often experience during the serenest weather violent agita- BOOK tions. In the Murche of Brandebaurg, the pool of Krestin often commences in fine weather to boil up in whirlpools, so as to engulph the little boats of the fishermen.* Perhaps the decomposition of calcareous stones has an influence upon some of these phenomena. In the general history of lakes, floating islands occupy a great space in the writings of some geographers. But when, on the one hand, we consider how many inaccessible marshes there are always floating in the water, and notwithstanding, covered with brushwood, and even trees; and on the other hand, when we consider those beds of vegetables, those immense forests that are found buried, and very recently buried, in turf pits, we may then easily form an idea of these floating islands which some geographers represent as wonders of nature. They are simply earth of the nature of a peat, but very light, sometimes only reeds and roots of trees interwoven together. After having been undermined by the waters, they detach themselves from the bank, and from their lightness and spongy consistency, joined to their inconsiderable thickness, they remain suspended and floating on the surface of the waters.f The delightful Loch Lomond, in Scotland, contains some of these floating islands, which are not very uncommon in Scotland or Ireland. A small lake in Artois, near Saint Omer, is covered with similar islands. The marshy lakes of Comacchio present a great number. The most considerable that are mentioned, are those of the lake of Gerdau, in Prussia, which furnish pasturage for 100 head of cattle; and that of the lake of Kolk, in the country of Osnabruck, covered with beautiful elms. There are some floating islands which appear and disap- Floating periodical pear alternately. The lake Ralang, in Smalande, a pro-islands. * Bernouille, Archives des Voyages, i. 325. Pliny, Natural History, ii. c. 95. Girolamo Silvestri, Treatise on Floating Islands, ancient and modern, (in Italian.) Kant. Géographie-Physique, ii. part i. p. 114. BOOK Floating islands which have become stationary. Temperature of lakes. vince of Sweden, encloses a floating island, which, from 1696 to 1766 has shewn itself ten times, generally in the months of September and October.* It is 280 feet long, and 220 broad. There is an island similar to it in Ostrogothia. The floating islands may have an influence upon the formation of the globe. Those which Pliny and Seneca saw floating in the lakes of Bolsena, Bressanello and others, have become fixed. West Friezland has a subterraneous lake, which appears to have been covered with floating islands, that gradually united together, and ended in the formation of a solid crust.† The shade of thick forests, or high mountains, may prevent certain lakes, like Loch Winnoch in Scotland, from getting rid of the perpetual ice which covers them in whole or in part. Other lakes, always ruffled by the winds, or stirred by the rivers which they receive, and the springs which feed them, brave all the rigours of a cold climate. The most extraordinary phenomenon would be to see lakes freeze during summer; and this has been related of some in China; the cause of which has been sought for in the saline nature of the neighbouring ground, but the fact appears to have been insufficiently observed, or incorrectly recorded.+ The depth of lakes varies infinitely, and cannot form a subject of general physical geography. We must be satisfied merely with contradicting the popular opinion, that there are lakes without a bottom. Those which have been considered as such owe this character solely to the existence of currents which carry along with them the lead attached to the sounding line. But we must not reckon as fabulous the accounts of lakes with double bottoms, which are said to be found in Jemptia or Jemptland, in Sweden and else where. It has been supposed, that a crust interwoven *Bergmann, Géographie Physique, ii. 238. † Annals of the Prussian Monarchy, 1799, p. 292, (in German.) with roots, similar to the floating islands, may exist at the BOOK bottom of a lake, and by either rising or sinking, may make XIII. the depth vary in appearance. waters. Such are the principal observations we have to make Chemical upon the origin and motion of springs, rivers, and lakes. nature of We are now to consider them with relation to their chemical nature. We have already remarked the property which water possesses of absorbing atmospheric air. It is estimated, that fresh water generally holds in solution of its weight of air. A certain time is necessary before it becomes saturated with salt, and all the elements of atmospheric air are not absorbed by water with equal readiness. Pure oxygen unites most easily with it: The good quality of fresh water consists in being completely saturated with oxygen, which must be frequently renewed by the running and agitation of the water. Its bad quality arises either from the alteration, or the superabundance of the oxygen; and cach of these states announces the presence of a heterogeneous substance in water, capable of absorbing more oxygen, or of altering it. These heterogeneous substances are earthy salts, sulphur, lime, gravel, and mud. of exposure upon wa These principles, completely established by modern che- Influence mistry, would lead us to believe that the influence of local exposure upon the nature of waters is as powerful as Hip- ter. pocrates has represented it.* Waters exposed to the rising sun, says he, are limpid, inodorous, soft, and agreeable to drink, because the sun at its rising corrects them by dissipating the fogs of the morning which may have mingled with them. Waters lying towards the setting sun are destitute of this advantage, and are not limpid. Those which flow towards the south, and are exposed to hot winds, should be brackish, not very deep, and consequently hot in summer and cold in winter, and likely to enervate man, and to render him liable to several maladies. Lastly, waters having a northern exposure, should in general be cold, hard Traité des Airs, des Eaux, et des Lieux, 9, 20, 22, 25. Edit. de Coray. XIII. BOOK and unpleasant, the use of them drains away the milk from women, and renders them barren. Such is the system of Hippocrates; but we ought not, with the blind admirers of that eminent physician, to extend its application too widely; for it is connected with his ideas upon the particular nature of winds, and these ideas contain only local truths, applicable to Greece and Asia Minor. The waters The waters of marshes, pools, and all those which stagof marshes. nate under ground, are unwholesome; they dissolve azotic and hydrogen gases, arising from the decomposition of plants, insects, and fishes. The surrounding atmosphere is loaded with these noxious gases. They who live near marshes, and who drink the waters, lead a miserable life, never acquire strength, and prematurely feel all the infirmities of age. Waters of mountains. In Salogne, (in France,) to go no farther in quest of an example, the stagnant humidity gives to the natives pale countenances, languishing eyes, and a weak voice.* Stagnant waters almost always absorb a great quantity of fixed air, or carbonic acid gas; for this gas is carried by its gravity towards the surface of the waters, and does not disengage itself. The water of hills and mountains, differs in its quality, hills and according as it filters through banks of pure rock, of schist, of quartz, or of sand, from all which substances it can scarcely derive any property whatever;-or, as it flows over beds of potter's earth, which it neither draws along with it nor dissolves;-or, lastly, as it traverses ground which is calcareous, marly, gypseous, impregnated with magnesia, salt, or bitumen. Waters of the kind last mentioned are always very much mixed with heterogeneous substances, and for the greatest part of the year are hard, turbid, and unwholesome, at least if daily used. Hippocrates, Homer, and Plutarch, have long ago condemned the use of them.t Those waters which have clayey bottoms are the most com + Mémoire de la Société Royale de Medicine, 1776, p. 61-72. Hippocrates, 1. c. 35. et le Commentaire de Coray, p. 107. mon; they unite those qualities which are essential to salu- BOOK brity. Those which flow from the hard rock are still more XIII. pure and limpid, as they must undergo a process of filtration in wearing their way over a stony bed. lakes. The waters of lakes being derived from springs and rivers, Waters of partake of their different qualities. There are some lakes whose waters are extremely limpid, such as the lake of Geneva, and that of Wetter in Sweden; in the latter, a farthing may be perceived at the bottom of the lake at 120 feet depth; but the lakes whose waters are motionless, or saline, or bituminous, may be looked upon as equally unwholesome with those of marshes. The waters of rivers contain some very heterogeneous River waelements, which seem necessarily to counteract each other; ter. and it is, perhaps, as much owing to this reciprocal destruction of pernicious principles, as to continual motion, that river water is so generally serviceable to the wants of man, and supports the freshness and purity of the atmosphere wherever they flow. It often, however, forms a sediment of gravel and mud; and Hippocrates imagined, that, when used as a constant beverage, it produced, amongst other maladies, that of the stone.* Well water, by remaining too long motionless, frequently well waacquires the bad qualities of stagnant waters. ter. Sea water acts as an emetic with us, but the inhabitants Sea water. of the Island of Paques, in the Pacific Ocean, make it their common beverage. Amongst the atmospheric waters, those Rain wacomposed of rain are the most wholesome on account of ter. their softness, variety, and lightness. Hippocrates has justly observed the admirable process which nature employs in distilling the vapours raised from the earth by the action of the sun. These vapours are agitated and rolled about in all directions; their more earthy and turbid part separate, and, sinking by their own weight, form fogs; the remainder, more subtle and more light, is still more completely dissolved by the solar heat. It is of this remainder that Hippocrates, 1. c. 51, and et le Commentaire de Coray, 134. |