CURRENT OBSERVATIONS.
There is considerable diversity in the design of floats
employed in current observations, dependant to some extent upon
whether it is desired to ascertain the direction of the surface
drift or of a deep current, it does not by any means follow
that they run in simultaneous directions. There is also
sometimes considerable difference in the velocity of the
current at different depths--the surface current being more
susceptible to influence of wind. A good form of deep float is
seen in Fig. 8. It consists of a rod 2 in by 2 in, or 4 sq in
The lower end of which a hollow wooden box about 6 in by 6 in
is fixed, into which pebbles are placed to overcome the
buoyancy of the float and cause it to take and maintain an
upright position in the water with a length of 9in of the rod
exposed above the surface. A small hole is formed in the top of
the box for the insertion the pebbles, which is stopped up with
a cork when the float is adjusted. The length of the rod will
vary according to the depth of water, but it will generally be
found convenient to employ a float about 10 ft and to have a
spare one about 6 ft deep, but otherwise it is similar in all
respects, for use in shallow water. A cheap float for gauging
the surface drift can be made from an empty champagne bottle
weighted with stones and partly filled with water. The top 12
in of rods and the cord and neck of the bottle, as the case may
be, should be painted red, as this colour renders floats more
conspicuous when in the water and gives considerable assistance
in locating their position, especially when they are at some
distance from the observer.
A deep-sea float designed by Mr. G. P. Bidden for ascertaining
the set of the currents along the base of the ocean has
recently been used by the North Sea Fisheries Investigation
Committee. It consists of a bottle shaped like a soda-water bottle,
made of strong glass to resist the pressure of the water, and
partly filled with water, so that just sufficient air is left
in it to cause it to float. A length of copper wire heavy enough
to cause it to sink is then attached to the bottle, which is then
dropped into the sea at a defined place. When the end of the wire
touches the bottom the bottle is relieved of some of its weight
and travels along with the currents a short distance above the bed
of the sea. About 20 per cent. of the bottles were recovered, either
by being thrown up on the beach or by being fished up in trawl nets.
[Illustration: FIG. 8.--DETAIL OF WOOD TIDAL FLOAT 10 FEET
DEEP.]
A double float, weighing about 10 lb complete, was used for the
tidal observations for the Girdleness outfall sewer, Aberdeen.
The surface portion consisted of two sheet-iron cups soldered
together, making a float 9 in in diameter and 6 in deep. The
lower or submerged portion was made of zinc, cylindrical in
shape, 16 in diameter and 16 in long, perforated at intervals
with lin diameter holes and suspended by means of a brass chain
from a swivel formed on the underside of the surface float.
In gauging the currents the float is placed in the water at a
defined point and allowed to drift, its course being noted and
afterwards transferred to a plan. The time of starting should
be recorded and observations of its exact position taken
regularly at every quarter of an hour, so that the time taken
in covering any particular distance is known and the length of
travel during any quarter-hour period multiplied by four gives
the speed of the current at that time in miles per hour.
The method to be employed in ascertaining the exact position of
the float from time to time is a matter which requires careful
consideration, and is dependent upon the degree of accuracy
required according to the importance of the scheme and the
situation of neighbouring towns, frequented shores, oyster
beds, and other circumstances likely to be injuriously affected
by any possible or probable pollution by sewage.
One method is to follow the float in a small boat carrying a
marine compass which has the card balanced to remain in a
horizontal position, irrespective of the tipping and rolling of
the boat, and to observe simultaneously the bearing of two
prominent landmarks, the position of which on the plan is
known, at each of the quarter-hour periods at which the
observations are to be taken. This method only gives very
approximate results, and after checking the value of the
observations made by its use, with contemporary observations
taken by means of theodolites on the shore, the writer
abandoned the system in favour of the theodolite method, which,
however, requires a larger staff, and is therefore more
expensive. In every case it is necessary to employ a boat to
follow the float, not only so as to recover it at the end of
each day's work, but principally to assist in approximately
locating the float, which can then be found more readily when
searching through the telescope of the theodolite. The boat
should be kept about 10 ft to 20 ft from the float on the side
further removed from the observers, except when surface floats
are being used to ascertain the effect of the wind, when the
boat should be kept to leeward of the float. Although obviously
with a large boat the observations can be pursued through
rougher weather, which is an important point, still the
difficulty of maintaining a large boat propelled by mechanical
power, or sail, sufficiently near the float to assist the
observers, prevents its use, and the best result will be
obtained by employing a substantial, seaworthy rowing boat with
a broad beam. The boatmen appreciate the inclusion of a mast,
sails, and plenty of ballast in the equipment to facilitate
their return home when the day's work is done, which may happen
eight or nine miles away, with twilight fast passing into
darkness. There should be two boatmen, or a man and a strong
youth.
In working with theodolites, it is as well before starting to
select observation stations at intervals along the coast, drive
pegs in the ground so that they can easily be found afterwards,
and fix their position upon a 1/2500 ordnance map in the usual
manner. It may, however, be found in practice that after
leaving one station it is not possible to reach the next one
before the time arrives for another sight to be taken. In this
case the theodolite must be set up on magnetic north at an
intermediate position, and sights taken to at least two
landmarks, the positions of which are shown on the map, and the
point of observation subsequently plotted as near as possible
by the use of these readings. Inasmuch as the sights will be
taken from points on the edge of the shore, which is, of
course, shown on the map, it is possible, after setting up to
magnetic north, to fix the position with approximate accuracy
by a sight to one landmark only, but this should only be done
in exceptional circumstances.
The method of taking the observations with two theodolites, as
adopted by the writer, can best be explained by a reference to
Fig. 9, which represents an indented piece of the coast. The
end of the proposed sea outfall sewer, from which point the
observations would naturally start, is marked 1, the numerals
2, 3, 4, etc., indicating the positions of the float as
observed from time to time. Many intermediate observations
would be taken, but in order to render the diagram more clear,
these have not been shown. The lines of sight are marked 1A,
1B, etc. The points marked A1, A2, etc., indicate the first,
second, etc., and subsequent positions of observer A; the
points B1, B2, etc., referring to observer B. The dot-and-dash
line shows the course taken by the float, which is ascertained
after plotting the various observations recorded.
It is very desirable to have a horse and trap in waiting to
move the observers and their instruments from place to place as
required, and each observer should be provided with small flags
about 2 ft square, one white and one blue, for signalling
purposes.
The instruments are first set up at A1 and B1 respectively, and
adjusted to read on to the predetermined point 1 where the
float is to be put in Then as soon as the boatmen have reached
the vicinity of this point, the observers can, by means of the
flags, direct them which way to row so as to bring the boat to
the exact position required, and when this is done the anchor is
dropped until it is time to start, which is signalled by the observers
holding the flags straight above their heads. This is also the
signal used to indicate to the men that the day's work is
finished, and they can pick up the float and start for home.
[Illustration: FIG. 9.--PLAN OF INDENTED COAST-LINE LLUSTRATING
METHOD OF TAKING CURRENT OBSERVATIONS WITH TWO THEODOLITES.]
Directly the float is put in the water, and at every even
quarter of an hour afterwards, each observer takes a reading of
its exact position, and notes the time. As soon as the readings
are taken to the float in position 2, the observer A should
take up his instrument and drive to A2, where he must set up
ready to take reading 3 a quarter of an hour after reading 2.
It will be noticed that he might possibly have been able to
take the reading 3 from the position A1, but the angle made by
the lines of sight from the two instruments would have been too
acute for accurate work, and very probably the float would have
been hidden by the headland, so that he could not take the
reading at all. In order to be on the headland A4 at the proper
time, A must be working towards it by getting to position A3 by
the time reading 4 is due. Although the remainder of the course
of the float can be followed from B1 and A4, the instruments
would be reading too much in the same line, so that B must move
to B2 and then after reading 5 and 6 he should move to B3. As
the float returns towards the starting point, A can remain in
the position A4 while B goes to B4 and then moves back along
the shore as the float progresses.
The foregoing description is sufficient to indicate the general
method of working, but the details will of course vary
according to the configuration of the shore and the course
taken by the float. Good judgment is necessary in deciding when
to move from one station to the next, and celerity in setting
up, adjusting the instrument, and taking readings is essential.
If the boatmen can be relied upon to keep their position near
the float, very long sights can be taken with sufficient
accuracy by observing the position of the boat, long after the
float has ceased to be visible through the telescope.
The lines of sight from each station should be subsequently
plotted on the 1/2500 ordnance map; the intersection of each
two corresponding sight lines giving the position of the float
at that time. Then if a continuous line is drawn passing
through all the points of intersection it will indicate the
course taken by the float.
It is very desirable that the observers should be able to
convey information to each other by signalling with the flags
according to the Morse code, as follows. The dashes represent a
movement of the flag from a position in front of the left
shoulder to near the ground on the right side and the dots a
movement from the left shoulder to the right shoulder.
TABLE 3.
MORSE ALPHABET.
E .
A .-
R .-.
L .-..
W .--
P .--.
J .---
I ..
U ..-
F ..-.
S ...
V ...-
H ....
T -
N -.
K -.-
C -.-.
Y -.--
D -..
X -..-
B -...
M --
G --.
Q --.-
Z --..
O ---
The signal to attract attention at starting and to signify the
end of the message is .. .. .. continued until it is
acknowledged with a similar sign by the other observer; that
for a repetition is .. -- .. which is signalled when any part
of the message is not understood, otherwise after each word is
signalled the receiver waves - to indicate he understands it.
Until proficiency is attained, two copies of the alphabet
should be kept by each observer for reference, one for
dispatching a message arranged in alphabetical order and the
other far reading a message arranged as set out above. The
white flag should be used when standing against a dark
background, and the blue one when on the skyline or against a
light background.
The conditions in tidal rivers vary somewhat from those
occurring on the coast. As the crest of the tidal wave passes
the mouth of the river a branch wave is sent up the river. This
wave has first to overcome the water flowing down the river,
which is acting in opposition to it, and in so doing causes a
banking up of the water to such a height that the inclination
of the surface is reversed to an extent sufficient to cause a
tidal current to run up the river. The momentum acquired by the
water passing up-stream carries it to a higher level towards
the head of the river than at the mouth, and, similarly, in
returning, the water flowing down the river gains sufficient
impetus to scoop out the water at the mouth and form a low
water below that in the sea adjoining. Owing to a flow of
upland water down a river the ebb lasts longer than the flood
tide by a period, increasing in length as the distance from the
mouth of the river increases; and, similarly to the sea, the
current may continue to run down a river after the tide has
turned and the level of the water is rising. The momentum of
the tide running up the centre of the river is in excess of
that along the banks, so that the current changes near the
shore before it does in the middle, and, as the sea water is of
greater specific gravity than the fresh, weighing 64 lb per
cubic foot against 62-1/2 lb, it flows up the bed of the river
at the commencement of the tide, while the fresh water on the
surface is running in the opposite direction. After a time the
salt water becomes diffused in the fresh, so that the density
of the water in a river decreases as the distance from the sea
increases. The disposal of sewage discharged into a river is
due primarily to the mixing action which is taking place;
inasmuch as the tidal current which is the transporting agent
rarely flows more rapidly than from two to four miles per hour,
or, say, twelve to fifteen miles per tide. The extent to which
the suspended matter is carried back again up stream when the
current turns depends upon the quantity of upland water which
has flowed into the upper tidal part of the river during the
ebb tide, as this water occupies a certain amount of space,
according to the depth and width of the river, and thus
prevents the sea water flowing back to the position it occupied
on the previous tide, and carrying with it the matter in
suspension. The permanent seaward movement of sewage discharged
into the Thames at Barking when there is only a small quantity
of upland water is at the rate of about one mile per day,
taking thirty days to travel the thirty-one miles to the sea,
while at the mouth of the river the rate does not exceed one-
third of a mile per day.