The average of the three stations in the above table is 125 inches per annum, and the average as deduced from the run-off observed during the investigations from 1868-72 is 68 inches. The proportion of run-off to rainfall may be considered high, but it must be remembered that the area of the catchment is comparatively small and in great part sheltered from the sun by forest, there are many cloudy and misty days during the year, the country is mainly ridges and ravines, and composed of rock lightly covered by soil, and tributaries have all rocky beds. For purposes of calculation both of floods and of the total available quantity of water the rainfall over the whole catchment has generally been taken at 100 inches per annum, since the fall during the South-West Monsoon was known to be greater, at the site of the dam, than the computation from run-off. There is reason however to believe 100 inches to be somewhat over the mark. The records maintained at the dam during its construction show an average of about 76 inches, and it was observed that the fall due to the South-West Monsoon decreased sensibly during its progress eastward. In the most easterly portions of the catchment the North-East Monsoon doubtless brings more rain than that recorded at the dam site, but its duration is comparatively so short that it probably does not make up for the easterly decrease of the South-West rainfall. Assuming the average to be 70 inches or 80 inches and the catchment to be 250 square miles, the whole of this quantity at any rate, less evaporation and small minor abstractions, falls into the Periyár, because of the impervious nature of the subsoil, and though the rainfall is variable it never fails altogether. There is obviously then an amount of water flowing down the Periyar sufficient for a large area of irrigation, could it only be diverted to the plains of Madura, and this was the object of the investigations now to be described.
The idea of diverting the Periyár into Madura has existed for an unknown time, but merely as an idea. No enquiry was made into its practicability till 1808, when the late Sir James Caldwell visited the neighbourhood and took a few levels. He however seems to have confined himself to a diversion, pure and simple, by means of a direct cutting from the Periyar through the watershed, and finding a rise of over 100 feet between these two points he condemned the project as "decidedly chimerical and unworthy of any further regard," which as thus conceived it undoubtedly was. The subject was mooted in a desultory manner from time to time, and in 1850 a small dam and
