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iron-bound sluice valves were made, and that it must have been no easy task to control the out-flow of the water at reservoirs which had a depth of thirty or forty feet, as was the case at several of the larger works. Yet the similarity of the designs of the bisōkotuwas at all periods proves that the engineers of the third century B.C., if not those of an earlier period, had mastered the problem so successfully that all others were satisfied to copy their designs.
An examination of the bisōkotuwas reveals two invariable and peculiar features in them: they are always rectangular, and the faces of their walls are never rough or uncut. The commonest type of them is an oblong enclosure, ten or twelve feet long by eight or ten feet wide, built across the culvert at a short distance nearer the water than the point where the water-level meets the slope of the bank. It has thick walls of brickwork laid in mortar, round which there is an excellent watertight backing of tempered clay, or 'puddle.' Where the plan is an oblong the longest sides are always built across the culvert. At most works the brickwork is faced or lined inside the well with admirably cut thin slabs of stone, laid horizontally, and invariably on their edges, which fit closely together. Usually they extend as monoliths along the whole length of each wall, and all have well-cut faces, free from any twist. In some cases there is no facing to the brickwork.
The wedging and accurate cutting of these long stone slabs, which are always of gneiss, must have proved a difficult work in pre-Christian times; we may guess that their preparation was the most arduous part of the construction of the sluices. As they have rarely a greater thickness than ten or twelve inches, even when they are two or three feet broad, and ten or twelve, or more, feet long, while the brick wall behind them is often six feet thick, it is clear that in most cases they were not used merely in order to increase the strength of the wall. They may have been inserted partly to protect the front of the brickwork, but the accurate cutting of their faces shows that this was not their only purpose.
In my opinion they were intended to permit the accurate
fitting, close to the face of the wall, of a further lining of woodwork. This alone will account for the excellent manner in which their faces were cut. It would transform the well into a nearly watertight box.
At a few sluices I have observed indications of the manner in which other woodwork was fixed inside these wells. It was evidently in the form of substantial beams or posts, the duty of which must have been partly to support the wooden lining of the walls, and partly to carry some form of liftinggear by which a door or valve might be raised or lowered, so as to regulate the discharge of the water.
This part of the woodwork appears to have varied in design at different sluices, but generally there was a vertical post about one foot square on each side of the entrance to the outlet culvert. These must have been supported by horizontal beams which also held up the wooden lining of the walls, some of them probably resting against other wooden posts standing near the corners. At one sluice at Minnēriya tank, a work of the third century A.D., square sockets were cut in the floor in order to receive tenons left at the ends of the vertical posts.
At a bisōkotuwa at Katiyawa, in the North-central Province, which tradition attributes to the time of Duṭṭha-Gāmiņi, that is, the second century B.C., there are two lateral recesses, two feet square, at the lower corners of the side walls next to the centre of the bank. In this, as at some few other sluices, there is a wide step of ashlar work in the bisōkotuwa, at each side of the inlet culvert, extending up to the side walls; its use is unknown. There is also a cut ten inches wide through the projecting coping stone, above the entrance to the outlet culvert. The age of the work is indicated by the bricks at another sluice; these are of the large type which belongs to the second half of the second century B.C., or the first part of the first century.
Wooden doors or valves, which might slide vertically in wide grooves, must have been placed so as to open or close both the culverts leading out of the wells. Most probably these were worked by means of levers supported by upright posts.
It must have been in order to reduce the friction at these valves that, while there was a single inlet culvert at Pāvatkulam, there were two outlets for discharging the water from the bisōkotuwa, each having a sectional area rather less than half that of the inlet, and thus permitting the use of a door or valve of much smaller size than would be otherwise necessary. This is unmistakable evidence that difficulty had been experienced in other works, before Pāvat-kulam was made, in overcoming the friction due to the pressure of the water on valves of larger area. A similar arrangement is found at many later works.
As no example of the woodwork of the sluices has been preserved, its exact details can only be conjectured; but it is clear from the indications given above that the purpose of these carefully-built bisōkotuwas was to act as true valvepits. Whatever form the design took it was a triumph of the ingenuity of the ancient Sinhalese engineers, and the more surprising when we find one of the earliest sluices furnished with it. Evidently from the first it was a device the general form of which later generations were unable to improve.
It was this invention alone which permitted the Sinhalese to proceed boldly with the construction of reservoirs that still rank among the finest and greatest works of the kind in the world. Without some efficient means of regulating the discharge of the water through the sluices, the provision of reservoirs for storing it could never have extended beyond the minor tanks. Thus, it may be inferred that the bisōkotuwa, with its valves, had not only been designed but had been found to work satisfactorily before the engineers would venture to undertake the construction of Pavat-kulam and Vavunik-kulam, both of which in many years would be of limited use without it. Whether the works of Panduwāsa Dēva or Paṇḍukābhaya were furnished with this means of
1 There are eight or nine post-Christian reservoirs in Ceylon which have areas exceeding 4000 acres ; detailed surveys have been made of one (Maha Kanadara-waewa) which covered 5670 acres, and of another (The Giant's Tank), which was not completed, that apparently would have had an area of 6400 acres; as now restored, the latter covers 4425 acres with water at a very low level.
regulating the outflow of the water is unknown. case it appears to date from either the fourth or the third century B.C.
Every engineer must feel astonishment to observe that the designer of this early sluice enlarged the sectional areas of his inlet and outlet culverts from their entrances to their outlets. He was evidently aware that as the water passed along the culverts the friction of the sides retarded its velocity, and thus rendered an increased space for it necessary in order to avoid undue pressure against the sides and roof. Without such enlargement the resulting increased pressure would tend to force the water through the joints of the masonry, along the back of which it would then flow, gradually removing the soil in suspension until in the end the bursting of the reservoir might be brought about. It is extremely likely that the existing breach at the great Padawiya tank was caused in this manner, and I feel no doubt whatever that other embankments gave way from the same cause; but the designer of Pāvaṭ-kulam cannot have had many opportunities of observing such effects, and it is therefore the more surprising to find him taking these precautions against them.
The use of well-tempered clay 'puddle' round masonry that was subject to water-pressure was perfectly understood at the time when Pāvat-kulam was constructed. It continued to be employed in similar positions at nearly all later sluices, and sometimes round the culverts also. It was always of excellent quality.
A smaller reservoir for storing water for irrigating rice lands was formed at an early date in the valley of the Sangili Kanadara-oya, a small river on the eastern side of the Malwattaoya valley. It had not special features like the last, but was a good average example of a class of reservoirs made solely for irrigation, and occupying a position between the larger village tanks and the great works like those last described.
The embankment, instead of running straight across the bed of the valley as usual, was raised for a great part of its
length in a north-and-south curve, having its convex side facing the reservoir. Its northern end was turned towards the west for 2000 feet, so as to carry part of the flood-waters clear away from the work; its southern end, on the other hand, was deflected sharply eastwards for 1700 feet, to meet high ground.
The bank has a total length of 8100 feet, or about a mile and a half; and its crest was 17 feet above the sills of the low-level sluices. Its top was 10 feet wide; and the sides sloped on the up-stream face at about 4 feet horizontal to I foot vertical, and on the outer face at 3.5 feet to 1. The slope adjoining the water was protected by small boulders up to a height of one foot above the level of the waste-weir.