The total annual rainfall on the whole surface of the earth amounts to about 28,000 cubic miles of water. Only about one-fourth of this amount ever reaches the ocean, but it is either absorbed for a time by animal and vegetable life or lifted through the process of evaporation into the air as invisible moisture, when it is carried over the region of rainfall and there condensed into water and falls back upon the earth--only to go through the same operation again. The whole surface of the earth is divided into drainage areas that lead either directly through rivulets and rivers to the ocean, or into some land-locked basin, where it either finds an outlet under ground or is kept within bounds through the process of evaporation, the same as is the case with our great oceans. In North America the amount of drainage area that has no outlet to the ocean amounts to about 3 per cent. of the whole surface. In other countries the percentage of inland drainage is much larger. The great Salt Lake in Utah is an instance where there is no outlet for the water except through the medium of evaporation. Inasmuch as all rivers and streams contain a certain proportion of salt,--especially in such strongly alkaline land regions as the Great Basin of the North American continent,--these inland lakes in time become saturated with this and other mineral substances. Salt is constantly being carried into the lake by the water of the stream that feeds it, and the water is continually being evaporated, leaving the salt behind. This process has been going on in the valley of Utah for so long a period that 17 per cent. of the contents of the lake is salt. The Humboldt River in Nevada, which empties into a small lake of the same name, and lies at the foot of the Humboldt Mountains, is said to have an underground outlet. This must be the case, because the area of the lake is very small as compared with Salt Lake, while the river that feeds the latter is very small compared with the one that flows into the former. That is to say, in the one case a very small stream empties into a large lake, while in the other case a much larger stream feeds a very small lake. Besides, Humboldt Lake, unlike the Great Salt Lake, is said to be a fresh-water lake; if it had no outlet it would become in time saturated with salt. The largest body of water in the world having no outlet to the ocean is the Caspian Sea, on the border between Asia and Russia in Europe, it being 180,000 square miles in extent.