FOR his Friday evening discourse at the Royal Institution, on November 12, Sir Daniel Hall took as his subject "Soil Erosion: the Growth of the Desert in Africa and Elsewhere". Soil, far from being stable, is easily set in motion by wind or rain if the cover of vegetation and the binding supplied by its roots and humus are unduly disturbed. Deforestation about the headwaters of the streams, followed by grazing by goats which prevent natural regeneration, has brought about the denudation of the hillsides in Levantine countries, has turned the river valleys into malarious swamps and choked the harbours at their mouths. Of recent years the duststorms that have swept across the United States represent the removal of the fertile soil from farming land west of the Mississippi, in many cases to such an extent as to cause the abandonment of the farms. It is not so much agriculture that is to blame, as the continuation of a wasteful system of farming and the breaking up of the sod on soils only fit for regulated grazing. Such wind destruction extends into Canada and has become serious in parts of Saskatchewan and Alberta.