In adverting to the advantages which a country derives from its rivers, we must first observe that the water is extensively used for the purposes of domestic economy. It is much purer than that of wells ; for river water in general contains much less saline matter than spring water ; it has also earthy particles in suspension, which may easily be separated by filtration, and which are deposited as a sediment when the water is left to stand for a short time. The water of wells generally contains a small quantity of some mineral salts in solution. The water of rivers is used, and is nearly equal to rain water, for all domestic purposes. The upper courses of rivers are generally inhabited by a small number of species of fish, and the whole amount is not great. But towards their mouths the number both of epeeies and individuals increases. The importance of a river fishery may be esti mated when we consider the quantity of salmon which is taken in the rivers of Britain, or of the beluga and sturgeon which is caught in the neighbourhood of Astrakhan. Many rivers, which are not adapted to the purposes of navigation, are converted into powerful instruments for assisting the industry of a country by the moving-power which they supply for mills and other heavy machinery. The advantage of such a natural moving-power primarily determines the seat of manu factures, as was the case in South Lancashire, where this advantage is combined with abundance of coal. The Atlantic States of North America are generally provided with abundance of streams, a circum stance which favours time establishment of manufactures.
The greatest advantages however which a country derives from its rivers are the facilities which they supply for conveying the produce of agriculture and of manufacturing industry to distant parts at a moderato expense. In this respect the rivers may be compared to the arteries and veins of the human body, which diffuse life and strength through all parts. Navigable rivers vivify, maintain, and excite the efforts of human industry. In many countries, where roads are neglected, it is estimated that the transport of goods by land is four times as expensive as that by means of navigable rivers, and thus many heavy and bulky commodities could not be brought to market but for the cheap conveyance of rivers. In considering the capacity of a river for navigation, two circumstances mainly require notice—how far seafaring vcesehs may ascend, and how far the river is navigable for river boats.
Seafaring vessels can ascend many rivers as far as the tides extend. Indeed some rivers, as the Amazonas, may be navigated by large vessels to a much greater distance than the tide ascends, but in others the waters become shallow long before the limit of tide-water is reached. Still high tides facilitate the navigation of rivers by large vessels, not only by producing a cum-rent contrary to that of the river, but also by temporarily increasing the depth of water so that vessels can sass over [shallows and sandbanks, which at low tides are nearly or quite dry. This is frequently the ease in rivers where the tides rise more than 12 feet. The tides in rivers are not of equal duration, as is the case in most parts of the sea ; but the ebb tides frequently last twice as long as the flowing tides. At Rotterdam the tides flow for about 4 hours and 5 minutes, but the ebb lasts; 7 hours and 55 minutes. The Meer wede at Dordrecht flows against the current of the river for 3 hours and 51 minutes, and with it 8 hours and 9 minutes. This difference is easily explained, when the force of the river current is taken into account. The mine circumstance explains the difference in the velocity of the ebbing and flowing tide. Between the North Sea and Hamburg, the flowing tide takes 5 minutes to run up a mile, but the ebb tide performs the same distance in less than 4 minutes. But it is
difficult to explain the well-established fact that the tides advance much farther into a river than might be expected. When the tide at the month of a river rises 4 feet, we might suppose that it would advance only to such a point in the river, where the surface is 4 feet above the sea, but it has been ascertained that it. advances further. It seems that the volume of water which is carried tp by the tide is pushed onwards by the mass behind it, and carried to a greater distance than the inclination of the river bed would seem to allow. It has also been observed, that during the flowing of the tide the surface of the water in the river presents a somewhat convex form, the water along the banks being a little lower than in the middle of the river, and that during the ebb the contrary takes place. The flowing tide raises the water from below, and thus sooner effects the main body of the river, where it has more room to operate, than the water near the margin. In accordance with this explanation it is observed that the flowing tide is perceptible in the middle, while it is still ebbing along the banks, and that vessels which are at. anchor near the banks are turned round before the water on the surface of the river near the banks begins to flow upward.
In a few rivers the tide ascends to a great distance from the sea. In the Amazonas it is perceptible in the Narrow of Fauxis near °hydra, a distance of nearly 500 miles from the mouth of the river, measured along its course. If we suppose that the tide in this river advances at the rate at which it runs in the Elbe between the North Sea and Ilamburg,—namely, nearly a mile in five minutes,—the tide can only reach thb Narrow of Pauxia in 42 hours, or in a space of time during which the direction of the tides has changed seven times at its mouth. It is therefore evident that the current of the Amazonas between the sea and the Narrow of Pawn's must, at the same time, in three or four different parts of its course, follow the impulse given to it by the tide, and run against the stream. We are of opinion, however, that the tide in the Amazonas advances more slowly than in the Elbe, owing to the stronger current of the Amazonas, and that the number of high tides in the Amazonas, between the two above mentioned points, will probably be found to be five or six. The tide rushes into some rivers with great impetuosity, and produces what is called It bOre, (BORE ; WAVE. j Human ingenuity, even in the lowest state of civilisation, has perceived the use of rivers as means of conveyance. Perhaps all rivers which have water enough to carry the smallest Loath of any shape or form are natigated, except where the nature of the current oppoees insuperable obstacles. These obstacle.. consist of cataracts or of rapids. When the river descends from a rock which rises several feet per pendicularly, it rushes down In a broken sheet of water, and is said to form a cataract. When the water descends with great velocity over an inclined plane of rock, it is said to form a rapid. A cataract may be descended when it is only a few feet high. Rapids may be ascended and descended in most cases with great labour and some danger, when they are not very long, and the bed of the river is free from projecting rocks, which however is rarely the case. The ascent of rapids is effected either by poling or by dragging the boats over the dangerous place by means of long ropes. Sometimes ropes are also used in the decent, as in the Rhine at Laufenburg in Switzerland. lint generally either the whole cargo or a part of it must be taken out of the boat, and carried a certain distance by land. Such a tract, over which the goods must be carried, is called a portage. At long and dangerous rapids the boats themselves must be carried or dragged over the portages.