This is the way in which all rivers and streams are formed. An enormous extent of land thus drained gives rise to thousands of rills, which collect into streams of goodly size, meandering through valleys and pouring their tribute of waters into a main stream, which at last accumulates by additions to its course, and becomes one of those great water-courses which form the highway through a continent. The Amazon, Hoang-Ho, Ganges, Nile, Danube, and Volga, may be cited as some of the greatest water-courses and highways of civilisation. These great rivers are navigable for large ships, even for many hundreds of miles, and it is most fortunate that this navigation is so seldom impeded by falls; for these, although among the grandest and most beautiful objects of nature, yet put a stop to the navigation of a river which would otherwise be passable for hundreds of miles further. Rivers which run through districts subject to periodical rains, or those whose sources lead from mountain slopes where vast quantities of snow are deposited in winter which melts on the approach of summer, are at such seasons of the year suddenly augmented, and generally overflow the banks and inundate the country for miles. This is in many cases a very great misfortune, as it spoils the land, reducing it to a swamp, but not in every case; the overflowing of the Nile brings down with it a great quantity of fertilising vegetable matter, and serves to manure the whole district, for the soil being extremely porous and the heat great, the moisture is soon got rid of, and hence it was a custom of the Egyptians to celebrate the rising of the Nile by feasts and ceremonies. Lakes are formed in the same way as rivers, but, being surrounded by high land, have no outlet for the waters so that they may flow back into the sea. If a lake were about to be formed it would be in this way--the rain descending on the surrounding high lands would run in rills to the lowest part, and there collect, forming a pool which would gradually increase in size as the water flowed into it, this increase continuing until the surface became so large that the evaporation from it exactly equalled the supply, it would then extend no further, but remain a permanent lake or inland sea, giving off vapour to refresh the surrounding country, and again receiving it back by streams and rivers, as does the ocean.