In order to secure a higher rate of discharge of sewage through the soil it is best to arrange an artificial bed which shall be made of coarse, sandy material which will allow a rate of at least 10 times that already given. The best material out of which to make such an artificial bed is a coarse sand; that is, a sand whose particles will not pass through a sieve which has 60 meshes to the inch and which would pass through a sieve of 10 meshes to the inch. Such an ideal sand will purify sewage at the rate of 50,000 gallons per acre per day, or an acre will take care of the sewage of at least 1000 persons. This means that it is necessary to provide about 50 square feet for each person in the family, or a family of 10 persons could have all the sewage taken care of on an area 25 feet square. The same principle of intermittency of application, however, must be observed by dividing the bed into three parts, so that the sewage may be alternated from one bed to another. Practice has indicated that it is better to shift from bed to bed about once a week and to deliver the sewage onto each bed intermittently; that is, to discharge a bucketful at a time with short intervals between, rather than to allow a small stream to flow continuously onto a bed. Such a bed should be about 3 feet deep, as already stated, and preferably should have light concrete side walls and bottom, as shown in the sketch (Fig. 68). Ordinarily, the surface of the sand will be level, and the dose of sewage applied to the bed will cover it a fraction of an inch deep, and in the course of an hour or so will disappear into the sand and reappear in the underdrains as clear water. [Illustration: FIG. 68.--Sewage beds.] In cold weather a thin sheet of sewage spread out over the surface of the sand would freeze before penetrating the bed; therefore, in the winter time, it is usual to furrow the beds; that is, dig furrows across the beds 2 or 3 inches wide at the bottom and about 10 inches deep, so that in the bottom of these furrows the sewage may be, partly at least, protected against frost. It has been found that, if sewage is discharged intermittently,--that is, in bucketfuls into such furrows,--the beds open and allow the filtration of the sewage. To be sure, the purification effected in cold weather is not quite that accomplished in warm weather, but the results are sufficiently satisfactory, and no nuisance ensues. _Subsurface tile disposal._ The other method of distributing sewage over land is by means of draintile placed in shallow trenches, so that the sewage may leach out into the soil through the open joints of the pipe. These draintiles receive the sewage intermittently, and by the constant rush of water are presumably filled throughout their length. The sewage then gradually works out of the joints into the surrounding soil, and the pipes are empty and ready to receive another dose when next delivered.