In large installations for cities and villages it is usual to install either plural alternating siphons or apparatus known as sewage feeds, by means of which the contents of the dosing chamber are discharged upon the different beds in rotation, there generally being four or five beds constructed in each unit. This requires a separate siphon or sewage feed for each bed, and entails considerable expense. However, for smaller plants such as are now being considered, two ordinary siphons may be placed in the same dosing chamber as described in Chapter III, and so primed as to discharge alternately. Then, by means of a diverting manhole or chamber through which the dose must pass, the effluent may be diverted onto two beds in rotation, allowing a third bed to rest, or, if there are five beds, it may be diverted onto two pairs of beds in rotation, allowing a fifth bed to rest. For instance, in the case of five beds, a diverting manhole may be constructed as shown in Fig. 45, and arrangements may be made to couple bed No. 3 with No. 2 or No. 4, allowing bed No. 1 or No. 5 to rest by means of the stop-plank to cut off the flow to either of these beds, as shown in the illustration. Then, when bed No. 3 is to be rested, stop-planks _A_ and _B_ are both closed, and the stop-planks against all pipe outlets are raised. If it is desired to throw bed No. 1 out of use, the stop-plank is placed against the end of the pipe leading to this bed, stop-plank A is raised, and stop-plank _B_ is lowered. One siphon will then discharge onto beds Nos. 4 and 5, and with the next filling of the siphon chamber the second siphon will discharge onto beds Nos. 2 and 3. By a proper combination of the stop-plank positions, any two sets of two beds each may receive alternately the discharge from the siphon chamber while the remaining single bed may be left resting. The method for operating the beds in rotation described above may, of course, be easily applied when only three beds are constructed. A provision for allowing one bed to be thrown out of use for a week or so at a time is very necessary for the reasons stated above. [Illustration: FIG. 44.—Plan of Diverting Manhole.]