If the ground is at first level, then rises and then again becomes level, a rain-gauge at the foot of the slope will, with the prevailing wind blowing up the slope, register too much, and a rain-gauge just beyond the top of the slope will register too little (_Ency. Brit._, Tenth Edition, vol. xxxiii.). 4. =Influence of Forests and Vegetation.=--When the ground is covered with vegetation, and especially forests, the _humus_ or mould formed from leaves, etc., absorbs and retains moisture. It acts like a reservoir, so that the run-off takes place slowly and the denudation and erosion of the soil is checked. The roots of the trees or other vegetation also bind the soil together. Vegetation and forests thus mitigate the severity of floods and reduce the quantity of silt brought into the streams. They also shield the ground from the direct rays of the sun and so reduce evaporation, and thus, on the whole, augment the available rainfall. Forests render the climate more equable and tend to reduce the temperature, and they thus, at least on hills, increase the actual rainfall to some extent. If a forest is felled and replaced by cultivation, the ploughing of the soil acts in the same way as the _humus_ of the forest, and the crops replace the trees; and it has been stated that in the U.S.A. the cultivation is as beneficial as the forests in mitigating floods and checking denudation of the soil (_Proc. Am. Soc. C.E._, vol. xxxiv.). But when forests are felled they are not, at least in hilly country, always replaced by cultivation. Measures to put a stop to the destruction of forests or to afforest or reforest bare land may enter into questions of the régime of streams or the supply of water. On the Rhine, increase in the severity of floods was distinctly traced to deforestation of the drainage area. It is usually said that forests act as reservoirs by preventing snows from melting. This is disputed in the paper above quoted, and it is stated that in the absence of forests the snow forms drifts of enormous depth, and these melt very gradually and act as reservoirs after the snow in the forests has disappeared. 5. =Heavy Falls in Short Periods.=--When rain water, instead of being stored or utilised, has to be got rid of, it is of primary importance to estimate roughly--exact estimates are impossible--the greatest probable fall in a short time. This bears a rough ratio to the mean annual fall. The maximum observed falls in twenty-four hours range, in the United Kingdom, generally from ·05 to ·10 of the mean annual fall--but on one occasion the figure has been ·20,--and in the tropics from ·10 to ·25. Actual figures for particular places can be extracted from the rain registers, but the probability of their being exceeded must be taken into account. The greatest fall observed in twenty-four hours in the United Kingdom is 7 inches, and in India 30 inches in the Eastern Himalayas.