121 



RIVERS. 



RIVERS. 



1J8 



Hamburg, the flowing tide takes 5 minutes to run up a mile, but the 

 ebb tide performs the same distance in lesa 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 mouth t>f 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 up by the tide is 

 pushed onwards by the mass behind it, and carried to a greater 

 e than the inclination of the river bed would seem to allow. 

 It has also beeu 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 affects 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 Pauxis near 

 Ubydos, 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 Kibe between the North 

 Sea and Hamburg, namely, nearly a mile in five minutes, the tide 

 can only reach the Narrow of Pauxis in 42 hours, or in a space of time 

 during which the direction of the tides has changed seven times at 

 it* mouth. It is therefore evident that the current of the Amazonas 

 between the sea and the Narrow of Pauxis must, at the same time, 

 in three or four different part* 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 produce* what is 

 called a ftotr. [Bonn ; WAVE.] 



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 boat* of any shape or 

 form are navigated, except where the nature of the current opposes 

 in-'iIHTable obstacles. These obstacle* consist of tataractt or <! 

 When the river descends from a rock which rises several feet per- 

 tilarly, 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, uln.'li however is rarely the case. The ascent of rapids is 

 Wwlid either by poling or by dragging the boats over the dangerous 

 place by means of long ropes. Sometimes ropes are also nsed in the 

 descent, as in the Rhine at Laufenburg in .Switzerland. But 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. 



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 shallow.and accordingly flat-bottomed 

 boats are used, like the coal-barges in London. Keel-boats 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 boat*. KafU generally consist of trees 

 fastened together with ropes or the flexible branches of trees, or, in 

 warm countries, by creeping plant* ; goods are placed upon the raft. 

 these rafts, with their cargoes, have arrived at their place of 

 destination, the raft itself is sold, either as timber or as firewood, 

 ling to its dimensions and quality, and the crews 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 ningly into the current, and towards the mouth chains are 

 laid acroM the river, above which the trunks collect, and whence they 

 are carried to their dc-xtination. This is frequently done in the rivers 

 lie southern districts of Norway. 



'i which traverse a mountain-region in some parts of their 

 course are either n.t navigalile in this part or only in some places. 

 Thus the Amazons* and Ganges, where they respectively flow within 

 the ranges of the Andes and Himalaya Mountains, are not navigable ; 

 ; Rhine and the Danube are navigable even within the moun- 

 tain-, in some parts for a considerable distance. The most extensive 

 system of internal navigation is presented by those rivers which have 

 e, and whose sources are situated at a comparatively small 

 elevation almve the sea. The Volga is navigable in the wli 



of its course, and the Mississippi up to the Falls of St. Anthony, a 

 distance of about 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 

 sources 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 unite, they generally form a small navi- 

 gable river, and such as, are uot 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, * 

 distance of 45 miles from the Norc, 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 nt 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 the tide, which is kept off from the 

 mouth of the river by the projecting coast of Kent between the two 

 Forelands, and there being consequently 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 ; but 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 28 S. lat., 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 crop. 



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 tin- 

 rivers are much less expensive, and are performed in a shorter time 

 than in summer in the ordinary way. This is the case on some of the 

 riven of New Brunswick and Lower Canada. 



It has been observed, that the outer borders of river-basins are 

 the most elevated parts which occur in some given places between 

 their respective beds, though it is not always the case that the water- 

 parting is formed by mountain-ridges. Owing to such a disposition of 

 the surface, the waters which are collected on or near the borders run 

 to one or the other of the two rivers. Up to the commencement of 

 this century it was thought improbable, if not impossible, that two 

 different river systems or basins could be united by a natural water 

 communication ; but it is now ascertained that a low tract of country 

 or a deep depression of the surface may occur, by which a portion of 

 the water of a river, after being diverted from its own channel, may 

 join a river which otherwise is not connected with that river from 

 which the water branches off. The instances in which this occurs arc 

 very few, and we shall therefore enumerate those whose existence is 

 beyond all doubt. The river Arno, in Tuscany, in that part where it 

 runs between the high ridges of the Apennines and approaches the 

 town of Arezzo, sends an arm southwards through a narrow valley, 

 under the name of Chiana, which falls into the Chiaro, an affluent of 

 the Tiber. The Chiana had been filled with sand, but its course has 

 been re-established by artificial means. Another case occurs in the 

 kingdom of Hanover, a few miles east of the town of Osnabnick, where 

 the river Haase divides into two branches, of which one, running west 

 to Osnabrtick, preserves its name, and, after a course of many miles, 

 joins the Ems ; the other, running east under the name of Elz, falls, 

 after a short course, into the Werre, an affluent of the Weser. In 

 .Sweden, two large rivers fall into the northern extremity of the Gulf 

 of Bothnia the Tornca Elf and the Calix Elf. About 100 miles from 

 the sea, the last-mentioned river sends off an arm to the south-east, 

 which, after a course of about twelve or fifteen miles, falls into the 

 Tornea Elf : this arm is called Tarenda.Elf. In tht-se caws the rivers 



