IV. LAKES AND RIVERS. 1. Lakes. An inland body of water not immediately connected with the ocean or any of its branches is called a lake; but some bodies of this description are also commonly called seas. They are generally fresh, but are salt when situated in districts of which the soil contains saline matter. 2. Classes of Lakes. There are four sorts of lakes. I. The first class includes those which have no outlet and receive no running water; these are usually very small. 11. The second class comprises those which have an outlet, but which do not receive any running water. They are generally in elevated situations, and are often the sources of large rivers; they are formed by springs rising up into a large hollow, until the water runs out over the lowest part of the edge of the basin. III. The third class embraces those lakes which receive and discharge streams of water, and is the most numerous. These lakes are the receptacles of the waters of the neighboring country, but in general have but one outlet which bears the name of the principal river that enters the lake. Such a river is said to traverse or flow through the lake, though not with strict propriety, since its current is commonly lost in the general mass of waters, and the outlet is in fact a newly formed river. The largest lakes of this class are the great lakes which lie on the northern frontier of the United States, and of which the St. Lawrence is the only outlet to the sea. IV. The fourth class of lakes includes those which receive, without discharging rivers. The largest of these is the Caspian Sea, which swallows up several large rivers; Lake Aral also belongs to this class. They are both salt, and this is the case with most of those which have no outlet. 3. Periodical Lakes. In tropical countries the violence of the rains often forms temporary lakes, covering spaces of several hundred miles in extent. South America has large lakes which are annually formed during the rainy season, and are therefore called periodical lakes; they are again dried up by the heats of a vertical sun. 4. Lagoons. The waters of one river or several rivers before reaching the sea sometimes spread out over a large surface, filling a shallow basin, which communicates with the ocean by a narrow channel. The eastern shore of the Southern States, and the coasts of the Gulf of Mexico present a great number of these basins, described under the various names of sounds, lakes, and bays; they may be more properly and conveniently denominated lagoons. 5. Rivers. Rivers are natural drains, which convey to the sea that portion of the waters falling upon the earth, which does not pass off by evaporation, or go to nourish organic bodies. The sources of rivers are generally springs, or small streams, fed by the melting of snow and ice upon the mountains, or by rains. 6. Basin. The district from which the waters of a river are derived, is called its basin. The basin is bounded by highlands, which are sometimes mountainous, and which divide it from other basins. The water descending from the water-shed or dividing ridge collects into brooks, the brooks unite into rivulets; the rivulets united form the main trunk or river, which conveys the waters of the whole to the sea. All these descend over inclined planes, so that the lowest point of each brook is that where it joins the rivulet; the lowest point of the rivulet that where it unites with the main stream; and the lowest point in the whole system that where the river falls into the sea. These basins form important natural divisions. Those streams which empty themselves into larger streams are called the tributaries of the latter. 7. Bed, Banks, &c. The cavity or channel, in which a river flows is called its bed, and generally has the appearance of having been cut or worn by the current itself. The borders of the channel are called the banks of the river; that bank which is to the right of a person descending the stream, or facing the mouth of the river, is called the right bank, and the opposite is the left bank. The mouth of a river is the point, where it enters into a lake, sea, or another river; in the latter case the point of junction of the two streams is called the confluence. 8. Estuary, Delta. Many of the largest rivers mingle with the sea by means of a single outlet, in which case they often spread into wide expanses, called estuaries or friths. Others before their termination divide into several branches, embracing a triangular space of land called a delta, from its resemblance to the shape of the fourth letter of the Greek alphabet (▲). 9. Falls, Rapids, Bore. When the bed of a river suddenly changes its level, so that the water plunges down a considerable descent, it forms a fall, cascade, or cataract. When its current is accelerated by a considerable inclination in its bed, or broken by a series of descents of little height, it forms rapids. Some streams rush with great force into the sea carrying a large mass of water, which encounters the ocean tide, advancing in the opposite direction; the collision of the opposing currents produces a tremendous shock; this phenomenon is called a bore. 10. Bars. The opposition which takes place between the tide and the currents of rivers causes, in many instances, collections of mud or sand at their mouths, which are called bars, on account of the obstruction which they offer to navigation. 11. Periodical Floods. All rivers are more or less subject to occasional or periodical floods or inundations. Within the tropics, these floods are produced by the annual rains, and occur during the summer months, but beyond the tropics, they occur at various seasons, and in high latitudes chiefly in the spring, when the snow and ice melt. In some cases where the river banks are high, the water merely rises in the bed of the river; but where the banks are but little above the level of the river-bed, the waters overflow them, forming vast lake-like expanses. 12. Alluvial Deposits. Rivers which pass through low and level tracts in their annual inundations, deposit the earth, sand, and gravel brought down by their waters, on their banks, and raise them gradually above the surrounding country, while a part of the matter carried to the sea extends the coast, or forms sand or mud banks, which rise by degrees above the water. It is thus that the Ganges, Po, Nile, Mississippi, and many other rivers flow on the top of ridges, behind which are cultivated and inhabited districts, lying lower than the level of the waters. During floods the elevated sides are sometimes burst through, and the waters which escape stagnate in temporary lakes, or return into the main stream lower down, or travel to the sea by a separate mouth. V. CLIMATE AND WINDS. 1. Climate. The term climate expresses the particular combination of temperature and moisture which characterises the atmosphere of any particular place. We may distinguish in general six different combinations or climates, which, however, are infinitely diversified in degree thus we have warm and moist, warm and dry, temperate and moist, temperate and dry, cold and moist, and cold and dry climates. 2. Causes of Climate. There are nine circumstances which determine the character of climate: 1. The sun's action upon the atmosphere; 2. the temperature of the earth; 3. the elevation of the ground above the level of the ocean; 4. the general slope of the ground and its particular exposure; 5. the position and direction of mountains; 6. the neighborhood and relative situation of great bodies of water; 7. the nature of the soil; 8. the degree of cultivation and density of population; and 9. the prevailing winds. 3. Seasons of the Torrid Zone. There are only two seasons in the torrid zone; the dry and the rainy or wet. The latter prevails in the tropical regions over which the sun is vertical, and is succeeded by the dry season when the sun retires to the other side of the equator. The rains are produced by the powerful action of a vertical sun, rapidly accumulating vapors by evaporation, which then descend in rains; this arrangement is wisely adapted to afford a shelter from the perpendicular rays of the sun. In some regions there are two rainy seasons, one of which is much shorter than the other. 4. Seasons of the Temperate Zones. The four seasons which we distinguish in this country are known only in the temperate zones, which alone are blessed with the varied charms of spring and autumn, the tempered beats of summer, and the salutary rigors of winter. In the part of the temperate zone bordering on the tropics the climate resembles that of the intertropical regions; and it is between 40° and 60° of latitude, that the succession of seasons is most regular and perceptible. 5. Seasons of the Frigid Zones. Beyond the 60th degree of latitude only two seasons take place; a long and severe winter is there suddenly succeeded by insupportable heats. The rays of the sun, notwithstanding the obliquity of their direction, produce powerful effects, because the great length of the days favors the accumulation of heat; in three days the snow is dissolved, and flowers at once begin to blow. 6. Wind is a current of air moving in some particular direction; the velocity and force of winds are various. The following table shows the degrees of velocity of different winds. Velocity.-4 or 5 miles an hour. Name of the wind.-Gentle wind, 10 to 15 30 to 35 50 80 to 100 Brisk Gale, 7. Permanent, Periodical and Variable Winds. Winds may be a.vided into three classes; permanent winds or those which flow constantly in the same direction; periodical winds, or those which flow in one direction only a certain part of the year, and variable winds, which are constantly changing their direction. 8. Trade-winds. The permanent winds blowing constantly between, and a few degrees beyond the tropics, from east to west, are called trade-winds. They prevail in the Pacific, Atlantic, and parts of the Indian ocean, to about 30o each side of the equator, being on the north a little from the north-east, and on the south from the south east. In sailing therefore from the Canaries to Cumana, or from Acapulco to the Philippines, the winds blow so steadily, that it is hardly necessary to touch the sails. 9. Monsoons. In the Indian ocean to the north of 10° S., and in the seas around Malaysia, there prevail periodical winds called monsoons, which blow half the year from one quarter, and the other half from the opposite direction; at the time of their shifting or breaking up, variable winds and violent storms prevail. On the north of the equator a south-west monsoon blows from April to October, and during the rest of the year a north-east monsoon; on the south of the equator a southeast wind prevails from April to October, and a north-west wind the other half of the year. 10. Land and Sea Breezes. There is another kind of periodical winds, common on islands and coasts in tropical countries. During, the day, when the air over the land is heated by the sun a cool breeze sets in from the sea; this blows from about 10 A. M. to 6 P. M. At night on the contrary a land-breeze prevails, that is the wind sets off from the land till about 8 A. M., when it dies away. 11. Hurricanes, Whirlwinds, &c. Hurricanes are violent storms of wind, blowing with great fury, often from opposite points of the compass, and causing dreadful devastations. They are rare beyond the tropics. Whirlwinds are sometimes caused by two winds meeting each from different directions, and then turning rapidly round upon a centre, and sometimes by the form of mountains, which occasions gusts of wind to descend with a spiral or whirling motion. The simoon of the desert of Sahara, the samiel of the Arabian deserts, the chamseen of Egypt, and the harmattan of Guinea, the solano of Spain, the sirocco of Italy, and the northwest wind of New South Wales, are noxious hot winds, some of which merely produce languor, while others if admitted into the lungs cause suffocation. VI. GEOGRAPHICAL DISTRIBUTION OF PLANTS. 1. Number and Distribution of Species. Each plant has generally a determinate climate to which it is best adapted; there are other climates, however, in which it can be raised, though less advantageously, but beyond certain limits it ceases to grow altogether. The whole number of species at present known amounts to 44,000, but it is estimated that the total number of existing species is about 80,000. The most simply organised plants, such as mosses, lichens, grasses, &c., which form the lowest order of the vegetable creation are the most widely diffused; the more perfect tribes are in general limited to particular regions, and, in some cases, as for example, the cedar of Lebanon, to a particular mountain or district. 2 Vegetation of the Frigid Zones. There are properly no plants which are peculiar to the frigid zone, because the mountains of the torrid zone, embracing every variety of climate between their base and summit, are capable of producing all the vegetables of the temperate and frigid regions. The number of vegetable species in the frigid zone is small; the trees are few and dwarfish, and as we advance towards the poles finally disappear. But mosses, lichens, ferns, creeping plants, & no some berry-bearing shrubs thrive during the short summer. 3. Vegetation of the Temperate Zones. In the high latitudes are the pine and the fir, which retain their verdure during the rigors of winter. To these, on approaching the equator, succeed the oak, elm, beech, lime, and other forest trees. Several fruit-trees, among which are the apple, the pear, the cherry, and the plum grow better in the higher latitudes; while to the regions nearer the tropics belong the olive, lemon, orange and fig, the cedar, cypress, and cork-tree. Between 300 and 50° is the country of the vine and the mulberry; wheat grows in 60°, and oats and barley a few degrees farther. Maize and rice are the grains more commonly cultivated in lower latitudes. 4. Vegetation of the Torrid Zone. The vegetation of the torrid zone, where nature supplies most abundantly moisture and heat, is the most remarkable for its luxuriance and the variety of its species. The most juicy fruits and the most powerful aromatics, the most magnificent and gigantic productions of the vegetable creation, are found in the intertropical regions. There the earth yields the sugar-cane, the coffee-tree, the palm, the bread-tree, the immense baobab, the date, the cocoa, the cinnamon, the nutmeg, the pepper, the camphor-tree, &c., with so many dye-woods and medicinal plants. At different elevations of soil the torrid zone exhibits, in addition to its peculiar forms, all the productions of the other regions of the earth. VII. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 1. Number and Distribution of Species. The number of species in the animal kingdom has been estimated at about 100,000. Each genus is generally confined to a particular region or climate, and where the same genus is found in different continents the species are different. Most of the domestic animals (the horse, cow, dog, sheep, goat, hog, and cat) thrive in nearly every variety of climate, although some of them become more or less degenerate in high latitudes. The camel and the elephant on the contrary cannot be naturalized in the colder climates. 2. Zoological Regions. The earth appears to be divided into at least eleven zoological regions or districts, of which each is the residence of a distinct set of animals; (1.) The Arctic region contains several tribes common to the eastern and western continents, a circumstance owing doubtless to the com munication between them afforded by means of ice. (2.) The temperate regions of the eastern continent are inhabited by peculiar races quite distinct from the kindred tribes of the (3.) corresponding zone in the American continent. The equatorial region contains four exten sive tracts, widely separated from each other by seas, and each peopled. by distinct races; these are (4.) the intertropical parts of Asia; (5.) those of Africa; (6.) those of America; (7.) the islands which constitute Malaysia; and (8.) Papua and the surrounding islands. (9.) The extensive region of New Holland forms a distinct zoological province, inhabited by several very singular tribes; and the southern extremities (10.) of America, and (11.) of Africa, separated from the northern temperate regions of their respective continents by the heats of the torrid zone, are each distinguished by peculiar races. 3. Animals of Islands. The animals of islands situated near continents are in general the same as those of the neighboring mainland. Small islands lying at a great distance from continents are nearly or quite destitute of quadrupeds, except such as appear to have been carried to them by man |