There are remains of baths and a cistern of Roman date.
When the sound ceases the cistern is known to be full, and the entrance of further water or syrup is stopped.
Large doors at the side of the cistern are then opened, and as soon as the bags are cool enough they are removed at the expense of very exacting labour and considerable time, and fresh bags and sheaths are fixed in their places ready for filtering fresh liquor.
P pe to water house cistern FIG.
After the sweets have come away, cold water is passed through the char until no trace of lime or sulphate of lime is found in it; then a large manhole at the bottom of the cistern is opened, and the washed and spent char is removed.
A small pipe entering below the false bottom allows the air in the cistern to escape as it is displaced by the water or syrup. In some refineries this pipe, which is carried up to a higher level than the top of the cistern, is fitted with a whistle which sounds as long as the air escapes.
A little to the south-west of the point where the road turns towards the Propylaea was found a large rock-cut cistern or reservoir which Dorpfeld identifies with the Enneacrunus.
A very usual size of cistern forming a convenient unit is one that will hold 20 tons of char.
The whole operation of thus changing a filter occupies about ten minutes, and there is no need for anyone to enter the hot cistern to detach the bags, which are removed in the open air above the mud tank.
Every installation is made up of a boiler or other water heater, a tank or cylinder to contain the water when heated, and a cistern of cold water, the supply from which to the system is regulated automatically by a ball valve.