LAKES. \ 



LAKES. 



78 



affluents nor outlet, others have affluents without any visible outlet, 

 some have an outlet without any visible affluents, and others again 

 have both affluents and an outlet. Lakes without outlets have the 

 level of their waters horizontal, that is, parallel to the curvature of the 

 earth ; while those with affluents and outfalls are on the contrary 

 more or less out of the horizontal level, sometimes, as in the Logo 

 Maggiore, as much as three inches in a mile. 



Lakes without apparent affluents or outlets are comparatively small, 

 and yet they are, relatively speaking, more permanent than larger 

 Lakes, because, being fed chiefly by subterraneous springs, they are not 

 liable to be filled by those deposits of earth and sand which are the 

 main cause of the rapid desiccation of such lakes as receive the troubled 

 waters of torrents and rivers. If we follow the usual custom and call 

 all natural sheets of water lakes, then there are many lakes without 

 affluents or outlet. Thus they are very numerous to the northward of 

 the Caspian and in the plains which extend between the Ural Moun- 

 tains and the Irtish, as also in the great Steppe of Baraba between the 

 Irtish and the Ob. But in truth the greater part of these are more 

 pmpi'riy ponds, formed of the accumulated waters from rain and 

 melted snow. The largest of them are not more than ten or twelve 

 miles in circumference and six or seven feet deep ; indeed many of 

 them are quite dried up towards the end of summer. Some are salt 

 and yield considerable profit. Their saltness is not easily accounted 

 for ; the more particularly as among and close to those that are salt 

 there are many whose waters are quite fresh. The opinions of natural- 

 ists on the subject of salt lakes are very various, and no satisfactory 

 theory has perhaps yet been offered. Small lakes of the kind of which 

 we have been speaking, that is to say, such as have neither affluents 

 nor outlet, sometimes occur in hollows resembling the craters of extinct 

 volcanoes. There has been much controversy as to the existence of 

 lakes in volcanic rocks. Dolomieu, Spallanzani, and others, asserted 

 their existence, while M. Desmarest absolutely denies the possibility of 

 lakes existing hi the craters of extinct volcanoes. But near the present 

 active volcano of Antuco (in the state of La Plata, not far from 

 Mendoza), there is a lake of immense depth, at a height of from 4000 

 to 5000 feet above the sea, which has such an intimate relation with 

 the volcano, that on every eruption the lake pours out a stream of 

 muddy water, wasting the adjacent district. The little lake of Nemi, 

 about 20 miles from Rome, is unquestionably formed in an old volcanic 

 crater, and that it haa increased since its first formation, is shown by 

 its having engulphed a building of the time of Tiberius. The cele- 

 brated lake of Averno is, according to Ferber and Breislak, situated in 

 an ancient crater ; as were those probably of Bolsena and Bracciano. 

 The subject of lakes, considered geologically, has not yet received the 

 attention it deserves. The connection of lakes with the main ocean is 

 an important but little known question.. They are all basins enclosed 

 by certain obstructions that act as dams, and which certain phenomena, 

 such as earthquakes or volcanoes, may destroy, and allow the water to 

 run ofE London, there is every reason to suppose, stands on the site 

 of what was once a lake of large size. 



Of lakes which receive affluents without having any visible outlet, 

 the largest is the Caspian. The Aral, and the Dead Sea, or Lake 

 Asphaltites, are also examples of this kind of lake, which is very com- 

 mon ii me of them are of vast extent, such for instance is 

 the lake Terkiri in Tibet, 27 leagues long and 9 leagues wide, and the 

 lake Hoho-nor, or Koko-nor, in the same country, whose surface is 240 

 square leagues. It was at one time thought that the saltness of certain 

 lakes was due to the circumstance of their receiving the saline impuri- 

 ties of then* affluents, which impurities could not escape for want of 

 an outlet ; but on the one hand, the Durrah in Segistan, which receives 

 the Helmund and has no outlet, is perfectly fresh ; and on the other, 

 there are many salt lakes which have no affluents, hence the saltness 

 of lakes must have some other cause. The question has sometimes 

 been asked, what becomes of the excess of water brought into lakes 

 having no outlet? Halley thought evaporation was all-sufficient to 

 carry it off. and his opinion is highly plausible. If, however, it shall 

 be found by actual experiment that a greater quantity of water is 

 brought into a lake without apparent issue than can be carried off by 

 evaporation, the natural conclusion will be, that the surplus is lost by 

 infiltration or sub-aqueous drainage. Several of these lakes have for- 

 merly had outlets, but water haa ceased to Bow from them, because 

 the lakes have sunk in consequence of receiving now a much smaller 

 quantity of water than formerly. There are many lakes in Europe at 

 the present day whose outlets are diminishing ; such among others are 

 the lakes Balaton and Neusiedel in Hungary. The extent of surface 

 of the former is very great compared with the quantity of water which 

 it receives, so that the evaporation is rapidly diminishing the lake, and 

 the river Schio, which used to carry off its superabundant waters and 

 pour them into the Danube, is now nothing more than a slip of bog ; 

 and as for the lake Neusiedel, it appears formerly to have communi- 

 cated with the Danube by the Raab, into which it emptied its waters, 

 and with which it has now no other communication than by a swamp. 

 The Aral also, it is generally believed, once communicated with the 

 Caspian. 



Those lakes which have an outlet without any apparent affluent are 



fed by subaquuumi springs, which, bursting out in a hollow, must fill 



it up before the waters can flow off in a stream. These lakes are 



My situated at considerable elevations above the level of the sea. 



Thus there is one on Monte Rotondo in Corsica, at an elevation of 

 9069 feet. From lakes of this kind some of the largest rivers take 

 their rise ; the Volga, for instance, springs from such a lake in the 

 government of Tver in Russia. The lake Titicaca in the Bolivian Andes, 

 is 12,000 feet above the sea-level, and is 120 fathoms deep, while the 

 Caspian is below the sea-level, according to some, 100 feet. The sur- 

 face of Lake Superior is 600 feet above the sea-level, and in certain 

 places the lake has a depth of 1200 feet ; while not only this but all 

 the other Canadian lakes show evident marks of their level having 

 been originally considerably higher, and that they have been lowered 

 by degrees. Lake Erie has a medium depth of only 60 to 70 feet, 

 and the shallowness there is little doubt is owing to the detritus 

 brought down from the lakes above, and there deposited. 



Lakes 'which receive one or more tributary streams and have a visible 

 outlet for their superabundant waters are the most common and the 

 largest ; such are the lakes of Switzerland and of the north of Italy, 

 the lakes Ladoga, Onega, Peipus, and llmen in Russia ; the Saima in 

 Finland, the Wener in Sweden, the Enara in Lapland, &c. In Asia 

 there are the Nor-Za'issan and the Baikal, &c. In Africa the existence 

 of large lakes in the interior has been well established ; first, by the 

 precise verification of lake Tchad, whose outfall is the Niger ; and more 

 lately by the discoveries of Dr. Livingstone of the Ngami, of which 

 the great outfall is the Zambezi ; by Captain Speke of the lake Nyassi, 

 or Nyanza, which is considered by him as the true source of the Nile. 

 In North America, Lake Superior, Lake Huron, Lake Erie, and Lake 

 Ontario are examples of this kind of lake ; each of them receives 

 several affluents ; and the grand outlet of the whole is the river St. 

 Lawrence. 



Lakes owe their origin to different circumstances : some from the 

 sinking of the soil by the falling in of subterraneous caverns such is 

 the supposed origin of the Baikal ; others are caused by earthquakes 

 such a lake was formed in the province of Quito in 1797 ; some by the 

 fall of mountains, as the Oschenen-see in the canton of Berne ; or by 

 lava currents damming up the stream, as the lakes Aidat and Cassiere 

 in Auvergne, hi France. Many are supposed to be the remains of the 

 universal ocean which once covered the earth, and their waters origi- 

 nally salt, have become fresh from their receiving constant supplies 

 of fresh water while the salt was continually let off by their outlets. 



Almost all lakes are in progress of diminution, although this is not 

 everywhere apparent. The detrital matter brought in by their affluents 

 is imperceptibly filling up their beds ; and if regidar observations were 

 made, many provinces which owe much of their prosperity to their 

 lakes, would find the time fast approaching when these pieces of water 

 will become mere pestilential marshes. 



Certain lakes exhibit remarkable phenomena : thus some have float- 

 ing islands in them, as is the case with a small lake near St. Omer. 

 The Lake Oerdau, in Prussia, has a floating island, on which a hundred 

 head of cattle may be seen pasturing. In the lake Kolk, in Osnabruck, 

 there is a floating island, on which fine elms are growing. Some of 

 these floating islands sink and rise again ; thus in the lake Ralang in 

 Smoland, a province of Sweden, there is a floating island which appeared 

 and disappeared ten successive times between the years 1696 and 1766. 

 Other floating islands are found in East Gothland and many other 

 places. Some subterranean lakes are supposed to have become so by 

 the formation and subsequent fixing of floating islands, which succes- 

 sively uniting have finished by forming a solid crust over the water. 



Some lakes have a double bottom, which rising and sinking alter- 

 nately changes the apparent depth of the lake ; there is a lake of this 

 kind at Jemtia in Sweden. 



Some lakes are said to have no bottom ; but this is an impossibility : 

 the fact is, that the sound does not reach the bottom, either for want 

 of sufficient weight of lead or length of line, or else it is carried away 

 by under-currents. 



In Poland there exists a lake said to render brown the skin of those 

 who bathe in it. Certain mineral waters impregnated with sulphu- 

 retted hydrogen are well known to change from white to brown the 

 skins of those persons who have been under a course of metallic medi- 

 cines, or who use metallic cosmetics, and some such circumstance may 

 be the case with the lake in question. 



Some lakes are intermittent : the most remarkable of this kind arc 

 those of Cirknitz in Illyria, and Kauten in Prussia. They are sup- 

 posed to be occasioned by a play of natural siphons, upon the same 

 principle as intermittent fountains. 



The Lake of Geneva is subject to a subaqueous wind, called the 

 Vaudaiae, which, rising to the surface, produces an agitation of the 

 water which is sometimes dangerous to the navigation of the lake. 

 Near Boleslaw in Bohemia there is a lake of unknown depth, from the 

 bottom of which there rise, in winter, such violent puffs of wind, that 

 they are said to send up into the air masses of ice of several hundred 

 pounds weight. The sudden escape of gases formed in the bowels of 

 the earth, and perhaps the air forcibly driven out from caverns by the 

 water rushing into and filling them up, may be among the causes of 

 this remarkable phenomenon. 



The Seiches are a phenomenon which has hitherto been observed 

 only hi the Lake of Geneva and some other of the Swiss and Italian 

 lakes, though it is probably common to many others. It consists in an 

 occasional undulation of the water, something like a tide wave, which 

 rises occasionally to the height of 5 feet. Its cause is not exactly 



