PHYSICAL GEOGRAPHY. 



but have no visible outlet, as the Caspian Sea ; 

 those which receive no running water (being fed 

 by springs), but have an outlet ; and such as 

 neither receive nor discharge running water. Lakes 

 are distributed over the globe according to the 

 inequalities of surface ; and all tend to annihila- 

 tion, partly by silting up their basins, and partly by 

 deepening their outlets, thereby effecting an entire 

 drainage of their waters. The most gigantic are 

 those of North America such as Superior, Huron, 

 Michigan, Erie, and Ontario, which respectively 

 occupy 32,000, 20,000, 16,000, 10,000, and 7200 

 square miles. Next in order are the lakes of 

 Africa, three of which Victoria Nyanza, Albert 

 Nyanza, and Tanganyika are estimated at 29,900, 

 25,400, and 10,400 respectively. The largest lakes 

 in Asia are Aral and Baikal ; the surface of the 

 former is estimated at 23,000, and the latter at 

 15,000 square miles. Europe can boast of a vast 

 number, which, though generally small, give 

 beauty and diversity to her landscapes. Those 

 of Ladoga and Onega, in Russia, are the largest ; 

 the former having a surface of 6330, and the latter 

 of 3280 square miles. A comparative estimate of 

 the extent of these vast sheets may be formed when 

 we mention that the area of Lake Geneva does 

 not exceed 340 square miles. 



Lakes subserve important purposes in the 

 economy of nature. They serve as reservoirs for 

 the waters which rivers would too speedily carry 

 away from the land ; they are the tanks, as it 

 were, in which the impurities of streams subside ; 

 they refresh and enliven the landscape ; and as 

 they all tend to silt up their own sites, these sites 

 become in time tracts of fertile alluvium, and such 

 has been the origin of some of our finest plains. 



Rivers, streams, springs whether flowing with 

 a volume several miles in breadth, or trickling in 

 a tiny rill which a child's hand might obstruct 

 constitute a class of the most valuable agencies in 

 the physical history of our globe. They are the 

 irrigators of its surface, adding alike to the beauty 

 of the landscape and the fertility of the soil ; they 

 carry off impurities and every sort of waste debris, 

 to be deposited in the ocean as the strata of future 

 continents ; and when of sufficient volume, they 

 form the most available of all channels of commu- 

 nication with the interior of continents. Rivers 

 originate in the rain and snow which descend from 

 the sky (see METEOROLOGY). Falling on the 

 surface, the water percolates the soil, finds its 

 way through the rents, fissures, and pores of the 

 rocky strata, and ultimately escapes at some lower 

 level in the form of springs. Some of these 

 springs are perennial, others temporary or inter- 

 mittent : some are limpid, and almost absolutely 

 pure ; others are impregnated with metallic, earthy, 

 and saline ingredients, according to the nature of 

 the strata through which they have percolated : 

 some are cold, others tepid; while many issue, 

 with bubbling and steam, near the ordinary boil- 

 ing-point of water. Springs, naturally tending to 

 lower levels, unite and form streams j and these, 

 again, falling still lower, conjoin in valleys, and 

 form rivers creating in their course rapids, 

 cataracts, and waterfalls, ravines and dells, lakes, 

 swamps, and marshes, alluvial plains, and low 

 terminating deltas. The valley in which a river 

 flows is usually termed its basin; and its drainage 

 is that portion of country drained by its streams 

 or tributaries ; the terms are often used syn- 



onymously. To compare merely the lengths of 

 rivers is far from conveying a correct idea either 

 of their physical or economical importance ; the 

 extent of their basins is an equally important item. 

 The following is a comparative view of the length 

 and drainage of some of the principal rivers on 

 the globe, headed by the Thames as a standard 

 of comparison. 



CLIMATOLOGY. 



The climatology of the globe relates to the degree 

 of heat and cold to which its respective countries 

 are subject, the dryness and moisture of the air, 

 and its salubrity or insalubrity as influenced by 

 these and other causes. As yet the minutiae of 

 climate are but imperfectly determined ; the fol- 

 lowing general causes, however, have been suffi- 

 ciently ascertained : i. The action of the sun upon 

 the soil and atmosphere ; 2. The internal heat of 

 the globe ; 3. The height of the place above the 

 sea ; 4. The general exposure of the region ; 5. The 

 direction of its mountains relatively to the cardinal 

 points ; 6. The neighbourhood of the sea, and its 

 relative position ; 7. The geological character of 

 the soil; 8. The degree of cultivation which 

 it has received; and 9. The prevalent winds. 

 These causes, acting together or separately, deter- 

 mine the character of a climate as moist and 

 warm, moist and cold, dry and warm, dry and 

 cold, &c. ; and this climatic character is the main 

 influence which determines the nature and amount 

 of vegetable and animal development The several 

 subjects belonging to this section are treated in 

 detail in the number on METEOROLOGY. 



DISTRIBUTION OF PLANTS AND ANIMALS. 



The life of the globe that is, its vegetable and 

 animal productions constitutes its most import- 

 ant and exalted feature as a creation. All the 



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