0 73 33. 0 91
—16.5 2 —6.5 20 3.5 38 13.5 56 23. 5 a 33. 5 92
—16.0 3 —6.0 21 4.0 39 14.0 57 24. 0 75 34.0 93
—15.5 4 —5.5 22 4.5 40 14.5 58 24. 5 76 34.5 94
—15.0 5 —5.0 23 5.0 41 15.0 59 25. 0 77 35. 0 95
—14.5 6 —4.5 24 5.5 42 15.5 60 Plast |p as} 35. 5 96
—14.0 7 —4.0 25 6.0 43 16. 0 61 26. 0 79 36. 0 97
—13.5 8 —3.5 26 6.5 44 16.5 62 26. 5 80 36.5 98
—13.0 9 —3.0 27 7.0 45 17.0 63 27.0 81 37.0 99
—12.5 10 —2.5 28 7.5 46 17.5 64 27.5 82 37.5 100
*—12.0 10 || *—2.0 28 *8.0 46 *18.0 64 *28. 0 82 *38. 0 100
—11.5 11 —1.5 29 8.5 47 18.5 65 28. 5 83 38. 5 101
—11.0 12 =1.0 30 9.0 48 19.0 66 29.0 84 39.0 102
—10.5 13 —0.5 31 9.5 49 19.5 67 29,5 85 39.5 | 103
| | !

* When two values in the column for the centigrade scale are given for the same whole degree in the
Fahrenheit scale the one indicated by the asterisk should be taken when converting the Fahrenheit readings
into the centigrade scale.

DENSITY

262. The unit of density is the density of fresh water at a tempera-
ture of 4° C. The actual density of the water at tide stations may
vary from a little less than unity for fresh water at a temperature of
more than 4° C. to approximately 1.031 for the heaviest sea water.
To provide for this entire range of density three hydrometers are
necessary—one with a scale ranging from 0.996 to 1.011, a second
with scale from 1.010 to 1.021, and a third with scale from 1.020 to
1.031. The particular hydrometers to be used at any tide station
will, of course, depend upon the density of the water, and the observer
will select the hydrometers which will float with the stem partly
immersed. ;

263. The hydrometers are numbered for identification and have
been tested by the Bureau of Standards. Tables of corrections will
be furnished when necessary. The number of the hydrometer used
for each observation should be entered in form 457.

264. The method of reading the scale of the hydrometer, which is
graduated downward, is illustrated in figure 30. Except for densities
Jess than unity, the first two figures will always be 1.0, as printed in

the column of density in form 457. The two figures immediately

84

U. S. COAST AND GEODETIC SURVEY-

following will be determined by the first numbered scale graduation
above the surface of the water, and if this is less than 10, it should

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be prefixed by a zero (0). The last figure, representing
the fourth decimal place in the reading, is determined
by the smallest subdivisions, each of which represents
a change of 0.0002 in density. If the density of the
water is less than unity, the printed figures 1.0 in the
form should be stricken out and the reading as illus-
trated in figure 30 substituted.

265. The water whose density is to be tested is poured
into the jar provided for the purpose and the hydrome-
ter floated in the same. A thermometer is then placed
in the jar to obtain the temperature for correcting the
density reading. A sufficient time should be allowed to
elapse to permit the hydrometer, thermometer, and re-
taining jar to acquire the same temperature as the water.
In reading the hydrometer the eye should. be brought
to the level of the surface of the water and the reading
taken which appears to coincide with the level surface.
After using, the jar and instruments are to be carefully
cleaned to prevent the accumulation of salt.

266. Reduction of density.—The density of sea
water as observed depends not only upon the amount
of solube matter contained in a unit volume but also
upon the temperature of the water at the time of obser- —
vations. It is, therefore; necessary to reduce the ob-
served clensities to some standard temperature in order
that they may be comparable and indicate the amount
of matter held in solution.

267. The table on pages 86-87 gives a series of differ-
ences to be applied to the observed densities in order
to reduce them to a standard temperature of 15° C.
The table, which is based upon data given in appendix 6 -
of the United States Coast and Geodetic Survey Report
for 1891, is applicable to readings of a hydrometer
standardized at a temperature of 15° C. with reference
to unit density at 4° C. If the hydrometer used is
standardized at some other temperature or refers to
another unit density, a further correction will be neces-
sary. The tabular differences include the correction

‘due to the expansion or contraction of the hydrometer

itself as well as the change in the density of the water
arising from changes in temperature, and should be
applied according to sign to the observed hydrometer
readings.

268. The differences in the table, which are expressed
in ten-thousandths of a unit, are given for each whole
degree of temperature from 0° to 35° C., and for each
change of 0.0010 in the density from unity to 1.0310.
For observed densities less than unity, the top line of
the table may be used without material error. The fol-
lowing example illustrates the use of the table: Suppose

a hydrometer reading of 1.0244 has been taken when the temperature

\

MANUAL OF TIDE OBSERVATIONS 85

of the water in the jar is 11.5° C. The nearest observed density read-
ing given in the table is 1.0240. Following this line,-we find differ-
ences of —7 and —5 for temperature 11° and 12°, respectively, giving
an interpolated difference of —6 for a temperature of 11.5°. This
diflerence of —6 applied to the original hydrometer reading of 1.0244
gives 1.0238 as the reduced value.

269. In the heading of form 457 the heaviest and lightest sea water
pertain to the reduced values.

270. Salinity.—The salinity of sea water is defined as the number
of grams of salts contained in 1,000 grams of sea water. While the
total amount of salts contained in a given volume of sea water varies

in different places, the relative portions of the different kinds of salts

is nearly constant in all parts of the ocean. For example, sodium
chloride or common table salt constitutes nearly 78 percent of all the
salts in any locality. Chemical analysis has shown that 1,000 grams
of sea water contain in solution an average of 35 grams of salts of
various kinds, of which about 27 grams is common table salt.

271. The salinity of sea water may be determined by several differ-
ent methods, one of the simplest methods being based upon the density
of the water as obtained from the use of the hydrometer. The
density depends upon the salinity and the temperature of the water.
The table on page 88 gives the salinity corresponding to different
densities at the standard temperature of 15° C., to which the den-
sities in form 457 are reduced. This table was compiled from
table 2 on page 38 of Coast and Geodetic Survey Special Publica-
tion No. 61, Physical Laws Underlying the Scale of a Sounding Tube.
Through the use of this table the values for the salinity in form 457
may be easily obtained from the reduced densities in the preceding
column.

U. S. COAST AND GEODETIC SURVEY

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MANUAL OF TIDE OBSERVATIONS

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88 U. S. COAST AND GEODETIC SURVEY

Corresponding densities and salinities

[Density at 15° C.—Salinity in parts per 1,000]

Density |Salinit,{ Density Density Salinity} Density |Salinity] Density | Salinity
0. 9991 0.0 . 0046 Ted! .0101 - 1.0156 21.4 1.0211 28.6 1. 0266 35.8
. 9992 .0 0047 | Tey? . 0102 y 1.0157 21.6 1.0212 28.8 1. 0267 35.9
. 9993 ok (Bo) . 0103 ; 1.0158 PAE e 1.0213 28.9 1. 0268 36.0
. 9994 3 7.5 . 0104 2 1.0159 21.8 1.0214 29.0 1.0269 36. 2
9995 .4 7.6 § 1.0105 : 1.0160 22.0 f 1.0215 29.1 | 1.0270 36.3
. 9996 ats path . 0106 14.9 | 1.0161 22.1 1.0216 29.3 1.0271 36. 4
. 9997 oe 7.9 . 0107 15.0 | 1.0162 222 1.0217 29. 4 1. 0272 36.6
. 9998 .8 8.0 | 1.0108 15.2 | 1.0163 22.4 f 1.0218 29.5 | 1.0273 36.7
. 9999 9 8.1 } 1.0109 15.3 | 1.0164 22.5 f 1.0219 29.7 9 1.0274 36.8
1. 0000 ies | 8.2 .0110 15.4 | 1.0165 22.6 1. 0220 29.8 1.0275 37.0
1.0001 11g. 8.4 -O111 15.6 | 1.0166 22.7 1.0221 29.9 1.0276 37.1
1.0002 1.3 8.5 .0112 15.7 | 1.0167 22.9 1.0222 30.0 1.0277 BAe
1.0003 1.4 8.6 | 1.0113 15.8 } 1.0168 23.0] 1.0223 30.2 | 1.0278 37.3
1.0004 1.6 8.8 | 1.0114 16.0 | 1.0169 23. 1 1.0224 30.3 | 1.0279 BY An)
1.0005 sey 8.9 | 1.0115 16.1 } 1.0170 23.3 | 1.0225 30.4 | 1.0280 37.6
1. 0006 1.8 9.0 .0116 16.2 | 1.0171 23.4 1. 0226 30.6 1, 0281 BY awd
1.0007 2.0 9.2 .0117 16.3 | 1.0172 23.5 1. 0227 30. 7 1.0282 37.9
1.0008 758 | 9.3 .0118 16.5 | 1.0173 23.7 1.0228 30.8 1. 0283 38.0
1. 0009 Pde. 9.4 .0119 16.6 | 1.0174 23.8 1.0229 31.0 1. 0284 38. 1
1.0010 2.4 9.6 | 1.0120 16.7 | 1.0175 23.9 f 1.0230 31.1 9 1.0285 38.2
1.0011 2.5 9.7 .0121 6.9 } 1.0176 24.0 1.0231 31.2 1. 0286 38. 4
1.0012 2.6 9.8 . 0122 -O | 1.0177 24.2 1. 0232 31.4 1.0287 38.5
1.0013 2.8 9.9 . 0123 .-1 | 1.0178 24.3 1. 0233 31.5 1.0288 38. 6
1.0014 2.9 ba . 0124 .3 | 1.0179 24.4 1. 0234 31.6 1.0289 38.8
1.0615 3.0 bee .0125 -4 9 1.0180 24.6 1.0235 31.8 1.0290 38.9
1.0016 sae iD . 0126 .5 | 1.0181 24.7 1.0236 31.9 1.0291 39.0
1.0017 3.3 Br . 0127 -6 § 1.0182 24.8 1. 0237 32.0 1.0292 39. 2
1.0018 3.4 .6 | 1.0128 .8 | 1.0183 25.0 | 1.0238 32.1 1. 0293 39.3
1.0019 3.5 ie . 0129 .9 Ff 1.0184 25. 1 1.0239 32.3 1. 0294 39. 4
1. 0020 Baa .8 | 1.0130 .O F 1.0185 25.2 § 1.0240 32.4 | 1.0295 39.6
1.0021 3.8 .0 .0131 .2 | 1.0186 25.4 1.0241 320.0 1.0296 39.7
1.0022 3.9 a . 0132 .3 | 1.0187 25, 5 1.0242 32. 7 1.0297 39.8
1. 0023 4.1 .2 | 1.0133 .4 | 1.0188 25.6 f 1.0243 32.8 | 1.0298 39.9
1.0024 4.2 .4 . 0134 .6 | 1.0189 25.8 1.0244 32.9 1.0299 40.1
1.0025 4.3 -5 | 1.0135 .7 | 1.0190 25.9 | 1.0245 33.0 | 1.0300 40.2
1. 0026 4.5 .6 | 1.0136 .8 | 1.0191 26.0 9 1.0246 33.2} 1.0301 40.3
1. 0027 4.6 .8 . 0137 -O F 1.0192 26. 1 1. 0247 33.3 1. 0302 40.4
1. 0028 4.7 9 . 0138 -1 | 1.0193 26.3 1.0248 33. 4 1. 0303 40.6 -
1. 0029 4.8 12.0 .0139 .2 7 1.0194 26.4 1.0249 33.6 1. 0304 40.7
1. 0030 5.0 122 . 0140 .4 7 1.0195 26.5 1. 0250 33. 7 1. 0305 40.8
1.0031 ae | 12.3 .0141 .5 F 1.0196 26.7 1.0251 33.8 1. 0306 41.0
1.0032 5.2 12.4 . 0142 -6 | 1.0197 26.8 1.0252 34.0 1. 0307 41.1
1.0033 5.4 12.6 . 0143 .7 | 1.0198 26.9 1.0253 34. 1 1. 0308 41.2
1, 0034 5.5 12.7 . 0144 .9 | 1.0199 2a 1. 0254 34. 2 1. 0309 41.4
1.0035 5.6 12.8 .0145 .O F 1.0200 2ts2 1. 0255 34. 4 1. 0310 41.5
1.0036 5.8 12.9 . 0146 . 1 § 1.0201 27.3 1. 0256 34.5 1.0311 41.6
1. 0037 5.9 13.1 . 0147 .3 f 1.0202 27.4 1, 0257 34.6 1.0312 41.8
1. 0038 6.0 13.2 . 0148 .4 4 1.0203 27.6 1.0258 34.7 1.0313 41.9
1. 0039 6.2 13.3 .0149 .5 | 1.0204 PH (ref | 1. 0259 34.9 1.0314 42.0
1, 0040 6.3 13.5 | 1.0150 .6 | 1.0205 27.8] 1.0260 35.0 J 1.0315 42.]
1.0041 6.4 13.6 .0151 .8 | 1.0206 28.0 1. 0261 3521 1.0316 42.3
1. 0042 6.6 13.7 . 0152 .9 | 1.0207 28. 1 1. 0262 35.3 1.0317 42.4
1. 0043 6.7 13.9 . 0153 .O | 1.0208 28, 2 1. 0263 35. 4 1.0318 42.5
1.0044 6.8 14.0 . 0154 z 1.0209 28. 4 1. 0264 35.5 1.0319 42.7
1.0045 (A 14.1 .0155 1.0210 | 28.5 1.0265 35. 6 1. 0320 42.8

INDEX
A Page
Page| Automatic tide gage, standard—
Abnormal tides due to configura- Continued.
HiOnNROf SHOTCS 22% = Sa 53 TENS Vay eee aie sr Se ee 17, 35
Automatic tide gage, portable_ 1, 18, 55 AN Kote eARCO PAD OW epee el 16
Attaching float__.___-__________ 57 Hiatt. welle see aes 26, 46, 50
WareworzclOCK Se es 58 Moat wwite! 2. Sane 2 mee 17, 35, 49
Cleaning float pipe___________ 59 Forwarding records to office__ 47
Cleaning stylus screw______-_ 59 Guide springs________________ 15
@leaning.. toolo22s... = 25

Hour-marking device__.. 16, 44, 49
50

Clock movement_______.____ 20, 58 Inspection: 2 e =) 2 hse eas
Comparative note____________ 59 J OMS ez Wee pio ee See 32
Counterpoise spring________ 21, 57 Limits of operation________ 18, 34
HCA er a= soE 23 Mainvoroller a2. 22. sone 15
Oni eee RS ee 24, 57, Marionanps20 cbse ies tee 15
Oa tee Ue ea 20 Names of parts..-_). 2) 772 6
Hiloaty) pipe. =n 2 24, 56, 59 Operating difficulties_________ 49
Float pipe installation________ 56 Papers 2 ee ee a 15, 42, 49
HlOatawite@s 2 2 22 fee 24, 57 POTN a es See na 16, 42
Ce ears a a VR 23, 57 Bencil arin eee See nS, 16
Ar S Gena Or ee 5D. Pencil (Screwea 15, 45, 49
Intake coupling. == 222. 24 Placing paper on gage________ 42
INamesiof parts 19 Pulleys, installation__________ 34
Onerations— 82 sos eee ss 55 Reading tide staff____________ 44
Record cylinder____________ 20, 58 Receiving roller_______________ 15
Record) papers i223. 22) 24, 58 Recording pencil_____________ 16
SOG) Sa eens ee ees 23 IROMERS oor x ees ae ade, 15
Siteylns Ses eae ee ea 20, 58 Scaleior gagena se “see. Bee 18
SOylUISpmanes on eae 20 Setting up gage 2 32
Stylus serew_______________ 20, 59 Springs) euides2 = Se 15
Table, scale ratios___________ 22 Standard times=)s2 sens) a 43
Automatic tide gage, standard ____ en Startinevrages-.--2 a sao 3E.
32, 41 Sng ohy: iOlesps 15
Adjusting pencils_____________ 42 Table, pulley installation_____ 34
Adjustment of hour-marking + Table, scale combinations____ 18
Gevicomimens 8 eo al 44 ROMSTOMs CONC oe ee eet et alee 2
Adjustment of wiring________ 34 Tension) weight = soy ay a mlign sy
Attaching counterpoise_______ 35 Tension weight drum ______ 15, 17
Attaching float=\< 6. - 1 35 DOrnepape rs. 52 = Swe) Mie es ra
Attaching tension weizht_____ 37 Weeklyaere porta ease ma. eens
Broken or tangled float wire__ 49 VAG, oe eae Oi eee gpa, 49
Changing paper on gage_____ 42
Cleaning pencil screw=______ 45 B
Clearing float well intake____ 46] Bench, marks____________ 37, 52
Clockstailunes suse sR aes 49 IS EAS @ escent Ronee eee ee ee 40
Wloclkohumiiss Gee 6 UL GIn SS es = eee ee ee SO 38
Clogged float well_____»______ 50 Coneie tenes Mae Mees crate eee abe 39
Comparative note________ 43 Descriptions ___________ Hed A a 40
Wounterpoise <2. a8 17, 35 Federal buildings_____________ 38
Counterpoise drum___________ 16 Im Spe ct Ons een eee eee 52
Datum pencils ee a 16 eveling 222. sue aiid. went 40, 52
Datum pencil holder_________ 16 Num per aan ese ans eins 39
Distortion of tide curve_______ 50 PETIA yes ee ee As 40
Dirnmyshath, «2.0 sees. ei’ ik 17 COUP WITT GF eS ere eat a aE 37
Failure of hour-marking de- EOC kyje-eneeieinre ee 5 sen Re NES Be 39
Nip Cle ae Fa is Cs ee eR 49 Standards Gis kiss wee suena 37
Failure to trace curve________ 50 Broken or tangled float wire_____ 49

90 INDEX
C Page
Page | Float wire, portable gage________ 24, 57
Cleaning pencil screw________--__-~ 45 Standard gage__________ ale & 35, 49
Cleaning stylus screw________-____ 59 | Forwarding records to office_____ 47
Cleaning nll sense eee ei 25, 27,46 | Furnishing information to public__ 48
Clearing float-well intake______ 46, 59
Clock movement, portable gage__ 20, 58 G
Clock unit, standard gage________ 6,49) "Gages, tide: 22-8 3 aa 1
Clogged float-well, indications___ 50] Gears, portable gage_______.___ 23, 57
Closing error, leveling____________ 41 | Glass tube for tide staff_-________ 2
Comparative note, portable gage__ 59] Greenwich lunitidal intervals_____ 69
Standard ‘gage! === 2 eer 43
Comparative readings, tabulations. 61 H
Comparison of ‘simultaneous ob- Half tide ‘level2._ = <U kee aes 13
SErVaAtIONS 4500 Ne alka 72, 75) Height allowance, tide reducers__ 80
Conversion of centigrade to Fahr- Height reductions___________ 61, 66, 73
enheit *scaleme 2S Leo See one 83} High and low water tabulation __ 63
Correction for longitude of moon’s High water. -io222:\_ 222 eneeeee 73
MOGE =. =n W ACA DRE ae ee 74 | Higher high water_______________ 74
Correction to plane of flotation, Hour-marking device_____________ 16
Tape ea es 2 en Re Adjustment = aes 44, 49.
Counterpoise, standard gage____ 17,35] Hourly height tabulation_________ 66
Counterpoise drum, standard gage. 16
Counterpoise __ spring, portable I
PALCHS Pane tae oe eS eee 21. 57. | Inequalities.2.- +s. eee
Inspection, primary tide station__ 50
D Installation:
aAvony (Penci) == 22 es Ae 16 Hloat.. well- 22 + 22312) ae 27, 56
Dataumsictidale: or etre ae See a 76 Portable automatic tide gage. 55
Density observations___________ 81, 83 Standard automatic tide gage. 32
Determination of plane of flota- Tape gages. 2222 eee 29
ion; “tane rage: ees 2 ee 30 Tide staff... 2) = Sa -29
Difficulties, operating, standard Intake coupling=-:2-. = 24, 26
SAEs se ei waned Pa LI 49 | Intake to float well____________ 24, 26
Diurnal high water inequality_.___ 74 27, 46, 59
Diurnal low water inequality_.__._ 74| Interpolations, tide record_______ 68
Duties of tide observer, primary Tron float. welln=22:=2- <4: 03ee eee 26
tide station == es 248 ee ae 41
K
Effect of wind_____. , Es a ae a 54 | Kerosene in float well____________ 28
Establishment, primary tide sta- L
tion ~~-~-~~-~_~-----__---_--=_- 25 | Leveling 40, 52
Secondary tide station________ of Teestion =... a Pe ;
Expenses, emergency_—-_---_---__ 48 Primary tide station________ 25, 51
F Secondary tide station________ 53
Low’ water. 22h. 2. 2 ee dana
Factors F(Mn) and Fy ________ 74, 75)|.Low water springs__..___.-- ay
Fair leader, portable gage________ 23 | Lower low: water.___=_-. a 74, 77
Rathometers2 oo se Pe) ee eee 5| Lunitidal intervals___.__._._________ 69
Hield> preductions=". = 4 sok 59 Comparison of simultaneous
Hrxeqdostati ts fk 1 amet 2 observations)... —9— eee 71
Float, portable gage______\___.__ 24, 57 Method of checking__________ 70
Standard ‘gape. o> 2 is it, oD :
Float drum, portable gage________ 20 M
standard gape oo So 16| Main roller, standard gage_______ 15
Float pipe, portable gage____ 24,56,59|Marigram________ 15
Moat well) ==> ee 26 | Mean low water_______.._________ 77
Clearing intike = a2 22 46,59] Mean low water springs__________ q7
_ Clogged well, indications_____ 50| Mean lower low water___________ [7
Freezing, precaution against__ 28] Mean range Of (tid @. 2-25 Sees 73
Inspection j= =! ese ee Nae 51|:‘Mean river level. _-- 22-3 17
Tnstallation= == 3 MRSS 2%, 56 | Mean) sea levelijsus2 2 2 Se 76
Da 2 Sa Tt OS Ee 27 | Measurements, inspection of tide
Eron Well. Ses eee eRe 26 station 234223) i cue See 52

INDEX. 91
N Page
Page| Standard automatic tide gage.
Names of parts, portable gage-____ 19 (See Automatic tide gage,
Standard: cages.) 2222s 6 standard.)
Standard disk bench mark_______ 37
O Standard time______________ 43, 60, 64
Onsenverscawnies. = es ee ee 41 ela portable gage-_---__--_- 20, a8
Operating difficulties_____________ 49 OSS a a ee =
Operation of tide station____--_-- 41 | Stylus screw_——--~---__--_--__- 20, 59
m Supplies for primary tide station__ 47
= | T
Paperwee see Skee 15, 24, 42, 49, 58] Tables:

Parts, names of, portable gage-_-__ 19 Allowable error in leveling... 41
Standard gage—_—- =) 5 = 6 Arrangement for hourly height
eran eee Ss ae 16, 42 tabulation 3.2228 iaG ees 68

reneilgarmne ts ee a 16 Conversion Of centigrade to
Iencil Sere was 2 22 Se be ae 15]. Fahrenheit scale___________ 83
CUS To ee 45, 49 Density and salinity__________ 88
Lite 20 oe 4 Differences for reducing den-
Plane of flotation, tape gage_____- 30 sity of sea water to 15° C__ 86
Planes at rererences=— 2 == - = 76 Factor F(Mn) for reducing
Portable automatic tide gage. (See HaAngsenor: tides swe ns q
Automatic tide gage, portable.) Factor F: for correcting DHQ
Portable tide staff____.___________- 2 ANC E.G. eh set q5
Precautions against freezing, float Factors for correcting plane of
ie ee Motation! 0s 2855 ea ee 32
LS ESSE a 5 Pulley installation__-________ 34
Erimary bench mark_—____- = —! __ 40 Reduction of Greenwich inter-
Primary tide station__________-__ 1, 25 vals to local intervals____-- 73
Purposes of tide observations____—~ 1 Seale combinations, standard
n reve ea ae eRe eas eee 18
R Scale ratios, portable gage____ 23
HanrevOn ideas. - ee a 8 73 Time required for tide wave
Reading tide staff_._______________ 44 TOstT AV eles) = bea ee ees. ~ 80
Receiving roller, standard gage___ 15| Tabulation and reduction__-__--- 59
Record cylinder, portable gage- 20,58 Checking height datum___-_- 61
Record paper, portable gage__-_ 24, 58 Checking tine] See 60
Standard gage__________ 15, 42, 49 Comparative readings________ 61
Recording stylus, portable gage_ 20, 58 Comparison of simultaneons
Reducers for soundings______-__- 78 observations__--___--__-_ 72, 75
Reductions. (See Tabulation and Correction for longitude of
reduction. ) MOONS! “nod eels sy ae 14
Report on inspection____-_---___- 50 Density reduction ____---____ 84
Requisition for supplies___________ 47 Diurnal high and low water
Rollers, standard gage__-_____--~- 15 inequalities=== 2 saat sae 74
Factors for reduction-_-_-__-_ 74
Ss + Hield reductions (221-2 °== 59
Nainntyeomeen Wer ON ee ENS 2 85 Height reductions_____—_ 61, 66, 73
Sealey avitmitied 222 ee fe ee 2 High and. low water tabu-
Scale of gage, portable___________ 23 TZ aK oS 0 eek tec core PRR ee EU 63
Sueur en reel ek ae ea 18 Hourly height tabulation_____ 66
Secondary tide station__________ 1,52 Interpola tions] ck ee eee 68
Shipment of Government property. 47 Lunitidal intervals__________-_ 69
Sign for tide house__________-____ 28 Preliminary work______-__-__ 59
Simultaneous observations, com- Sailint yreeet eatery 85
(DST O Wa peer 2s eS 72, 15 Tidalivdatwmss = eee _ 6
Soundings, location of gage______ 53 Time) to; be) useda.22e. = 60, 64
Abnormal tides due to config- Pan emg ae Oia ties re hae 3
uration of shore_-_________ 53 FNSPeCtl Ons se a ee 51
Effect of wind_____-___________ 54 installation= see. ee 29
Exposed channel approaches__ 53 Plane of flotation____________ 30
Outerscoast== 232 eee ee 53 | Temperature and density observa-
Upper reaches of tidal rivers. 53 EL QS ese rey eee Sit 81
Spring range of tide______________ Fe MRM STON COG se aa ees See 17

Statip rea im gee ei See

44 | Tension weight____--_______--__- 17, 37

92 INDEX
Page Page
Tidal bench marks. (See Bench (Ride: stafi---=- += 2 eee 2, 29, 51
marks. ) Tide. ‘staff support. == > 2
Tidal datums. (See Datums, tidal.) Time to be used_____-_______ 43, 60, 64
Nide: cages pou sbevest eels Eee
Automatic. (See Automatic V
tide gage.)
Bathometerus: k= 22 ks gh 5 | Vitrified scale for tide staff______ 2
2a 1 pee ncaa APRESS meena foes. ee tt 4
Diressures 2 Se Site a eee ses 5 WwW
Els gf a NS Sea ee ee 3, 29
Witke-stai. fo 3 528 2) Weekly report___=_---_--3 eee 47
Rie NOUSE= = 2 elise eh ek eee, 28 | Weight, counterpoise___________ 17, 35
ide LObSERVer..2= Ss ba et 41, 51 Téension2 .<eo eeee aw Eis Lh
Tide reducers for soundings__-~__~~- 48'| Wind: effect_.2>. "3 eee
Graphie method: 2os?it sees 78 | Wire, portable gage__-___________ 24, 57
Height allowance____-__- ~-_-- 80 Standard gage__-_______ 17, 35, 49
Time allowance: ce = es ees 80 Wooden float-well________-________ 27