The most important data respecting the depth of rainfall in a given district, for practical purposes, are, the least annual rainfall; mean annual rainfall; greatest annual rainfall; distribution of the rainfall at different seasons, and especially, the longest continuous drought; greatest flood rainfall, or continuous fall of rain in a short period. The available rainfall of a district is that part of the total rainfall which remains to be stored in reservoirs, or carried away by streams, after deducting the loss through evaporation, through permanent absorption by plants and by the ground, and other causes. The proportion borne by the available to the total rainfall varies very much, being affected by the rapidity of the rainfall and the compactness or porosity of the soil, the steepness or flatness of the ground, the nature and quantity of the vegetation upon it, the temperature and moisture of the air, which will affect the rate of evaporation, the existence of artificial drains, and other circumstances. The following are examples: Available Rainfall. Ground. ÷ Total Rainfall. Steep surfaces of granite, gneiss, and slate, nearly 1 Moorland and hilly pasture from ·8 to ·6 Flat cultivated country from ·5 to ·4 Chalk 0 Deep-seated springs and wells give from ·3 to ·4 of the total rainfall. Stephenson found that for the chalk district round Watford the evaporation was about 34 per cent., the quantity carried off by streams 23·2 per cent., leaving 42·8 per cent., which sank below the surface to form springs. In formations less absorbent than the chalk it can be calculated roughly, that streams carry off one-third, that another third evaporates, and that the remaining third of the total rainfall sinks into the earth. Such data as the above may be used in approximately estimating the probable available rainfall of a district; but a much more accurate and satisfactory method is to measure the actual discharge of the streams, and the quantity lost by evaporation, at the same time that the rain-gauge observations are made, and so to find the actual proportion of available to total rainfall. The following Table gives the mean annual rainfall in various parts of the world;-- TABLE OF RAINFALL. Collected by G. J. Symons.