Two essential points must be considered in the successful operation of such a plant: the grade of the tile and the length of the tile. The grade of the tile must be properly adjusted to the porosity of the soil; that is, in open, porous, and gravelly soils a grade must be steeper than in loamy and dense soils. The reason is manifest. In a gravel soil, the sewage is at first rapidly absorbed, so that as the sewage goes down the pipe line the first joints take up the water and deliver it to the soil, where it disappears, and probably no flow reaches the end of the line at all. This means that the soil surrounding the first joints does the work which the entire pipe line was intended to do and thus becomes overworked. When overworked, the soil always refuses to do anything, so that when the succeeding joints take up the sewage and in their turn become overworked, the line is useless. If, on the other hand, the grade had been steep enough to carry the sewage down the pipe line gradually so as to secure a uniform distribution, then the same or approximately the same amount of sewage would be taken out of the pipe at each joint, securing a long life for the system. In loamy soil, on the contrary, there is not the same absorption at the joints, and so on a steep grade there is the tendency for all the sewage to follow down the pipe line to the lower end and there escape to clog the soil and thus spoil the system. As a general average, it may be said that the proper grade for such a subsurface distribution pipe line in a fairly good sandy loam should be 5 inches in 100 feet; less than this as the loam becomes clay and more as the loam becomes gravel.