INTRODUCTION. 



29 



(4.) The fourth class of lakes includes tnose which receive without discharguig 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. 



The following table exhibits the dimensions of tJie principal lakes. 



JVames. 



Caspian 



Superior 



Michigan 



Huron 



Ballcasli 



Maracaybo 



Sg. ms. 



120,000 

 35,000 

 25,000 

 20,000 

 14,400 

 14,000 



JVames. 



Slave 



Winnipeg 



Bailial 



Erie 



Aral 



Onega 



Sq. ms. 

 13,500 

 12,500 

 12,000 

 10,000 

 9,000 

 9,000 



JVamcs. Sq. ms. 



Ontario 8,200 



Ladoga 6,000 



Nicaragua 5,000 

 Lalie of tire Woods 2,800 



Wenner 2,400 



Cliamplain 900 



JVames. 

 Maeler 

 Wetter 

 Geneva 

 (\'ay uga 

 Constance 



Sq. m». 

 1,600 

 1,000 

 500 

 480 

 400. 



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. Springs. Springs are small reservoirs, which receive their waters from the neighboring 

 ground through small channels, and which are usually discharged by overflowing. The origin 

 of springs must be referred to various causes ; among these the most common are the falling 

 of rain and dew, and the melting of ice and snow. Mountains attract the fogs and clouds 

 which float around them ; consequently more rain and snow fall upon them than upon the 

 plains. The waters descend, forming innumerable streams, some in the shape of rivers, and 

 others in the form of springs. 



Spouting springs are formed by having their source considerably elevated above the aperture 

 at which they issue. They are constructed on the same principles as artificial fountains, — the 

 column of water being thrown upwards by the pressure from above. Hot spouting springs 

 doubdess obey the same laws, the water being heated by volcanic masses in the earth. Boiling 

 springs are found in various countries. Periodical, or intermitting springs, are very curious 

 works of nature. At Como, in Italy, there is one which rises and falls every hour. There is 

 a spring in Provence, in France, which rises eight times in an hour, and another in Languedoc, 

 the period of whose elevation each day, is fifty minutes later than the preceding day. England 

 furnishes examples of several springs which rise and fall with the ebb and flow of the sea. 



6. Glaciers. Glaciers, which crown the tops of the highest mountains, have a close con- 

 nexion with the origin of springs. The snows which have accumulated for whole centuries, 

 sink down and are consolidated by alternate thaws and frosts. Thus the tops of the mountains 

 are covered with ice ; these masses gradually extend, and the high valleys become filled. The 

 masses continuing to increase, sometimes for a series of years, even extend to the lower parts 

 of the mountains. They have in Switzerland filled up whole valleys, buried villages, and shut 

 up the pass between Le Valais and the Canton of Berne. The scenes which these bodies of 

 ice present to the traveller, are various and striking ; sometimes they resemble the waves of 

 the sea ; again they appear like mirrors of brilliant glass. Here a thousand spires glitter in the 

 sunshine ; sometimes they assume the appearance of ruins fallen and crushed ; and again vast 

 pyramids and obelisks are presented to the astonished beholder. 



These glaciers are of essential service in furnishing to the adjacent regions, slowly and in 

 almost a regular manner, waters, which without this congelation would be precipitated with 

 impetuosity from the heights of the mountains, so as to overflow and devastate the countries 

 which they now fertihze. 



7. Mineral Springs. The water of springs always contains air and some saline ingredients. 

 In good drinkable water the air is usually equal to about one twenty-fifth of its volume, and the 

 saline contents form about one 6000th of its weight. Such springs are called soft, and 

 answer well for domestic purposes ; when the solid ingredients exceed this quantity, the water 

 is hard, and is less fitted for cooking, dissolving soap, &c. When these substances are in 



