Many old soil-pipes are trapped at the foot. This trap is a source of danger, especially if two or more closets on different floors are connected to the soil-pipe. When the uppermost closet is discharged, the water acts as a piston, and must expel part of the air in the soil-pipe either through the trap at the foot or through the trap of one of" the lower closets. As the latter generally offers less resistance, the air (fouled by the soil-pipe) is driven into the building. The force of the upper discharge is sometimes so great as to eject a considerable portion of the contents of the lower trap on to the floor of the room; in other cases, the water in the trap may be merely raised in the basin, but the momentum gained in falling back will carry part of the water out of the trap into the soil-pipe. Thus the trap may be either partially or wholly unsealed.