River boats differ greatly in shape and construction, being always adapted to the nature of each river. Most rivers contain numerous shoals, on which the water is very slaallow,and accordingly flat-bottomed boats are used, like the coal-barges in London. Keebboata can only be used where the river has a depth of a few feet, and is free from shoals and sand-banks. When a river is shallow and rapid, but of considerable width, rafts are substituted for boats. Rafts generally consist of trees fastened together with ropes or the flexible branches of trees, or, in warm countries, by creeping plants; goods are placed upon the raft. When these rafts, with their cargoes, have arrived at their place of destination, the raft itself is sold, either as timber or as firewood, according to its dimensions and quality, and the crows return by land. When a river is too full of cataracts and rapids to allow either boats or rafts to descend, it may still be used for floating down timber or firewood. The trunks of trees, after being deprived of their branches, are thrown singly into the current, and towards the mouth chains are laid across the river, above which the trunk. collect, and whence they are carried to their destination. This is frequently done in the rivers of the southern districts of Norway.
Rivers which traverse a mountain-region in some parts of their course are either not navigable in this part or only in some places. Thus the Amazons and Ganges, where they respectively flow within the ranges of the Andes and Ititnalaya Mountains, are not navigable; but the Rhine and the Danube are navigable even within the moun tains, in some parts for a considerable distance. The most extensive system of internal navigation is presented by those rivers which have a long course, and whose sources aro situated at a comparatively small elevation above the sea. The Volga is navigable in the whole length of its course, and the Mississippi up to the Falls of St. Anthony, a distance of ;bout 1800 miles, measured along the river. Both these rivers, as already observed, have the greater part of their course between hills of small elevation, and they do not traverse a mountain region.
The rivers of England supply the means of an extensive system of inland navigation—a circumstance partly due to their small fall, their soirees being only a few hundred feet higher than their mouths, and partly to the abundant supply of water from rain, mists, and springs. Accordingly, if two rivulets mite, they generally form a small navi gable river, and such are not navigable become useful as feeders to canals. The navigation of most of the rivers of England has been much improved by artificial means.
The Thames is navigable for large sea-vessels to London Bridge, IL distance of 45 miles from the Nore, though the whole course of the river, measured along its windings, hardly exceeds 200 miles. No
river in the world, perhaps the Amazonas excepted, is navigable for vessels of such dimensions for one-fourth of its course. This circum stance is not due solely to the height of the tides, which is about 19 feet at London Bridge, but mainly to the fact that there are no sand-banks at its mouth which prevent the access of large vessels. The river probably brings down sufficient earthy matter to form a bar but owing to the direction of tho tide, which is kept off from tbo mouth of the river by the projecting coast of Kent between the two Forelands, and there being eousequently nothing to oppose the current of the river at its mouth, the earthy matter is carried farther from the coast, and deposited in deep water.
The advantages hitherto enumerated are common to rivers in all parts of the globe; hut there are some countries in which the value of rivers is much increased by the use which is made of the water for irrigation. This occurs in those countries in which it either does not rain at all, or in which rain occurs only at a certain period of the year, and even then only for two or three months. The first class of such countries—for instance, the western coast of South America, between 5' and 29° S. fat, would be uninhabitable but for the rivers which descend from the western declivity of the Andes, and in their course to the sea have furrowed the surface with deep depressions or valleys, in which agriculture is carried on with success as far as the water of the river can be dispersed over the level part of the valleys by small canals. In those warm climates where the rains occur periodically, though only in two or three months of the year, the fields would certainly produce a crop, even without irrigation ; but for more than half the year the soil would produce nothing for want of water. By using the water of the rivers for irrigating their lands, the inhabitants of those countries are enabled to get two, and in many cases three, crops annually. Even in the southern countries of Europe, where rain is very scarce in summer, and not sufficient to maintain vege tation, whilst the heat is excessive, irrigation is practised, and two crops of Indian corn are thus annually obtained, or one crop of wheat and a green erop.
In those countries in which the temperature for three or four months is under the freezing-point, the rivers during that time are covered with ice, and in this state they afford to the inhabitants, in some degree, the advantages which other countries derive from rail ways. Travelling and the transport of goods on the smooth ice of the rivers are much less expensive, and are pelonned in a shorter time than in summer in the ordinary way. This is the case on sonic of the rivers of New Brunswick and Lower Canada.