Before providing shelter it is as well to study the behaviour of wind in the open and where some shelter is present. A knowledge of this subject is invaluable even when considering the planting of a new hedge in the smallest garden. Prevailing winds, as every gardener knows, cart be the most damaging, but the atmosphere is under pressure from all directions, and winds, which are bodies of air under pressure, flow from all directions. Thus all-round protection is needed in every garden, and, unless it is provided naturally it needs to be provided by plants or structures if a successful garden is to be created. Over a perfectly level surface such as water or a large flat plain, air flows direct and unhindered from high to low pressure areas and only changes direction as the areas of pressure alter. However, no garden is completely flat and every solid projection distorts the flow of air. Hills, houses, forests and so on down to the smallest plants change the direction of the wind to a greater or lesser extent. The flowing air may rise above or go round each obstacle. This tends to increase the wind speed close to the top or side of the obstacle and slightly reduce it on its lee side. A small degree of vacuum is created in the lee of solid barriers and the turbulence that ensues is frequently noticed when dry leaves or snow are there to indicate the swirling effects that are produced. In the same way the higher wind speeds at the corners of buildings and between two solid barriers are often noted. PRACTICAL OBJECTIVES Solid barriers, as we have seen, create their own problems of changing wind direction, increases of speed above or beside them and turbulence behind them. Open screens and thin hedges filter the air flow and thus reduce the turbulence. Such screens reduce the wind speed and allow some air to flow through and fill the vacuum that otherwise might be created. We can arrange for, on the one hand, complete diversion of the wind with consequent turbulence on the lee side of the barrier or, at the other extreme, have a very open screen which has very little effect on wind speeds. For the gardener the optimum amount of reduction in both wind speed and turbulence is provided by screens that have a ratio of about 60 per cent solid to 40 per cent aperture. Thus 2lin(6cm) planks spaced i|in(4cm) apart would come near the optimum for use as a screen to prevent damage by wind. Lath fences made of iin(2.5cm) laths spaced iin(2.5cm) apart and mounted on 2in(5cm) timber with diagonal cross branching, are a most satisfactory, practical design. These have long been used in windy districts. It is more difficult to measure the effectiveness of living hedges as windbreaks; they vary in permeability according to the season and the stiffness or flexibility of their branches and twigs, while deciduous plants are obviously more permeable in winter than in summer. A rough general rule is that a suitable shelter hedge is one where movement can be distinguished when looking through it but the identity of the moving objects cannot be distinguished.