Graham's Magazine, Vol. XLI, No. 5, November 1852. VariousЧитать онлайн книгу.
was fought, the Avon, Trent, and Nen receive affluents, which reach the ocean at opposite coasts of the island, through the Humber, the Wash, and the Bristol Channel. The field in question is an elevated piece of table-land in the centre of England. The district referred to, where rivers proceeding to the Baltic and the Euxine take their rise, is a plateau about a thousand feet above the level of the sea. The springs of the Missouri and the Columbia are in the Rocky Mountains; and it is generally the case, that those parts of a country from which large rivers flow in contrary directions, are the most elevated sites in their respective districts, consisting either of mountain-chains, plateaus, or high table-lands. There is one remarkable exception to this in European Russia, where the Volga rises in a plain only a few hundred feet above the level of the sea, and no hills separate its waters from those which run into the Baltic. The great majority of the first-class rivers commence from chains of mountains, because springs are there most abundant, perpetually fed by the melting of the snows and glaciers. They have almost invariably an easterly direction, the westward-bound streams being few in number, and of very subordinate rank. Of rivers flowing east, we have grand examples in the St. Lawrence, Orinoco, Amazon, Danube, Ganges, Amour, Yang-tse-Kiang, and Hoang Ho. The chief western streams are the Columbia, Tagus, Garonne, Loire and Neva, which are of far inferior rank to the former. The rivers running south, as the Mississippi, La Plata, Rhone, Volga and Indus, are more important, as well as those which proceed to the north, as the Rhine, Vistula, Nile, Irtish, Lena and Yenisei. The easterly direction of the great rivers of America is obviously due to the position of the Andes, which run north and south, on the western side of the continent, while the chain of mountains which traverses Europe and Asia, from west to east, cause the great number of rivers which flow north and south. In our own island, the chief course of the streams is to the east. This is the case with the Tay, Forth, Tweed, Tyne, Humber and Thames, the Clyde and Severn being the most remarkable exceptions to this direction. The whole extent of country from which a river receives its supply of water, by brooks and rivulets, is termed its basin, because a region generally bounded by a rim of high lands, beyond which the waters are drained off into another channel. The basin of a superior river includes those of all its tributary streams. It is sometimes the case, however, that the basins of rivers are not divided by any elevations, but pass into each other, a connection subsisting between their waters. This is the case with the hydro-graphical regions of the Amazon and Orinoco, the Cassiaquaire, a branch of the latter, joining the Rio Negro, an affluent of the former. The vague rumors that were at first afloat respecting this singular circumstance, were treated by most geographers with discredit, till Humboldt ascertained its reality, by proceeding from the Rio Negro to the Orinoco, along the natural canal of the Cassiaquaire.
Rivers have a thousand points of similarity, and of discordance. Some exhibit an unbroken sheet of water through their whole course, while others are diversified by numerous islands. This peculiarly characterizes the vast streams of the American continent, and contributes greatly to their scenical effect, of which our illustration gives us an example, selected from the beautiful Susquehanna, the largest Atlantic river of the United States. The St. Lawrence, soon after issuing from the Lake Ontario, presents the most remarkable instance to be found of islands occurring in a river channel. It is here called the Lake of the Thousand Islands. The vast number implied in this name was considered a vague exaggeration, till the commissioners employed in fixing the boundary with the United States actually counted them, and found that they amounted to 1692. They are of every imaginable size, shape, and appearance; some barely visible, others covering fifteen acres; but in general their broken outline presents the most picturesque combinations of wood and rock. The navigator in steering through them sees an ever-changing scene, which reminds an elegant writer of the Happy Islands in the Vision of Mirza. Sometimes he is inclosed in a narrow channel; then he discovers before him twelve openings, like so many noble rivers; and soon after a spacious lake seems to surround him on every side. River-islands are due to original surface inequalities, but many are formed by the arrest and gradual accretion of the alluvial matter brought down by the waters.
There is great diversity in the length of rivers, the force of their current, and the mass and complexion of their waters; but their peculiar character is obviously dependent upon that of the country in which they are situated. As it is the property of water to follow a descent, and the greatest descent that occurs in its way, the course of a river points out generally the direction in which the land declines, and the degree of the declination determines in part the velocity of its current, for the rapidity of the stream is influenced both by its volume of water and the declivity of its channel. Hence one river often pours its tide into another without causing any perceptible enlargement of its bed, the additional waters being disposed of by the creation of a more rapid current, for large masses of water travel with a swift and powerful impetus over nearly a level surface, upon which smaller rivers would have only a languid flow. In general, the fall of the great streams is much less than what would be supposed from a glance at their currents. The rapid Rhine has only a descent of four feet in a mile between Shaffhausen and Strasburg, and of two feet between the latter place and Schenckenschauts; and the mighty Amazon, whose collision with the tide of the Atlantic is of the most tremendous description, falls but four yards in the last 700 miles of its course, or one-fourth of an inch in 1¼ mile. In one part of its channel the Seine descends one foot in a mile; the Loire, between Pouilly and Briare, one foot in 7,500, and between Briare and Orleans one foot in 13,596; the Ganges, only nine inches; and, for 400 miles from its termination, the Paraguay has but a descent of one thirty-third of an inch in the whole distance. The fall of rivers is very unequally distributed; such, for instance, as the difference of the Rhine below Cologne and above Strasburg. The greatest fall is commonly experienced at their commencement, though there are some striking exceptions to this. The whole descent of the Shannon, from its source in Loch Allen to the sea, a distance of 234 miles, is 146 feet, which is seven inches and a fraction in a mile, but it falls 97 feet in a distance of 15 miles, between Killaloe and Limerick, and occupies the remaining 219 miles in descending 49 feet. When water has once received an impulse by following a descent, the simple pressure of the particles upon each other is sufficient to keep it in motion long after its bed has lost all inclination. The chief effect of the absence of a declivity is a slower movement of the stream, and a more winding course, owing to the aqueous particles being more susceptible of divergence from their original direction by impediments in their path. Hence the tortuous character of the water-courses, chiefly arising from the streams meeting with levels after descending inclined planes, which so slackens their speed that they are easily diverted from a right-onward direction by natural obstacles, to which the force of their current is inferior. The Mæander was famed in classical antiquity for its mazy course, descending from the pastures of Phrygia, with many involutions, into the vine-clad province of the Carians, which it divided from Lydia near a plain properly called the Mæandrian, where the bed was winding in a remarkable degree. From the name of this river we have our word meandering, as applied to erratic streams.
This circumstance increases prodigiously the extent of their channels, and renders their navigation tedious, but the absence of that velocity of the current which would make it difficult is a compensation, while a larger portion of the earth enjoys the benefit of their waters. The sources of the Mississippi are only 1250 miles from its mouth, following a straight line, but 3200 miles, pursuing its real path; and the Forth is actually three times the length of a straight line drawn from its rise to its termination. The rivers which flow through flat alluvial plains frequently exhibit great sinuosities, their waters returning nearly to the same point after an extensive tour. The Moselle, after a curved course of seventeen miles, returns to within a few hundred yards of the same spot; and a steamer on the Mississippi, after a sail of twenty-five or thirty miles, is brought round again, almost within hail of the place where it was two or three hours before. In high floods, the waters frequently force a passage through the isthmuses which are thus formed, converting the peninsulas into islands, and forming a nearer route for the navigator to pursue. By the “grand cut off” on the Mississippi, vessels now pass from one point to another in half a mile, in order to accomplish which they had formerly a distance of twenty miles to traverse.
Rivers receive a peculiar impress from the geological character of the districts through which they flow. Those of primary or transition countries, where sudden declivities abound, are bold and rapid streams, with steep and high banks, and usually pure waters, owing to the surface not being readily abraded, generally emptying