As the amount of the annual rainfall varies greatly according to the country, and, even in England, considerably in a small area, the earth will be more affected in one place than another; for instance, 48 to 50 inches is approximately the annual average rainfall upon the extreme S.S.W. coast of England, being greatest at the highest level nearest the sea and to leeward; it diminishes gradually from W. to E. to from 26 to 24 inches, the minimum of about 20 inches being in Essex. The differences of quantity must therefore be regarded; but such rainfall is as nothing compared with that of tropical lands, for the fall often continues many hours, and yet equals and perhaps exceeds an inch per hour. The heaviest fall and its usual time of appearance should be ascertained, as earthworks may have to be constructed in a peculiar district where the rainfall may be more than double that of the average wettest district, and it will often be much greater at the foot or the top of neighbouring hills than on a flat coast. Local information from reliable sources is the best guide when confirmed by general knowledge. In the tropics 100 to 200 inches in depth of rain instead of 20 to 30 inches has to be treated, and frequently half the total annual rainfall in England comes down in twenty-four hours. It is almost superfluous to name that the protective works which would be amply sufficient in one country may be useless in another, simply because of the variation in the amount of the rainfall and the capacity of different earths to resist or invite the percolation of water. Obviously percolation will vary considerably with the seasons, and a succession of wet periods or a continuous downpour will increase the quantity of infiltrating water; but the effect of a fall may not be experienced until some time after it occurs, as in districts or rivers that are fed with water from the thawing of snow upon surrounding or distant hills water reaches the lower tracts of country in hot and sunny, and therefore generally dry weather, when evaporation is the greatest, and not in winter. Again, there is generally very little rainfall over flat deserts, but an excess upon mountain ranges which may surround a desert, and especially in tropical countries experience has proved that storms and rainfall are often local and extend over a small area, one district being more liable to such a visitation than another; they are also, as usual, of irregular duration and severity.