India, make enormous use of Tanks or reservoirs for the purpose of
irrigation, the undulating and broken nature of the country as clearly
indicating their use, as the flat plains of Northern India appear to
demand canals. Many of these tanks have been in use for ages, and in
the single province of Mysore alone there are not less than 30,000;
hence, of course, they vary much in size, from those with an area of a
few square yards up to those which contain several square miles, and
which form artificial lakes on which you can navigate. The revenue of
the country depends on the maintenance of these tanks, as in the case
of canals elsewhere, and the chief duty of the Madras engineers is
their inspection and repair, in concert with the civil officials of the
district.
However much tanks may differ in locality and size, they may all be
said to consist of the following parts:—1. A Dam or bund, natural or
artificial, or both, by which the water is restrained. 2. A Stream, by
which the tank is fed, unless it is fed by direct rainfall. 3. A Sluice
or sluices, by which the ponded water is drawn off to be used for
irrigation. 4. A Waste Weir, overfall, or _Calingula_, by which, when
the tank is full, surplus water can escape safely. The Dam is generally
the most important work in a tank, and the determination of its site,
and the construction of it when chosen, often demand engineering skill
of a high order. It is, of course, first of all necessary to take a
very careful series of levels in order to determine whether the amount
of water that can be ponded up by a given length and height of dam will
be enough to pay for the cost of the work. Next, you must determine
whether there is a sufficient area of land in the vicinity demanding
water, and whether you are pretty sure of a good supply of water for
the tank.
The Dam may be built of earth or stone, or of a combination of the
two; and in the case where a great depth of water is ponded up, the
foundations of the dam must, of course, be constructed in the most
substantial and careful manner, for a failure may involve not merely
the destruction of the tank, but the loss of hundreds of lives. The
question of the best form of retaining wall to withstand the pressure
of great depths of water, has excited a good deal of discussion, and
you will find some of the best sections in the ‘Papers,’ as well as a
good description and diagram of calculation of the Moota Dam in Bombay.
The other chief point in a tank besides the dam is the Waste Weir,
which is generally built at the exit from the tank of the original
channel by which it is fed. It must be made of sufficient length to
discharge the maximum quantity of water which your calculations have
shown you is likely to pass over it, and it must be strong enough,
both in body and foundation, to resist both the pressure and the shock
of this water. For this latter purpose, the principle of the vertical
canal fall, already described, is often used, and the falling water is
received into a cistern, by which its shock is broken.
You will find a chapter devoted to tanks in the ‘Roorkee Treatise,’ and
several examples of the different kinds employed.
As to the irrigating duty of tank water, it is roughly calculated that
a cubic yard of water is required for every square yard of land; this,
however, refers to rice, which takes far more water than wheat. We have
not time to pursue the subject further.
Before closing the subject of irrigation works, I may perhaps be
allowed to name one or two of those engineers who have added most to
our knowledge of that science. Foremost stands the name of the late
Sir Proby Cautley, F.R.S., of the late Bengal Artillery, the designer
and constructor of the Ganges Canal; then of Sir A. Cotton, of the
Madras Engineers, his great rival, head and founder of the Madras
school of irrigation; Colonel Baird Smith, F.R.S., chief engineer at
the siege of Delhi; and then the late Colonel Dyas, of the Bengal
Engineers, one of the most scientific officers in India.
* * * * *
Though not exactly under the head of irrigation works, the question of
_River Works_, or river improvements, is not very far removed from it,
and though we have but little time to spare for it, it would not be
right to exclude such a subject altogether from your notice, for many
of you will probably have a good deal to say to it sooner or later.
An engineer is generally called upon to devise works for the
improvement of a river, either to increase its navigable facilities
or to prevent its waters from inundating the country. With regard to
the first object, the work in India generally consists in removing
obstacles, such as shoals, kunkur banks, sunken trees or boats, and I
cannot do better than draw your attention to the admirable paper on the
Gogra River Works, sent to me by the late Lieutenant Carroll, R.E.,
whom I have already mentioned. The systematic improvement of rivers by
the lock and dam method, as in the Thames and other small streams, is
hardly applicable to such rivers as the Ganges and Indus, except at a
cost which puts it altogether out of the question. It is greatly to
be regretted that it is so, for the navigable capabilities of Indian
rivers are far inferior to what we should suppose, judging from their
length and size; much inferior to those of America, for example, though
the Mississippi has much the same general characteristics as the Ganges.
During the rainy season, Inundations in the lower part of the course of
many of these Indian rivers often extend far inland; the delta of the
Ganges is, in fact, a vast sheet of water at that time of year, and in
the dry season it is an enormous jungle intersected with watercourses,
and inhabited only by tigers, snakes, and other wild animals, with the
exception of a few wretched fishermen or woodcutters.
Higher up the country, again, rivers, such as the Damooda, are
restrained within artificial embankments, which, unfortunately, have
not been systematically planned, but have been erected from time to
time by the different villages threatened with inundation along its
course. There are often many miles of such embankment (or levees, as
they are termed in America,) under a young engineer’s charge, and
very anxious work it is, as he has to guard a long line from the
attack of an insidious foe, often with a very imperfect garrison.
Such embankments as these are always constructed of earth, and when
consolidated by time and covered with grass, a very thin earthen bank
will successfully resist until it is absolutely overtopped, while new
earthwork of twice and three times the sectional area will be soaked
through and breached readily. Another source of danger arises from
rats and other vermin, which perforate your embankments in hundreds of
places.
Rivers are also trained, either to prevent inundations or to improve
their navigation, by the use of Spurs or groins, which are much
employed in India. They are generally run out from the shore at an
angle of 45° with the current, so as to deflect it towards the opposite
side when needful. By a series of such spurs, a long line of bank may
be protected, and a corresponding series on the opposite side may often
be necessary to give the stream a set in the right direction. Such
spurs may be built of stone where there is any, but in India are more
often constructed of a double row of piling, filled in with fascines;
the nose of the spur is often revetted with sand-bags. Sometimes ropes
are anchored, and trees or brushwood tied along the rope, by which the
surface current is acted upon sufficiently for the purpose. In the
Markunda River, near Umballa, well-cylinders were sunk down at a great
expense, to act as anchors, and were connected by stout iron chains or
wire cables, to which trees and branches were attached. The advantage
of spurs founded on the bottom over floating spurs or breakwaters is
chiefly in the great deposits of silt which occur both above and below
the latter, and which soon form an adequate protection to them from the
stream.
For all this kind of work it is impossible to lay down any fixed rules;
experience and local knowledge are our chief guides, though it may be
said generally that rivers are like women—more easily led than driven!
* * * * *
I have now run over—very hastily and imperfectly, it is true—the chief
branches of civil engineering which those going to India may be called
upon to engage in. There are several other important branches, such as
Lighthouses, Harbour works, Gas, Water, and Sewage works, on which I
have not touched, both because you will rarely have anything to do with
them, and because there is very little that is _special_ about them so
far as India is concerned. But before I conclude, I wish to give you a
brief account of the Indian Survey, partly because the results of that
survey are of great importance to the engineer, and because many of our
own officers are employed in it.
The _Indian Survey Department_ is divided into the three branches of
the Great Trigonometrical, the Topographical, and the Revenue, Survey
branches. To understand something of the work done by the first, it is
necessary to sketch briefly the history of its operations. It was in
the year 1800 that Major Lambton commenced to run a line of triangles
across India from Madras to the west coast, with a view simply to a
local survey. In the course of this work it was discovered that there
was an error of no less than 40 miles in the assumed breadth of the
peninsula at that point. The original Base Line was measured near
Madras, and a base of verification at Bangalore, and from this latter
base a line of triangles was subsequently carried down south to Cape
Comorin, and up north to Dera, 40 miles from Roorkee. This line,
carried along the centre of the continent, formed the Great Indian Arc,
and is the largest arc of longitude ever yet measured on the earth’s
surface. Another great line of triangles has since been carried from
Kurrachee, in the west, to Calcutta, in the east, bases being measured
at each end. From Kurrachee, again, the Indus series of triangles
extends to Attock, where another base was measured, and from Attock
another chain comes down the Punjab back to Dera. There are several
other series of triangles with which I need not trouble you.
In the earlier days of the survey, the instruments used, especially
those for base measurement, were necessarily imperfect, but Colonel
Colby’s compensation apparatus, used in the Irish Ordnance survey, was
afterwards sent out to India, and two 36-inch theodolites, made by
Troughton and Simms, which, I believe, are the very best instruments
ever turned out. Subsequently, several 24-inch theodolites on the
Everest pattern were sent out, besides a first-rate Zenith Sector, and
many other instruments; and all the old work has been re-measured,
while fresh series of triangles are yearly being added. The published
results of the work show that it is singularly accurate, and may bear
comparison, in this respect, with our own or the Russian survey, while
in the magnitude of its operations it surpasses both. I should mention
that for the principal triangulation stations in the plains, high
towers of masonry are built, from which the angles are taken. The
stations on the peaks of hills are marked by small pillars of stone.
Besides the ordinary triangulation, and the determination of the
Latitudes and Longitudes of the chief stations by careful Astronomical
observations, a series of very valuable Pendulum observations has been
lately executed for the Royal Society at intervals along the Great Arc,
the object being to determine the effects of local attraction on the
plumb-line caused by the great mountain chain of the Himalayahs. It was
while engaged in these that Major Basevi, Royal Engineers, died alone
in the solitudes of Thibet.
In addition to the above, a very extensive series of Spirit-Levelling
operations has been undertaken within the last few years by the
Survey Department, the object of which is to have an independent mode
of determining the height of the principal stations in the interior
above the mean sea-level at Kurrachee, and to establish a series of
Bench Marks all over the country, by which the levels taken for any
engineering project may be referred to the Kurrachee datum.
Major Lambton was succeeded by the late Sir George Everest and Sir
Andrew Scott Waugh, F.R.S., now retired, both of the late Bengal
Engineers. The present Superintendent of the Great Trigonometrical
Survey is Colonel J. T. Walker, F.R.S., late of the Bombay Engineers,
who some years ago executed an admirable military survey of the
Trans-Indus frontier under circumstances of great difficulty and peril.
His second in command is Major Montgomerie, F.R.S., late Bengal
Engineers, who received a gold medal from the Royal Geographical
Society for his admirable survey of the kingdom of Cashmere, one of the
completest and most difficult surveys ever executed. Among the other
subordinates is Lieutenant Herschel, F.R.S., late Bengal Engineers, a
son of the great astronomer, and already well known to the scientific
world for his spectroscopic observations on the last two total eclipses
of the sun.
The Topographical Survey is confined to those large tracts of country
which, from their hilly and jungly configuration, would not pay to
be surveyed in the elaborate manner employed in the plains. The
instruments used for the triangulation are 7-inch Everest theodolites,
the stations being always connected with the most convenient points
of the Great Trigonometrical Survey. The details of the country are
filled in by the Plane Table, which is similar to the sketch block,
and though not so portable, is more convenient and perhaps admits of
greater accuracy. Many very beautiful maps have been turned out by this
Department, the head of which for some years was Colonel Robinson, late
Bengal Engineers, one of the most accomplished military draughtsmen in
the service, now Director-General of the Indian Telegraph Department.
The Revenue Survey is, as its name denotes, the survey of land for the
purpose of revenue assessment. It takes up the chief points determined
by the Great Trigonometrical Survey, runs secondary lines of triangles
where necessary, and fills in details by the Gale Traverse system,
which is specially adapted to a flat, open country. The Plane Table
supplements the Traverse work, and the result is a series of maps
which, for minute detail, are scarcely equalled by the 25-inch English
Ordnance maps, for every field is determined and even the nature of the
soil and the crop laid down.
The Revenue Survey is presided over by the Surveyor-General of India,
Colonel Thuillier, F.R.S., late Bengal Artillery, who has for many
years devoted himself specially to this work, and to whom India is
indebted for many improvements.
* * * * *
And now, Gentlemen, I have finished my lectures, and would fain hope
that they have given you half as much pleasure in listening to them as
I have had in preparing them. The field was so vast that I could easily
have extended their length and their number; but my object has been
rather to awaken your interest and stimulate your curiosity than to
supersede your reading.
Footnotes:
[A] I remember a native’s answer to a remark of mine, which shows
that there are always two sides to a question, even a question of
improvement. His Persian wheel, near to which my tent had been pitched,
kept me awake all night with its abominable creaking, and I asked him
in the morning why he didn’t use grease to stop the noise? He said, “I
am accustomed to it, and if the creaking ceases I wake up; I know the
wheel has stopped because the bullock-driver has gone to sleep, and I
go out and beat him.”
[B] The Grand Trunk Road having been first metalled with kunkur when
Lord William Bentinck was Governor-General, that distinguished nobleman
was called by some facetious engineer, William the Kunkurer.
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