Air is free, and the owner of a river bank has only tenuous rights over the water that flows past his property. What nobody owns may be polluted with impunity, unless the law forbids it and is rigorously enforced. Luckily air and water are in constant movement and shed their pollutants easily: they fall from the sky, are deposited as silt, or are swept out to sea. There is therefore a natural tendency for air and water to cleanse themselves if given a chance. Ill treatment of land has far worse consequences: in the short run, land is fixed both in amount and position; a municipal rubbish tip, a slag heap, or twelve feet of alkali waste will ruin it for years. A road, a factory, or a housing estate will also ruin it for agricultural purposes, but by adding value to the land will not arouse quite the same sense of loss. In the long ages of geological time wind, rain and frost can obliterate any damage, but the process is immeasurably slower than the cleansing of air or water. Land has however one important protection that is lacking to air or water — it is not a free good, but for the most part (in Britain) is owned either privately or by a corporation. To some small extent land can be injured by atmospheric pollution or by toxic flooding, but on the whole it is fair to say that if a firm, or public authority, intends to change the character of land it will first have to buy it; in other words, it will incur costs and not evade them by leaving the community to pick up the bill, as happens with the pollution of air or water. The study of land loss and reclamation will therefore take a different course from that followed in preceding chapters. The emphasis will fall not on pollution but on changing land use.