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the west side of America, when it is moderate; but as the waters advance westward, their motion is accelerated ; and after having traversed the globe, they strike with great violence on the eastern shore of America. Being stopped by that continent, they rush, in the form of au impetuous current called by navigators the Gulf-stream, into the Gulf of Mexico, and thence proceed along the coast of North America, till they come to the south side of the great bank of Newfoundland, when they turn suddenly off and run down through the Azores, or Western Isles. This motion is most probably owing to the diurnal revolution of the earth on its axis, which is in a direction contrary to the current of the sea. The third motion is the tide, which is a regular swell of the ocean every 121 hours. This motion is now ascertained to be owing to the attractive influence of the moon, and also partly to that of the sun. There is always a flux and reflux at the same time, in two parts of the globe, and these are opposite to each other; so that when our antipodes have high water, we have the same. When the attractive powers of the sun and moon act in the same direction, which happens at the time of new and full moon, we have the highest or spring tides ; but when their attraction is opposed to each other, which happens at the quarters, we have the lowest or neap tides.
The origin of the numerous springs that break forth from beneath the earth's surface cannot be referred to one exclusive cause. The internal reservoirs by which they are supplied are, in many cases, derived from the water which the earth absorbs from rain and melted snow; and from these reservoirs, wherever there is uneven or mountainous ground, the water flows out by minute fissures in the sides of the hills. But when we see' springs rising up in plains, it is evident that they must have ascended, that is, travelled, in a direc
tion contrary to that produced by the force of gravity, in order to reach the surface. This, no doubt, is sometimes to be attributed to water flowing under ground from distant elevations, and to the natural tendency of a liquid to find its level. But the rising up of springs in plains cannot always be accounted for in this manner; and it has, therefore, been supposed that the earth .contains capillary tubes, which attract the water upwards. It is also evident that such springs as suffer no diminution even from the longest continued dry weather, must be derived from a source quite independent of rains and other external means of supply. They must, therefore, proceed from some vast body of water within the earth ; and it has, with apparent reason, been concluded, that many springs arise from the ocean, filtering through the pores of the earth, the salt particles being lost in the passage. Springs, which have their waters combined with mineral substances, and are, from that circumstance, called mineral, are very numerous, and of various kinds. Warm and hot springs are also common, especially in volcanic countries, where they are some times distinguished by violent ebullitions. Iceland is noted for these curious phenomena: its celebrated boiling fountain, the great Geyser, frequently throws out its contents to the height of more than a hundred feet: sometimes to twice that elevation.
Rivers are to be traced to springs, or to the gradual meltings of the ice and snow, which perpetually cover the summits of all the most elevated ranges of mountains upon the globe. The union of various springs, or of these meltings, forms rivulets; these last follow the declivity of the ground, and commonly fall, at different stages into one great channel, called a river, which at last discharges its waters into the sea, or some great inland lake. The declivities along which descend the various streamsi that flow into one particular river are called its basin; á term, therefore, which includes the whole extent of country from which the waters of the river are drawn. As mountainous regions abound in springs, we find that most rivers, more especially those of the first class, commence from a chain of mountains :
each side of a chain also has its springs, and the rivers which originate on one side flow in the opposite direction to those which rise on the other. As it is the property of water to follow the most rapid descent that comes in its way, the courses of streams naturally point out the various declivities of the earth's surface, and the line, from which rge rivers flow in contrary directions will generally mark out the most elevated parts of the earth. When rivers proceed through a mountainous and rugged country, they frequently fall over precipices and form cataracts, in some cases several hundred feet in depth. The most celebrated falls in the world are those of Niagara, in North America. In the tropical regions, most of the rivers are subject to periodical overflowings of their banks, in consequence of the rains which annually fall in such abundance in these countries during the wet season. The overflow of the Nile was considered by the ancients, who were ignorant of its cause, as one of the greatest mysteries in nature, because in Egypt, where the overflow takes place, no rain ever falls. The apparent mystery is easily explained from the circumstance of the rains descending and the snow melting upon the mountains in the interior of Africa where the Nile rises. The consequent accumulation of the waters among the high grounds gradually swells the river along its whole extent, and in about two months from the commencement of the rains, occasions those yearly inundations, without which Egypt would be a desert. Rivers, in their junction with the sea, present several appearances worthy of notice. The opposition which takes place between the tide and their own currents occasions, in many instances, the collection at their mouths of banks of sand or mud, called bars, on account of the obstruction which they offer to navigation. Some streams rush with such force into the sea, that it is possible to distinguish for a considerable distance their * waters from those of the sea. Many of the largest rivers, as the St. Lawrence and the Rio de la Plata, mingle with the ocean by means of a single outlet, while others, as the Nile, the Ganges, the Volga, the Rhine, and the Orinoco, before their termination, divide into
several branches. In some of the sandy plains of the torrid zone, the rivers divide into branches, and, from the nature of the soil and the heat of the climate, are absorbed and evaporated, and thus never reach the sea. Lakes may
be classed into four distinct kinds. The first class includes those which have no outlet, and which do not receive any running water. They are usually very small; and some appear to be the craters of extinct volcanoes. The second class are those which have an outlet, but which receive no running water. They have been formed by springs flowing into some large hollow:
: upon the water rising up to the top of the hollow, it would, of course, run over the lowest part of the edge, and thus find an outlet; and these outlets are, in some cases, the beginning of very large rivers. The third class, which embraces all those which both receive and discharge streams of water, is much more numerous than any. Though they are the receptacles of many streams from the neighbouring country, they usually have each but one outlet, which often takes its name from the principal river that runs into the lake. The largest lakes of this class are the immense bodies of water in North America, between Canada and the United States. There are five, (Superior, Michigan, Huron, Erie, and Ontario,) almost all like seas in extent, connected together, and having their purity maintained by means of the continual flow of water which is kept up from one to another. Their final outlet to the Atlantic Ocean is the great river St. Lawrence. Lake Baikal, in Asiatic Russia, is also remarkable for its size; it sends forth a large stream which joins the Yenisei. The fourth class of lakes comprises a very small number, but they are the most singular of all in their character. They are those which receive streams of water, and often great rivers, but have no visible outlet. The most celebrated are the Caspian Sea, Lake Aral, and the Deait Sea, all situated in the west of Asia. The Caspian is between 600 and 700 miles long, and, in one part, between 300 and 400 miles in width. It receives some very large rivers, the chief of which are the Volga, the Ural or Yaik, and the Kur. Lake Aral is much smaller
than the Caspian, but possesses the same peculiarities; and, from the character of the isthmus which separates them, it is supposed that they formerly composed one body of water. They are both salt lakes, and are distinguished by marine productions; from which it has been conjectured that they must, at a very remote period, have been connected with the Black Sea. The Dead Sea is still smaller than the Lake Aral, it is also salt and exceedingly bitter.
Library of Useful Knowledge.
CHANGES IN THE SURFACE OF THE EARTH.
From the quiet and regular succession of natural events to which we are accustomed, and the repugnance we feel to the idea that it is possible for the course of nature to suffer interruption, we might, without due investigation, almost persuade ourselves that the phy: sical features and condition of the globe possess an unchangeable character. So far, however, is this from being the case, that there is no country wherein traces are not discoverable of the violent revolutions of which the earth has formerly been the theatre: and even yet it is experiencing changes of a very perceptible kind. Of the several agents which contribute to these changes water has the widest sphere of activity. Streams which descend along the flanks of elevated grounds carry along with them some portion of the materials of their respective slopes, especially when swelled into violence by rains or the melting of snows; and such as come from mountains sweep down with them even some of the fragments of rock that have been collected in the high valleys. In proportion, however, as these streams reach the more level country, and their channels become more expanded, they deposit the fragments and stones, till at last their waters convey along only particles of mud of the minutest kind. If, therefore, these waters do not