The amount of rain varies enormously in different parts of the world, some districts being either absolutely rainless, or having only a very few inches of rain in the year, whereas others have some hundreds of inches in the year. Even in England itself there is considerable variation. The average rainfall for the whole country is about 30 inches a year, but the amount in different parts of the country varies from about 20 inches to nearly 200 inches a year. The eastern side of England, as Field remarks, has much less rain than the western side, and, roughly speaking, if a line be drawn from Portsmouth to Newcastle-on-Tyne, it will divide the country into a dry portion and a wet portion. The portion of the country on the east of this imaginary line will (with the exception of the south coast, which is wetter) have only 25 inches of rain or less, and the portion on the west of the line will have from 30 to 50 inches, with much larger amount in the Cumberland and Welsh mountains, and at Dartmoor. The rainfall of the wettest year is about double that of the driest year. This gives a very useful rule for roughly ascertaining the extreme rainfalls, which are really more useful for the purpose of water supply than the rainfall for an average year. The fall in the driest year may be assumed to be one-third less than the average, and for the wettest one-third more. Thus, with an average rainfall of 30 inches, the fall of the driest year would be 20 inches, and that of the wettest year 40 inches. A portion only of the total rain which falls is available for water supply, as there is always more or less loss. In the case of rain falling on roofs, the loss is comparatively small, but in the case of rain falling on the surface of the earth the loss is considerable. The latter is disposed of in three different ways: part of it runs directly into open watercourses and streams, part is taken up by vegetation or lost by evaporation, and part percolates through the surface ground and accumulates in the water-bearing strata which feed the springs and wells.