substitution of the earth dam formerly proposed, by one of granite masonry and concrete. It is very necessary, even though at the risk of prolixity, to follow and recapitulate all the changes and improvements which have followed the original design for these water-works. It is to these structural amendments and improve- ments that we owe those marked increases in the estimate of cost which have caused surprise and which but for this explanation might be unjustly ascribed solely to miscalculation or extravagance.
The subject of masonry versus earth reservoir-dams is one which has greatly occupied the attention of the engineering profession during the last fifteen years.
Within this period a consensus of scientific opinion appears to have declared in favour of masonry notwithstanding the greatly increased cost, as against earthen and puddled clay dams, for heights of 100 feet or exceeding 100 feet. Lofty earthen structures of this type are known only in England. The French and other Con- tinental engineers have always rejected them as things of chance, and with us, I apprehend their allurement has been only their comparative economy of cost. They are not of ancient origin in the United Kingdom, having been borrowed only since our conquest of India, from the practice of the Hindoos, with whom however bunds of earth for the creation of artificial lakes and tanks have seldom as a rule exceeded a height of 100 feet. No doubt the bursting of the Bradfield reservoir dam near Sheffield in 1864, did much to modify previous opinions as to the efficacy of lofty dams of clay and earth. In this case a leak had crept into the heart of the bank by following the outside surface of the large iron supply-pipes that traversed the structure, and this leak having succeeded in penetrating across the clay heart on to the other side, the final bursting of the dam became simply a matter of moments. Unfortunately this happened in the night, and the Bradfield reservoir suddenly liberated, was hurled down the valley sweeping before it with irresistible force-villages, mills and houses and causing a loss of life and property that has made the catastrophe historical. But however deeply to be deplored, it has been asserted that this calamity has been productive indirectly of much good, for it caused many municipal and other water authorities throughout the country, to look to the condition and stability of such of their own reservoir dams as were made of earth and clay. At the present time many of the deeper reservoirs of England are never filled to their proper height for fear of accidents. In order to prevent rupture, the greatest caution is exercised in the management of these dams, the water being kept 20 or 30 feet below the level originally designed. It would seem also that in addition to these experiences, the construction of lofty earthen dams received shortly before his death, the condemnation of Professor MACQUORN RANKINE who then stood at the head of the civil engineering profession of England in its theore- tical branch. In a paper which he wrote in connection with the then proposed water-supply of Bombay, Professor RANKINE laid it down, that it was doubtful whether any earthen dam was to be relied on where the depth of water exceeded 100 or 120 feet.
In these circumstances, when in 1882 the old proposition of 1873 for an earthen embankment at Tytam came again under review, the flood of new light that had been shed on this particular branch of reservoir construction during the long intervening sleep of nine years, decided me to recommend a masonry dam in spite of the increased cost. It is true that I had to deal only with a height of ninety feet and might still therefore have adhered to an earthen fabric without incurring danger, but I conceived that in future years, as the necessities of the Colony increased with the growth of Victoria and the increase of the population, my successors in office would be called upon to raise the height of the bank in order to impound more water, and that consequently, the structure should be designed ab initio as one that must eventually be carried up over the 100 feet in height. A leak through the Tytam dam if built of earth and puddled clay, would mean the destruction and total sweeping away of the entire structure and the con- sequent loss of the capital sunk in its construction. A leak through the same dam if made of masonry or cement-concrete, means only so much loss of water and does not in any way affect its safety. In Colonial works, unavoidably subject to inter- rupted or to imperfect supervision and to the many other risks and difficulties affecting ultimate stability unknown to engineers at home, it is unquestionably truer economy, if there is to be any venturing of capital, to adopt the safest methods of construction even though involving increased cost.
These also would appear to have been the views of Mr. CHADWICK who in his preliminary report to the Imperial Government, written from Hongkong, on the subject of the water-supply advocated independently of anything I had advanced, a masonry dam in lieu of the
99