If all the lands were of the same height, the rain would generally come in largest proportion upon their coastal belt, or those portions of the shore-line districts over which the sea winds swept. But as these winds vary in the amount of the watery vapour which they contain, and as the surface of the land is very irregular, the rainfall is the most variable feature in the climatal conditions of our sphere. Near the coasts it ranges from two or three inches in arid regions--such as the western part of the Sahara and portions of the coast regions of Chili and Peru--to eight hundred inches about the head waters of the Brahmapootra River in northern India, where the high mountains are swept over by the moisture-laden airs from the neighbouring sea. Here and there detached mountainous masses produce a singular local increase in the amount of the rainfall. Thus in the lake district in northwestern England the rainfall on the seaward side of mountains, not over four thousand feet high, is very much greater than it is on the other slope, less than a score of miles away. These local variations are common all over the world, though they are but little observed. In general, the central parts of continents are likely to receive much less rainfall than their peripheral portions. Thus the central districts of North America, Asia, and Australia--three out of the five continental masses--have what we may call interior deserts. Africa has one such, though it is north of the centre, and extends to the shores of the Mediterranean and the Atlantic. The only continent without this central nearly rainless field is South America, where the sole characteristic arid district is situated on the western slope of the Cordilleran range. In this case the peculiarity is due to the fact that the strong westerly setting winds which sweep over the country encounter no high mountains until they strike the Andean chain. They journey up a long and rather gradual slope, where the precipitation is gradually induced, the process being completed when they strike the mountain wall. Passing over its summit, they appear as dry winds on the Pacific coast. Even while the winds frequently blow in from the sea, as along the western coast of the Americas, they may come over water which is prevailingly colder than the land. This is characteristically the case on the western faces of the American continent, where the sea is cooled by the currents setting toward the equator from high latitudes. Such cool sea air encountering the warm land has its temperature raised, and therefore does not tend to lay down its burden of moisture, but seeks to take up more. On this account the rainfall in countries placed under such conditions is commonly small.