the sailing boats often seem as if they were in the middle of the fields.
At present these rivers are restrained in their courses by banks. When left free they are continually changing their beds; and their courses, at first sight, seem to follow no rule, but as it is termed from a celebrated river of Asia Minor—they seem to "Meander" along without aim or object, though, in fact, they follow very definite laws.
For a considerable part of its course the curves of the Mississippi are so regular that they are said to have been used by the Indians as a measure of distance.
If the country is flat, a river gradually raises the level on each side; the water which overflows during floods, being retarded by trees, bushes, sedges, and thousand other obstacles, gradually deposits the solid matter which it contains, and, thus raising the surface, becomes at length suspended, as it were, above the general level. When this elevation has reached a certain point, the river, during some flood, overflows and cuts through its banks, and, deserting its old bed, takes a new course along the lowest accessible level. This, then, it gradually fills up, and so on, coming back from time to time if permitted, after a long cycle of years, to its first course.
The most celebrated floods are those of the Nile. The river commences to rise towards the beginning of July; from August to October it floods all the low lands, and early in November it sinks again. At its greatest height the volume of water sometimes reaches twenty times that when it is lowest, and yet, perhaps, not a drop of rain may have fallen. Though we now know that this annual variation is due to the melting of the snow, and the fall of rain on the high lands of Central Africa, still, when we consider that the phenomenon has been repeated annually for thousands of years, it is impossible not to regard it with wonder. In fact, Egypt itself may be said to be the bed of the Nile in flood time.
Some rivers, on the other hand, offer no such periodical difference. The lower Rhone, for instance, below the junction with the Saone, is nearly the same all through the year, and yet we know that the upper portion is greatly derived from the melting of the Swiss snows. In this case, however, while the Rhone itself is on this account highest in summer and lowest in winter, the Saone, on the contrary, is swollen by the winter's rain, and falls during the fine weather of summer. Hence the two just counterbalance one another.
Periodical differences are, of course, comparatively easy to deal with. It is very different with floods due to irregular rainfall. Here, also, however, the mere quantity of rain is by no means the only matter to be considered. For instance, a heavy rain in the watershed in the Seine, unless very prolonged, causes less difference in the flow of the river, say at Paris, than might at first have been expected, because the height of the flood in the nearer affluents has passed down the river before that from the more distant ones has arrived. The highest floods are when the rain in the districts drained by the various affluents happen to be so timed that the different floods coincide in their arrival at Paris.
