The bottom and sides of such a tank should be made with the best Portland cement and finest granite chippings wrought to a smooth surface, so that the sludge may be easily swept clean away with a squeegee to its outlet valve, as it is very necessary to have the tank thoroughly washed after each emptying if my view of the _clean_ mode of sewage disposal is to be carried out. But with the dirty mode, on the contrary, some of the sludge only should be drawn off and the septic anaerobic action preserved continuously in the tank itself, whereas I prefer that action to have its early and less offensive course in the tank and its completion in a drying bed mixed if possible with farm-yard manure. 4. The aerobic process. The one essential point in this final process, whether in land or “contact beds,” is _sufficient_ aeration (excess as by blowing has no result commensurate with cost of its introduction), and it can be attained by intermittence of sewage and rest, or by continuous passage of sewage through a bed of coarse medium _kept always just moist in all its atoms_ by a rain-like dropping on the surface so carefully adjusted as to moisten all parts and not to form a water-seal in any part of the bed. Intermittence is easily arranged on any scale of working, and continuous filtration, on the contrary, is difficult even for a few thousand gallons a day. [Sidenote: Anticipation of a coming reaction against “fads” and overpressure in sanitation.] Since the above was written our grand old philosopher Herbert Spencer has published a volume of “Facts and Comments”[3] containing a chapter on “Sanitation in Theory and Practice,” which points to a coming reaction against the movement begun, some fifty years ago, by the late Sir Edwin Chadwick and followed up by many enthusiastic exploiters of the popular dread of “germs,” which he associated with bad smells. Of course the professor’s practical acquaintance with Chadwick’s hobby is, as he says, very limited, and his argument, that because sewage and manure smells are harmless in the open air of the country, they should be equally innocuous in a town, falls to the ground when brought to the test of experience, and I trust that Mr. Spencer will forgive me for pointing out that sewer-gas, drawn into a dwelling room, in town or country, through scullery waste pipe or other connection with a sewer in which the air is of lower temperature than that of the dwelling room, is really prejudicial to health whether accompanied or not by disease germs.