Flush - tanks for trough - closets and latrines are almost invariably of the siphonic type, and ought to be designed so that the action will be started by a drop-by-drop supply. In some cases the tank is filled by repeated discharges of a tipper, and when the tank is filled to the top of the siphon-pipe another discharge of the tipper starts the siphonic action; an additional supply-pipe controlled by a tap can be used to fill the tank more rapidly if required. Sometimes large tippers are used instead of siphon - tanks. These discharge the water with great velocity, and may be used for ordinary trough-closets, but do not give the after-flush required for recharging siphonic latrines.

As a rule, not more than six closets are fixed in one range, but occasionally as many as ten have been flushed from one tank. Where more than six closets are used, the tank must be at or near the middle of the range. For such ranges the tanks are of large size, and care must be taken in designing the building to allow sufficient height at the points where the tanks must be fixed. The height from the floor to the bottom of the tank is generally about 6 ft., but some makers recommend 8 ft. or even more,* to which must be added the depth of the tank and sufficient space above for removing the parts of the siphon in case repairs are required; the dome of an annular siphon is very nearly equal to the depth of the cistern, and space must be allowed for removing this. The following dimensions of the "Tubal" closet-range may prove useful:-

The hoppers are usually of salt-glazed stoneware or enamelled fireclay, and the connecting-pipes and traps of similar materials or of glass-enamelled cast-iron.

No definite rule can be laid down as to the number of flushes per day to be given to a range of trough-closets or latrines. As a rule, a stopcock is fitted on the supply-pipe to the flush-tank, so that the supply can be regulated to give that number of flushes which experience proves to be necessary, and so that the supply can be shut off* when required. The stopcock should be placed in a box fitted with a lock and key. Some water companies stipulate that the connection to every automatic flush-tank shall be by means of a union in which a disc is inserted; the size of the hole in the disc is regulated by the company according to the number of flushes required during the day, and after the disc has been inserted, the union is sealed. The tanks must be supplied directly from the communication-pipe or from the low service or basement cistern, and not from a cistern at any higher level, and must be placed so as to be easily accessible for inspection.

If the tank of a trough-closet is fixed too high, the splashing may be so great as to render the inlet end of the trough quite useless. In one building designed by the writer, three ranges of closets of the type shown in fig. 126 were used, and the manager of the works insisted on the cistern of one range being fixed about 12 ft. above the floor. The result was that the water splashed over the first seat to such an extent that no one would use it.