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.