The effluent from the tank is comparatively clear and inoffensive, and not liable to any subsequent fermentation, the work of decomposition being already done. In this state, there osa l>e no reasonable objection to its discharge into tidal water. It is eminently fitted for utilization on land, containing, as it does, all the constituents of the sewage having any manurial value, in a form immediately available as food for plants; while its freedom from suspended matter removes the difficulty met with in irrigation with crude sewage. It is also in a fit.state for filtration.

The filtration of sewage or sewage-effluent is not a mere straining action. If it were so. the filters would soon clog and become useless. Moreover, the effluent from the septic tank, being free from solids, is not susceptible of improvement by -training. The work to l>e done consists in the oxidation of the ammonia formed in the tank. This is by filtration converted into nitric acid, which at once combines with the bases present to form nitrates. The filtering

. must be divided into at least two part-, each of which will be filled in turn while the other i- emptying and at rest

An automatic alternating gear for turning the sewage from one filter to the other forms |part of the apparatus, and may be described as follows: - The supply of effluent to each filter, and the discharge of the clear water after filtration, are controlled by valves, all connected to one rocking shaft; the clear water from each filter passes into a bed of gravel underlying it, from which it is led by drains into a collecting-well; as the effluent fills the filter, the clear water rises in the collecting-well, and when the filter becomes full, a small quantity of clear Mater overflows from the collecting-well into a bucket carried by the shaft; the water thus thrown into the bucket bears it down the rocking shaft, and thereby actuates all the valves; the flow of effluent to the filter already full is stopped, and it- discharge-valve opened, the effluent being turned on to the empty filter, whoso discharge-valve is at the same time shut down. The water, rushing out from the filter last in use, draw- down after it through the filtering material the charge of air required for dealing with the next dose of effluent. When the bucket which rocks the shaft -ink- into its lower position, its contents are di-rharged through a counterbalance chamber, in which a part of the water remains to hold the valves in place until the other filter shall be full. The overflow from this second filter passes into another bucket, which was raised into position by the sinking of the first, and by means of which the valves are brought back into their original position. The first set of alternating gear installed was naturally regarded with the wholesome distrust engendered by sad experience of automatic devices, but six months' working has demonstrated its absolute reliability.

Plate XV. shows the general arrangements of the septic tank and filter-beds, with the ingenious arrangement whereby the filters are automatically changed so as to give each one a period of net

The first working installation of the septic system consisted of a small tank only, which dealt with the sewage of over thirty houses, as well as that of a large reformatory. It also received. a considerable amount of storm-water. This tank was in use for thirteen months, during which it gave an uniformly clear effluent. At no time was any solid matter taken out of it, nor did it require cleaning at the end of that period. Since then a large installation has been put down, and is working splendidly.

A question naturally suggests itself with regard to pathogenic organisms. If, in order to purify our sewage, we provide conditions favourable to the growth of micro-organisms, will not the germs of disease be thereby retained alive or even increase in number? This question can be answered with a decided negative. Micro-organisms in general, and those of disease in particular, are peculiarly sensitive beings, requiring certain well-defined conditions as to food, temperature, and so on. During the passage of sewage through the tank and filters, any organisms contained in it are subjected to complete changes of environment. First we have the dark airless tank, then free exposure to light au<l air in the effluent-channels, and, lastly, the subjection to strong oxidizing agents in the depths of the filters. During each of these stages, any organisms originally present in the sewage are liable to be preyed on by others better adapted to the conditions in which they are placed. The experiments of the Massachusetts State Board of Health prove that the chances of microbes surviving the passage through a filter are infinitesimal.

In the matter of cost, the advantage of the septic system is not less marked than in that of efficiency. The capacity of tanks and filters required is not greater than with chemical processes, and the whole cost of machinery for preparing and mixing chemicals, and dealing with the sludge, and buildings for the accommodation of the machinery, etc, and for storing the sludge, is saved. The only additional item of cost with the septic tank system is the alternating gear, the valves themselves being common to both systems. The annual cost of working is practically nil.

Not the least advantage of the system is its power of dealing temporarily with extraordinary volumes of sewage far in excess of the normal capacity of the plant, a feature which renders it possible to abandon the risky expedient of discharging slightly-diluted sewage without treatment, whenever the volume of dry-weather flow is slightly exceeded.

With this system the diffienltj often experienced in finding a suitable site for works of sewage-disposal is reduced to a minimum, for a septic apparatus can be placed in situations where a plant on any other system would be impracticnble.