These two chambers are connected with the reservoir by three brick inlets , 10 feet in diameter . And thy are to hold water to a depth of 54 feet , yet the outer walls ( which are en- closed in an artificial embankment ) have a uniform thick- ness , from the base to top of but eight feet , while safe construction demands that they should be about four times that thickness at the base . These walls which had been built to a height of fifty feet , would not have sufficient strength to resist the water pressure that would come against them , and their de- struction might result in a great losss of water from the reservoir and the demolition of the embankment , by the rush of water in falling a height of 150 feet to reach the valley below . On Dec. 18 , 1902 , on the writer's recom- mendation the Aqueduct Commissioners approved a plan providing for the reinforcement of these walls . The preliminary estimate of the cost to construct the dam under the original plan was $ 4,150,573 . This will be increased to about $ 6,400,000 . The great increase in the cost of the work is due principally to the four above de- scribed additions and modifications namely : the extra depth excavated under the main stone dam , the first ex- tension of 110 feet of the main stone dam , the extension of the main stone dam necessitating the removal of the embankment and core wall and lastly the additional ex- cavation necessary for the foundation of the last exten- sion . Some idea of what these changes have necessitated may be gained from the consideration of but two items , that of the amount of earth excavation which will be trebled and that of the amount of masonry which will be increased from 550,000 to about 800,000 cubic yards .